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[website.git] / docs / protocol / draft-riikonen-silc-spec-09.txt

Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-spec-09.txt                          15 January 2007
Expires: 15 July 2007


                 Secure Internet Live Conferencing (SILC),
                          Protocol Specification
                     <draft-riikonen-silc-spec-09.txt>

Status of this Draft

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Abstract

   This memo describes a Secure Internet Live Conferencing (SILC)
   protocol which provides secure conferencing services over insecure
   network channel.  SILC provides advanced and feature rich conferencing
   services with security as main design principal.  Strong cryptographic
   methods are used to protect SILC packets inside the SILC network.
   Three other specifications relates very closely to this memo;
   SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
   Protocols [SILC3] and SILC Commands [SILC4].









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Table of Contents

   1 Introduction ..................................................  3
     1.1 Requirements Terminology ..................................  4
   2 SILC Concepts .................................................  4
     2.1 SILC Network Topology .....................................  5
     2.2 Communication Inside a Cell ...............................  6
     2.3 Communication in the Network ..............................  7
     2.4 Channel Communication .....................................  7
     2.5 Router Connections ........................................  8
   3 SILC Specification ............................................  9
     3.1 Client ....................................................  9
         3.1.1 Client ID ........................................... 10
     3.2 Server .................................................... 11
         3.2.1 Server's Local ID List .............................. 11
         3.2.2 Server ID ........................................... 12
         3.2.3 SILC Server Ports ................................... 12
     3.3 Router .................................................... 13
         3.3.1 Router's Local ID List .............................. 13
         3.3.2 Router's Global ID List ............................. 14
         3.3.3 Router's Server ID .................................. 15
     3.4 Channels .................................................. 15
         3.4.1 Channel ID .......................................... 16
     3.5 Operators ................................................. 17
     3.6 SILC Commands ............................................. 17
     3.7 SILC Packets .............................................. 17
     3.8 Packet Encryption ......................................... 18
         3.8.1 Determination of the Source and the Destination ..... 18
         3.8.2 Client To Client .................................... 19
         3.8.3 Client To Channel ................................... 20
         3.8.4 Server To Server .................................... 21
     3.9 Key Exchange And Authentication ........................... 21
         3.9.1 Authentication Payload .............................. 22
     3.10 Algorithms ............................................... 24
         3.10.1 Ciphers ............................................ 24
                3.10.1.1 CBC Mode .................................. 24
                3.10.1.2 CTR Mode .................................. 25
                3.10.1.3 Randomized CBC Mode ....................... 27
         3.10.2 Public Key Algorithms .............................. 27
                3.10.2.1 Multi-Precision Integers .................. 28
         3.10.3 Hash Functions ..................................... 28
         3.10.4 MAC Algorithms ..................................... 28
         3.10.5 Compression Algorithms ............................. 29
     3.11 SILC Public Key .......................................... 29
     3.12 SILC Version Detection ................................... 32
     3.13 UTF-8 Strings in SILC .................................... 33
         3.13.1 UTF-8 Identifier Strings ........................... 33
     3.14 Backup Routers ........................................... 34



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         3.14.1 Switching to Backup Router ......................... 36
         3.14.2 Resuming Primary Router ............................ 37
   4 SILC Procedures ............................................... 39
     4.1 Creating Client Connection ................................ 39
     4.2 Creating Server Connection ................................ 41
         4.2.1 Announcing Clients, Channels and Servers ............ 42
     4.3 Joining to a Channel ...................................... 43
     4.4 Channel Key Generation .................................... 44
     4.5 Private Message Sending and Reception ..................... 45
     4.6 Private Message Key Generation ............................ 46
     4.7 Channel Message Sending and Reception ..................... 47
     4.8 Session Key Regeneration .................................. 47
     4.9 Command Sending and Reception ............................. 48
     4.10 Closing Connection ....................................... 49
     4.11 Detaching and Resuming a Session ......................... 49
     4.12 UDP/IP Connections ......................................  51
   5 Security Considerations ....................................... 52
   6 References .................................................... 53
   7 Author's Address .............................................. 55
   Appendix A ...................................................... 55
   Appendix B ...................................................... 56
   Appendix C ...................................................... 57
   Appendix D ...................................................... 57
   Full Copyright Statement ........................................ 58

List of Figures

   Figure 1:  SILC Network Topology
   Figure 2:  Communication Inside cell
   Figure 3:  Communication Between Cells
   Figure 4:  Router Connections
   Figure 5:  SILC Public Key
   Figure 6:  Counter Block
   Figure 7:  CTR Mode Initialization Vector


1. Introduction

   This document describes a Secure Internet Live Conferencing (SILC)
   protocol which provides secure conferencing services over insecure
   network channel.  SILC can be used as a secure conferencing service
   that provides rich conferencing features.  Some of the SILC features
   are found in traditional chat protocols such as IRC [IRC] but many
   of the SILC features can also be found in Instant Message (IM) style
   protocols.  SILC combines features from both of these chat protocol
   styles, and can be implemented as either IRC-like system or IM-like
   system.  Some of the more advanced and secure features of the
   protocol are new to all conferencing protocols.  SILC also supports



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   multimedia messages and can also be implemented as a video and audio
   conferencing system.

   Strong cryptographic methods are used to protect SILC packets inside
   the SILC network.  Three other specifications relates very closely
   to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
   Authentication Protocols [SILC3] and SILC Commands [SILC4].

   The protocol uses extensively packets as conferencing protocol
   requires message and command sending.  The SILC Packet Protocol is
   described in [SILC2] and should be read to fully comprehend this
   document and protocol.  [SILC2] also describes the packet encryption
   and decryption in detail.  The SILC Packet Protocol provides secured
   and authenticated packets, and the protocol is designed to be compact.
   This makes SILC also suitable in environment of low bandwidth
   requirements such as mobile networks.  All packet payloads in SILC
   can be also compressed.

   The security of SILC protocol sessions are based on strong and secure
   key exchange protocol.  The SILC Key Exchange protocol is described
   in [SILC3] along with connection authentication protocol and should
   be read to fully comprehend this document and protocol.

   The SILC protocol has been developed to work on both TCP/IP and UDP/IP
   network protocols.  However, typical implementation would use only TCP/IP
   with SILC protocol.  Typical implementation would be made in client-server
   model.


1.1 Requirements Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
   MAY, and OPTIONAL, when they appear in this document, are to be
   interpreted as described in [RFC2119].


2. SILC Concepts

   This section describes various SILC protocol concepts that forms the
   actual protocol, and in the end, the actual SILC network.  The mission
   of the protocol is to deliver messages from clients to other clients
   through servers and routers in secure manner.  The messages may also
   be delivered from one client to many clients forming a group, also
   known as a channel.

   This section does not focus to security issues.  Instead, basic network
   concepts are introduced to make the topology of the SILC network
   clear.



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2.1 SILC Network Topology

   SILC network forms a ring as opposed to tree style network topology that
   conferencing protocols usually have.  The network has a cells which are
   constructed from a router and zero or more servers.  The servers are
   connected to the router in a star like network topology.  Routers in the
   network are connected to each other forming a ring.  The rationale for
   this is to have servers that can perform specific kind of tasks what
   other servers cannot perform.  This leads to two kinds of servers; normal
   SILC servers and SILC router servers.

   A difference between normal server and router server is that routers
   knows all global information and keep the global network state up to date.
   They also do the actual routing of the messages to the correct receiver
   within the cell and between other cells.  Normal servers knows only local
   information and receive global information only when it is needed.  They do
   not need to keep the global network state up to date.  This makes the
   network faster and scalable as there are less servers that needs to
   maintain global network state.

   This, on the other hand, leads into a cellular like network, where
   routers are in the center of the cell and servers are connected to the
   router.

   The following diagram represents SILC network topology.

          ---- ---- ----         ---- ---- ----
         | S8 | S5 | S4 |       | S7 | S5 | S6 |
         ----- ---- -----       ----- ---- -----
        | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
         ---- ------ ----       ---- ------ ----
         | S6 | S3 | S1 |       | S1 | S3 | S2 |         ---- ----
          ---- ---- ----         ---- ---- ----         | S3 | S1 |
             Cell 1.   \             Cell 2.  | \____  ----- -----
                        |                     |        | S4 | S/R4 |
            ---- ---- ----         ---- ---- ----       ---- ------
           | S7 | S4 | S2 |       | S1 | S3 | S2 |      | S2 | S5 |
           ----- ---- -----       ----- ---- -----       ---- ----
          | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
           ---- ------ ----       ---- ------ ----
           | S8 | S5 | S3 |       | S6 | S7 | S8 |     ... etc ...
            ---- ---- ----         ---- ---- ----
               Cell 3.                Cell 5.

                     Figure 1:  SILC Network Topology


   A cell is formed when a server or servers connect to one router.  In



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   SILC network normal server cannot directly connect to other normal
   server.  Normal server may only connect to SILC router which then
   routes the messages to the other servers in the cell.  Router servers
   on the other hand may connect to other routers to form the actual SILC
   network, as seen in above figure.  However, router is also able to act
   as normal SILC server; clients may connect to it the same way as to
   normal SILC server.  This, however is not a requirement and if needed
   router servers may be hidden from users by not allowing direct client
   connections.  Normal server also cannot have active connections to more
   than one router.  Normal server cannot be connected to two different
   cells.  Router servers, on the other hand, may have as many router to
   router connections as needed.  Other direct routes between other routers
   is also possible in addition of the mandatory ring connections.  This
   leads into a hybrid ring-mesh network topology.

   There are many issues in this network topology that needs to be careful
   about.  Issues like routing, the size of the cells, the number of the
   routers in the SILC network and the capacity requirements of the
   routers.  These issues should be discussed in the Internet Community
   and additional documents on the issue may be written.


2.2 Communication Inside a Cell

   It is always guaranteed that inside a cell message is delivered to the
   recipient with at most two server hops.  A client which is connected to
   server in the cell and is talking on channel to other client connected
   to other server in the same cell, will have its messages delivered from
   its local server first to the router of the cell, and from the router
   to the other server in the cell.

   The following diagram represents this scenario:


                         1 --- S1     S4 --- 5
                                  S/R
                          2 -- S2     S3
                              /        |
                             4         3


                   Figure 2:  Communication Inside cell


   Example:  Client 1. connected to Server 1. send message to
             Client 4. connected to Server 2. travels from Server 1.
             first to Router which routes the message to Server 2.
             which then sends it to the Client 4.  All the other



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             servers in the cell will not see the routed message.


   If the client is connected directly to the router, as router is also normal
   SILC server, the messages inside the cell are always delivered only with
   one server hop.  If clients communicating with each other are connected
   to the same server, no router interaction is needed.  This is the optimal
   situation of message delivery in the SILC network.


2.3 Communication in the Network

   If the message is destined to client that does not belong to local cell
   the message is routed to the router server to which the destination
   client belongs, if the local router is connected to destination router.
   If there is no direct connection to the destination router, the local
   router routes the message to its primary route.  The following diagram
   represents message sending between cells.



                1 --- S1     S4 --- 5            S2 --- 1
                         S/R - - - - - - - - S/R
                 2 -- S2     S3           S1
                     /        |             \
                    4         3              2

                   Cell 1.               Cell 2.


                  Figure 3:  Communication Between Cells


   Example:  Client 5. connected to Server 4. in Cell 1. sends message
             to Client 2. connected to Server 1. in Cell 2. travels
             from Server 4. to Router which routes the message to
             Router in Cell 2, which then routes the message to
             Server 1.  All the other servers and routers in the
             network will not see the routed message.


   The optimal case of message delivery from the client point of view is
   when clients are connected directly to the routers and the messages
   are delivered from one router to the other.







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2.4 Channel Communication

   Messages may be sent to group of clients as well.  Sending messages to
   many clients works the same way as sending messages point to point, from
   message delivery point of view.  Security issues are another matter
   which are not discussed in this section.

   Router server handles the message routing to multiple recipients.  If
   any recipient is not in the same cell as the sender the messages are
   routed further.

   Server distributes the channel message to its local clients which are
   joined to the channel.  Router also distributes the message to its
   local clients on the channel.


2.5 Router Connections

   Router connections play very important role in making the SILC like
   network topology to work.  For example, sending broadcast packets in
   SILC network require special connections between routers; routers must
   be connected in a specific way.

   Every router has their primary route which is a connection to another
   router in the network.  Unless there is only two routers in the network
   must not routers use each other as their primary routes.  The router
   connections in the network must form a ring.

   Example with three routers in the network:


                    S/R1 - < - < - < - < - < - < - S/R2
                     \                               /
                      v                             ^
                       \ - > -  > - S/R3 - > - > - /


                       Figure 4:  Router Connections


   Example:  Network with three routers.  Router 1. uses Router 2. as its
             primary router.  Router 2. uses Router 3. as its primary router,
             and Router 3. uses Router 1. as its primary router.  When there
             are four or more routers in th enetwork, there may be other
             direct connections between the routers but they must not be used
             as primary routes.

   The above example is applicable to any amount of routers in the network



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   except for two routers.  If there are only two routers in the network both
   routers must be able to handle situation where they use each other as their
   primary routes.

   The issue of router connections are very important especially with SILC
   broadcast packets.  Usually all router wide information in the network is
   distributed by SILC broadcast packets.  This sort of ring network, with
   ability to have other direct routes in the network can cause interesting
   routing problems.  The [SILC2] discusses the routing of packets in this
   sort of network in more detail.


3. SILC Specification

   This section describes the SILC protocol.  However, [SILC2] and
   [SILC3] describes other important protocols that are part of this SILC
   specification and must be read.


3.1 Client

   A client is a piece of software connecting to SILC server.  SILC client
   cannot be SILC server.  Purpose of clients is to provide the user
   interface of the SILC services for end user.  Clients are distinguished
   from other clients by unique Client ID.  Client ID is a 128 bit ID that
   is used in the communication in the SILC network.  The client ID is
   based on the user's IP address and nickname.  User use logical nicknames
   in communication which are then mapped to the corresponding Client ID.
   Client IDs are low level identifications and should not be seen by the
   end user.

   Clients provide other information about the end user as well. Information
   such as the nickname of the user, username and the host name of the end
   user and user's real name.  See section 3.2 Server for information of
   the requirements of keeping this information.

   The nickname selected by the user is not unique in the SILC network.
   There can be 2^8 same nicknames for one IP address.  As for comparison to
   IRC [IRC] where nicknames are unique this is a fundamental difference
   between SILC and IRC.  This typically causes the server names or client's
   host names to be used along with the nicknames on user interface to
   identify specific users when sending messages.  This feature of SILC
   makes IRC style nickname-wars obsolete as no one owns their nickname;
   there can always be someone else with the same nickname.  Also, any kind
   of nickname registering service becomes obsolete.  See the section 3.13.1
   for more information about nicknames.





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3.1.1 Client ID

   Client ID is used to identify users in the SILC network.  The Client ID
   is unique to the extent that there can be 2^128 different Client IDs,
   and IDs based on IPv6 addresses extends this to 2^224 different Client
   IDs.  Collisions are not expected to happen.  The Client ID is defined
   as follows.

      128 bit Client ID based on IPv4 addresses:

      32 bit  Server ID IP address (bits 1-32)
       8 bit  Random number or counter
      88 bit  Truncated MD5 hash value of the nickname

      224 bit Client ID based on IPv6 addresses:

      128 bit  Server ID IP address (bits 1-128)
        8 bit  Random number or counter
       88 bit  Truncated MD5 hash value of the nickname

      o Server ID IP address - Indicates the server where this
        client is coming from.  The IP address hence equals the
        server IP address where the client is connected.

      o Random number or counter - Random number to further
        randomize the Client ID.  Another choice is to use
        a counter starting from the zero (0).  This makes it
        possible to have 2^8 same nicknames from the same
        server IP address.

      o MD5 hash - MD5 hash value of the case folded nickname is
        truncated taking 88 bits from the start of the hash value.
        This hash value is used to search the user's Client ID
        from the ID lists.  Note that the nickname MUST be prepared
        using the stringprep [RFC3454] profile described in the
        Appendix A before computing the MD5 hash.  See also the
        section 3.13.1 for more information.

   Collisions could occur when more than 2^8 clients using same nickname
   from the same server IP address is connected to the SILC network.
   Server MUST be able to handle this situation by refusing to accept
   anymore of that nickname.

   Another possible collision may happen with the truncated hash value of
   the nickname.  It could be possible to have same truncated hash value
   for two different nicknames.  However, this is not expected to happen
   nor cause any serious problems if it would occur.  Nicknames are usually
   logical and it is unlikely to have two distinct logical nicknames



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   produce same truncated hash value.  Use of MD5 in nickname hash is not
   a security feature.


3.2 Server

   Servers are the most important parts of the SILC network.  They form the
   basis of the SILC, providing a point to which clients may connect to.
   There are two kinds of servers in SILC; normal servers and router servers.
   This section focus on the normal server and router server is described
   in the section 3.3 Router.

   Normal servers MUST NOT directly connect to other normal server.  Normal
   servers may only directly connect to router server.  If the message sent
   by the client is destined outside the local server it is always sent to
   the router server for further routing.  Server may only have one active
   connection to router on same port.  Normal server MUST NOT connect to other
   cell's router except in situations where its cell's router is unavailable.


3.2.1 Server's Local ID List

   Normal server keeps various information about the clients and their end
   users connected to it.  Every normal server MUST keep list of all locally
   connected clients, Client IDs, nicknames, usernames and host names and
   user's real name.  Normal servers only keeps local information and it
   does not keep any global information.  Hence, normal servers knows only
   about their locally connected clients.  This makes servers efficient as
   they do not have to worry about global clients.  Server is also responsible
   of creating the Client IDs for their clients.

   Normal server also keeps information about locally created channels and
   their Channel IDs.

   Hence, local list for normal server includes:

      server list        - Router connection
         o Server name
         o Server IP address
         o Server ID
         o Sending key
         o Receiving key
         o Public key

      client list        - All clients in server
         o Nickname
         o Username@host
         o Real name



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         o Client ID
         o Sending key
         o Receiving key
         o Public key

      channel list       - All channels in server
         o Channel name
         o Channel ID
         o Client IDs on channel
         o Client ID modes on channel
         o Channel key


3.2.2 Server ID

   Servers are distinguished from other servers by unique 64 bit Server ID
   (for IPv4) or 160 bit Server ID (for IPv6).  The Server ID is used in
   the SILC to route messages to correct servers.  Server IDs also provide
   information for Client IDs, see section 3.1.1 Client ID.  Server ID is
   defined as follows.

      64 bit Server ID based on IPv4 addresses:

      32 bit  IP address of the server
      16 bit  Port
      16 bit  Random number

      160 bit Server ID based on IPv6 addresses:

      128 bit  IP address of the server
       16 bit  Port
       16 bit  Random number

      o IP address of the server - This is the real IP address of
        the server.

      o Port - This is the port the server is bound to.

      o Random number - This is used to further randomize the Server ID.

   Collisions are not expected to happen in any conditions.  The Server ID
   is always created by the server itself and server is responsible of
   distributing it to the router.


3.2.3 SILC Server Ports

   The following ports has been assigned by IANA for the SILC protocol:



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          silc            706/tcp    SILC
          silc            706/udp    SILC


   If there are needs to create new SILC networks in the future the port
   numbers must be officially assigned by the IANA.

   Server on network above privileged ports (>1023) SHOULD NOT be trusted
   as they could have been set up by untrusted party.


3.3 Router

   Router server in SILC network is responsible for keeping the cell together
   and routing messages to other servers and to other routers.  Router server
   may also act as normal server when clients may connect to it.  This is not
   requirement and router servers may be hidden from clients.

   However, router servers have a lot of important tasks that normal servers
   do not have.  Router server knows everything and keeps the global state.
   They know all clients currently on SILC, all servers and routers and all
   channels in SILC.  Routers are the only servers in SILC that care about
   global information and keeping them up to date at all time.


3.3.1 Router's Local ID List

   Router server as well MUST keep local list of connected clients and
   locally created channels.  However, this list is extended to include all
   the informations of the entire cell, not just the server itself as for
   normal servers.

   However, on router this list is a lot smaller since routers do not need
   to keep information about user's nickname, username and host name and real
   name since these are not needed by the router.  The router keeps only
   information that it needs.

   Hence, local list for router includes:

      server list        - All servers in the cell
         o Server name
         o Server ID
         o Router's Server ID
         o Sending key
         o Receiving key

      client list        - All clients in the cell
         o Client ID



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      channel list       - All channels in the cell
         o Channel ID
         o Client IDs on channel
         o Client ID modes on channel
         o Channel key


   Note that locally connected clients and other information include all the
   same information as defined in section section 3.2.1 Server's Local ID
   List.  Router MAY also cache same detailed information for other clients
   if needed.


3.3.2 Router's Global ID List

   Router server MUST also keep global list.  Normal servers do not have
   global list as they know only about local information.  Global list
   includes all the clients on SILC, their Client IDs, all created channels
   and their Channel IDs and all servers and routers on SILC and their
   Server IDs.  That is said, global list is for global information and the
   list must not include the local information already on the router's local
   list.

   Note that the global list does not include information like nicknames,
   usernames and host names or user's real names.  Router does not need to
   keep these informations as they are not needed by the router.  This
   information is available from the client's server which maybe queried
   when needed.

   Hence, global list includes:

      server list        - All servers in SILC
         o Server name
         o Server ID
         o Router's Server ID

      client list        - All clients in SILC
         o Client ID

      channel list       - All channels in SILC
         o Channel ID
         o Client IDs on channel
         o Client ID modes on channel








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3.3.3 Router's Server ID

   Router's Server ID is equivalent to normal Server ID.  As routers are
   normal servers same types of IDs applies for routers as well.  See
   section 3.2.2 Server ID.




3.4 Channels

   A channel is a named group of one or more clients which will all receive
   messages addressed to that channel.  The channel is created when first
   client requests JOIN command to the channel, and the channel ceases to
   exist when the last client has left it.  When channel exists, any client
   can reference it using the Channel ID of the channel.  If the channel has
   a founder mode set and last client leaves the channel the channel does
   not cease to exist.  The founder mode can be used to make permanent
   channels in the network.  The founder of the channel can regain the
   channel founder privileges on the channel later when he joins the
   channel.

   Channel names are unique although the real uniqueness comes from 64 bit
   Channel ID.  However, channel names are still unique and no two global
   channels with same name may exist.  See the section 3.13.1 for more
   information about channel names.

   Channels can have operators that can administrate the channel and operate
   all of its modes.  The following operators on channel exist on the
   SILC network.

      o Channel founder - When channel is created the joining client becomes
        channel founder.  Channel founder is channel operator with some more
        privileges.  Basically, channel founder can fully operate the channel
        and all of its modes.  The privileges are limited only to the
        particular channel.  There can be only one channel founder per
        channel.  Channel founder supersedes channel operator's privileges.

        Channel founder privileges cannot be removed by any other operator on
        channel.  When channel founder leaves the channel there is no channel
        founder on the channel.  However, it is possible to set a mode for
        the channel which allows the original channel founder to regain the
        founder privileges even after leaving the channel.  Channel founder
        also cannot be removed by force from the channel.

      o Channel operator - When client joins to channel that has not existed
        previously it will become automatically channel operator (and channel
        founder discussed above).  Channel operator is able to administrate the



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        channel, set some modes on channel, remove a badly behaving client
        from the channel and promote other clients to become channel
        operator.  The privileges are limited only to the particular channel.

        Normal channel user may be promoted (opped) to channel operator
        gaining channel operator privileges.  Channel founder or other
        channel operator may also demote (deop) channel operator to normal
        channel user.




3.4.1 Channel ID

   Channels are distinguished from other channels by unique Channel ID.
   The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
   collisions are not expected to happen in any conditions.  Channel names
   are just for logical use of channels.  The Channel ID is created by the
   server where the channel is created.  The Channel ID is defined as
   follows.

      64 bit Channel ID based on IPv4 addresses:

      32 bit  Router's Server ID IP address (bits 1-32)
      16 bit  Router's Server ID port (bits 33-48)
      16 bit  Random number or counter

      160 bit Channel ID based on IPv6 addresses:

      128 bit  Router's Server ID IP address (bits 1-128)
       16 bit  Router's Server ID port (bits 129-144)
       16 bit  Random number or counter

      o Router's Server ID IP address - Indicates the IP address of
        the router of the cell where this channel is created.  This is
        taken from the router's Server ID.  This way SILC routers know
        where this channel resides in the SILC network.

      o Router's Server ID port - Indicates the port of the channel on
        the server.  This is taken from the router's Server ID.

      o Random number or counter - To further randomize the Channel ID.
        Another choice is to use a counter starting from zero (0).
        This makes sure that there are no collisions.  This also means
        that in a cell there can be 2^16 different channels.






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3.5 Operators

   Operators are normal users with extra privileges to their server or
   router.  Usually these people are SILC server and router administrators
   that take care of their own server and clients on them.  The purpose of
   operators is to administrate the SILC server or router.  However, even
   an operator with highest privileges is not able to enter invite-only
   channels, to gain access to the contents of encrypted and authenticated
   packets traveling in the SILC network or to gain channel operator
   privileges on public channels without being promoted.  They have the
   same privileges as any normal user except they are able to administrate
   their server or router.


3.6 SILC Commands

   Commands are very important part on SILC network especially for client
   which uses commands to operate on the SILC network.  Commands are used
   to set nickname, join to channel, change modes and many other things.

   Client usually sends the commands and server replies by sending a reply
   packet to the command.  Server MAY also send commands usually to serve
   the original client's request.  Usually server cannot send commands to
   clients, however there MAY be commands that allow the server to send
   commands to client.  By default servers MAY send commands only to other
   servers and routers.

   Note that the command reply is usually sent only after client has sent
   the command request but server is allowed to send command reply packet
   to client even if client has not requested the command.  Client MAY
   choose to ignore the command reply.

   It is expected that some of the commands may be misused by clients
   resulting various problems on the server side.  Every implementation
   SHOULD assure that commands may not be executed more than once, say,
   in two (2) seconds.  However, to keep response rate up, allowing for
   example five (5) commands before limiting is allowed.  It is RECOMMENDED
   that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
   SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
   as they require heavy operations.  This should be sufficient to prevent
   the misuse of commands.

   SILC commands are described in [SILC4].


3.7 SILC Packets

   Packets are naturally the most important part of the protocol and the



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   packets are what actually makes the protocol.  Packets in SILC network
   are always encrypted using, usually the shared secret session key
   or some other key, for example, channel key, when encrypting channel
   messages.  It is not possible to send a packet in SILC network without
   encryption.  The SILC Packet Protocol is a wide protocol and is described
   in [SILC2].  This document does not define or describe details of
   SILC packets.


3.8 Packet Encryption

   All packets passed in SILC network MUST be encrypted.  This section
   gives generic description of how packets must be encrypted in the SILC
   network.  The detailed description of the actual encryption process
   of the packets are described in [SILC2].

   Client and its server shares secret symmetric session key which is
   established by the SILC Key Exchange Protocol, described in [SILC3].
   Every packet sent from client to server, with exception of packets for
   channels, are encrypted with this session key.

   Channels have a channel key that are shared by every client on the channel.
   However, the channel keys are cell specific thus one cell does not know
   the channel key of the other cell, even if that key is for same channel.
   Channel key is also known by the routers and all servers that have clients
   on the channel.  However, channels MAY have channel private keys that are
   entirely local setting for the client.  All clients on the channel MUST
   know the channel private key beforehand to be able to talk on the
   channel.  In this case, no server or router knows the key for the channel.

   Server shares secret symmetric session key with router which is
   established by the SILC Key Exchange Protocol.  Every packet passed from
   server to router, with exception of packets for channels, are encrypted
   with the shared session key.  Same way, router server shares secret
   symmetric key with its primary router.  However, every packet passed
   from router to other router, including packets for channels, are
   encrypted with the shared session key.  Every router connection MUST
   have their own session keys.


3.8.1 Determination of the Source and the Destination

   The source and the destination of the packet needs to be determined
   to be able to route the packets to correct receiver.  This information
   is available in the SILC Packet Header which is included in all packets
   sent in SILC network.  The SILC Packet Header is described in [SILC2].

   The header MUST be encrypted with the session key of who is the next



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   receiver of the packet along the route.  The receiver of the packet, for
   example a router along the route, is able to determine the sender and the
   destination of the packet by decrypting the SILC Packet Header and
   checking the IDs attached to the header.  The IDs in the header will
   tell to where the packet needs to be sent and where it is coming from.

   The header in the packet MUST NOT change during the routing of the
   packet.  The original sender, for example client, assembles the packet
   and the packet header and server or router between the sender and the
   receiver MUST NOT change the packet header.  Note however, that some
   packets such as commands may be resent by a server to serve the client's
   original command.  In this case the command packet sent by the server
   includes the server's IDs as it is a different packet.  When server
   or router receives a packet it MUST verify that the Source ID is
   valid and correct ID for that sender.

   Note that the packet and the packet header may be encrypted with
   different keys.  For example, packets to channels are encrypted with
   the channel key, however, the header is encrypted with the session key
   as described above.  Most other packets have both header and packet
   payload encrypted with the same key, such as command packets.


3.8.2 Client To Client

   The process of message delivery and encryption from client to another
   client is as follows.

   Example:  Private message from client to another client on different
             servers.  Clients do not share private message delivery
             keys; normal session keys are used.

   o Client 1 sends encrypted packet to its server.  The packet is
     encrypted with the session key shared between client and its
     server.

   o Server determines the destination of the packet and decrypts
     the packet.  Server encrypts the packet with session key shared
     between the server and its router, and sends the packet to the
     router.

   o Router determines the destination of the packet and decrypts
     the packet.  Router encrypts the packet with session key
     shared between the router and the destination server, and sends
     the packet to the server.

   o Server determines the client to which the packet is destined
     to and decrypts the packet.  Server encrypts the packet with



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     session key shared between the server and the destination client,
     and sends the packet to the client.

   o Client 2 decrypts the packet.


   Example:  Private message from client to another client on different
             servers.  Clients have established a secret shared private
             message delivery key with each other and that is used in
             the message encryption.

   o Client 1 sends encrypted packet to its server.  The packet header
     is encrypted with the session key shared between the client and
     server, and the private message payload is encrypted with the
     private message delivery key shared between clients.

   o Server determines the destination of the packet and sends the
     packet to the router.  Header is encrypted with the session key.

   o Router determines the destination of the packet and sends the
     packet to the server.  Header is encrypted with the session key.

   o Server determines the client to which the packet is destined
     to and sends the packet to the client.  Header is encrypted with
     the session key.

   o Client 2 decrypts the packet with the secret shared key.

   If clients share secret key with each other the private message
   delivery is much simpler since servers and routers between the
   clients do not need to decrypt and re-encrypt the entire packet.
   The packet header however is always encrypted with session key and
   is decrypted and re-encrypted with the session key of next recipient.

   The process for clients on same server is much simpler as there is
   no need to send the packet to the router.  The process for clients
   on different cells is same as above except that the packet is routed
   outside the cell.  The router of the destination cell routes the
   packet to the destination same way as described above.


3.8.3 Client To Channel

   Process of message delivery from client on channel to all the clients
   on the channel.

   Example:  Channel of four users; two on same server, other two on
             different cells.  Client sends message to the channel.



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             Packet header is encrypted with the session key, message
             data is encrypted with channel key.

   o Client 1 encrypts the packet with channel key and sends the
     packet to its server.

   o Server determines local clients on the channel and sends the
     packet to the Client on the same server.  Server then sends
     the packet to its router for further routing.

   o Router determines local clients on the channel, if found
     sends packet to the local clients.  Router determines global
     clients on the channel and sends the packet to its primary
     router or fastest route.

   o (Other router(s) do the same thing and sends the packet to
      the server(s).)

   o Server determines local clients on the channel and sends the
     packet to the client.

   o All clients receiving the packet decrypts it.


3.8.4 Server To Server

   Server to server packet delivery and encryption is described in above
   examples. Router to router packet delivery is analogous to server to
   server.  However, some packets, such as channel packets, are processed
   differently.  These cases are described later in this document and
   more in detail in [SILC2].


3.9 Key Exchange And Authentication

   Key exchange is done always when for example client connects to server
   but also when server and router, and router and another router connect
   to each other.  The purpose of key exchange protocol is to provide secure
   key material to be used in the communication.  The key material is used
   to derive various security parameters used to secure SILC packets.  The
   SILC Key Exchange protocol is described in detail in [SILC3].

   Authentication is done after key exchange protocol has been successfully
   completed.  The purpose of authentication is to authenticate for example
   client connecting to the server.  However, clients MAY be accepted
   to connect to server without explicit authentication.  Servers are
   REQUIRED to use authentication protocol when connecting.  The
   authentication may be based on passphrase (pre-shared secret) or public



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   key based on digital signatures.  All passphrases sent in SILC protocol
   MUST be UTF-8 [RFC3629] encoded.  The connection authentication protocol
   is described in detail in [SILC3].


3.9.1 Authentication Payload

   Authentication Payload is used separately from the SKE and the Connection
   Authentication protocols.  It can be used during the session to
   authenticate with a remote.  For example, a client can authenticate
   itself to a server to become server operator.  In this case,
   Authentication Payload is used.

   The format of the Authentication Payload is as follows:

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Payload Length         |     Authentication Method     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Public Data Length       |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                           Public Data                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Authentication Data Length  |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                       Authentication Data                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5:  Authentication Payload


      o Payload Length (2 bytes) - Length of the entire payload.

      o Authentication Method (2 bytes) - The method of the
        authentication.  The authentication methods are defined
        in [SILC2] in the Connection Auth Request Payload.  The NONE
        authentication method SHOULD NOT be used.

      o Public Data Length (2 bytes) - Indicates the length of
        the Public Data field.

      o Public Data (variable length) - This is defined only if
        the authentication method is public key.  If it is any other



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        this field MAY include random data for padding purposes.
        However, in this case the field MUST be ignored by the
        receiver.

        When the authentication method is public key this includes
        128 to 4096 bytes of non-zero random data that is used in
        the signature process, described subsequently.

      o Authentication Data Length (2 bytes) - Indicates the
        length of the Authentication Data field.  If zero (0)
        value is found in this field the payload MUST be
        discarded.

      o Authentication Data (variable length) - Authentication
        method dependent authentication data.


   If the authentication method is passphrase-based, the Authentication
   Data field includes the plaintext UTF-8 encoded passphrase.  It is safe
   to send plaintext passphrase since the entire payload is encrypted.  In
   this case the Public Data Length is set to zero (0), but MAY also include
   random data for padding purposes.  It is also RECOMMENDED that maximum
   amount of padding is applied to SILC packet when using passphrase-based
   authentication.  This way it is not possible to approximate the length
   of the passphrase from the encrypted packet.

   If the authentication method is public key based (or certificate)
   the Authentication Data is computed as follows:

     HASH = hash(random bytes | ID | public key (or certificate));
     Authentication Data = sign(HASH);

   The hash() and the sign() are the hash function and the public key
   cryptography function selected in the SKE protocol, unless otherwise
   stated in the context where this payload is used.  The public key
   is SILC style public key unless certificates are used.  The ID is the
   entity's ID (Client or Server ID) which is authenticating itself.  The
   ID encoding is described in [SILC2].  The random bytes are non-zero
   random bytes of length between 128 and 4096 bytes, and will be included
   into the Public Data field as is.

   The receiver will compute the signature using the random data received
   in the payload, the ID associated to the connection and the public key
   (or certificate) received in the SKE protocol.  After computing the
   receiver MUST verify the signature.  Also in case of public key
   authentication this payload is always encrypted.  This payload is
   always sent as part of some other payload.




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3.10 Algorithms

   This section defines all the allowed algorithms that can be used in
   the SILC protocol.  This includes mandatory cipher, mandatory public
   key algorithm and MAC algorithms.


3.10.1 Ciphers

   Cipher is the encryption algorithm that is used to protect the data
   in the SILC packets.  See [SILC2] for the actual encryption process and
   definition of how it must be done.  SILC has a mandatory algorithm that
   must be supported in order to be compliant with this protocol.

   The following ciphers are defined in SILC protocol:

   aes-256-cbc          AES in CBC mode, 256 bit key            (REQUIRED)
   aes-256-ctr          AES in CTR mode, 256 bit key            (RECOMMENDED)
   aes-256-rcbc         AES in randomized CBC mode, 256 bit key (OPTIONAL)
   aes-192-<mode>       AES in <mode> mode, 192 bit key         (OPTIONAL)
   aes-128-<mode>       AES in <mode> mode, 128 bit key         (RECOMMENDED)
   twofish-256-<mode>   Twofish in <mode> mode, 256 bit key     (OPTIONAL)
   twofish-192-<mode>   Twofish in <mode> mode, 192 bit key     (OPTIONAL)
   twofish-128-<mode>   Twofish in <mode> mode, 128 bit key     (OPTIONAL)
   cast-256-<mode>      CAST-256 in <mode> mode, 256 bit key    (OPTIONAL)
   cast-192-<mode>      CAST-256 in <mode> mode, 192 bit key    (OPTIONAL)
   cast-128-<mode>      CAST-256 in <mode> mode, 128 bit key    (OPTIONAL)
   serpent-<len>-<mode> Serpent in <mode> mode, <len> bit key   (OPTIONAL)
   rc6-<len>-<mode>     RC6 in <mode> mode, <len> bit key       (OPTIONAL)
   mars-<len>-<mode>    MARS in <mode> mode, <len> bit key      (OPTIONAL)
   none                 No encryption                           (OPTIONAL)

   The <mode> is either "cbc", "ctr" or "rcbc".  Other encryption modes MAY
   be defined to be used in SILC using the same name format.  The <len> is
   either 256, 192 or 128 bit key length.  Also, additional ciphers MAY be
   defined to be used in SILC by using the same name format as above.

   Algorithm "none" does not perform any encryption process at all and
   thus is not recommended to be used.  It is recommended that no client
   or server implementation would accept "none" algorithm except in special
   debugging mode.


3.10.1.1 CBC Mode

   The "cbc" encryption mode is the standard cipher-block chaining mode.
   The very first IV is derived from the SILC Key Exchange protocol.
   Subsequent IVs for encryption is the previous ciphertext block.  The very



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   first IV MUST be random and is generated as described in [SILC3].


3.10.1.2 CTR Mode

   The "ctr" encryption mode is Counter Mode (CTR).  The CTR mode in SILC is
   stateful in encryption and decryption.  Both sender and receiver maintain
   the counter for the CTR mode and thus can precompute the key stream for
   encryption and decryption.  By default, CTR mode does not require
   plaintext padding, however implementations MAY apply padding to the
   packets.  If the last key block is larger than the last plaintext block
   the resulted value is truncated to the size of the plaintext block and
   the most significant bits are used.  When sending authentication data
   inside packets the maximum amount of padding SHOULD be applied with
   CTR mode as well.

   In CTR mode only the encryption operation of the cipher is used.  The
   decryption operation is not needed since both encryption and decryption
   process is simple XOR with the plaintext block and the key stream block.

   The counter block is used to create the key for the CTR mode.  The format
   of the 128 bit counter block is as follows:

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Truncated HASH from SKE                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Sending/Receiving IV from SKE                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Packet Counter                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Block Counter                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 6:  Counter Block

      o Truncated HASH from SKE (4 bytes) - This value is the first 4
        bytes from the HASH value that was computed as a result of SKE
        protocol.  This acts as session identifier and each rekey MUST
        produce a new HASH value.

      o Sending/Receiving IV from SKE (4 bytes) - If the CTR mode is fully
        stateful this field MUST include the first 4 bytes from the Sending
        IV or Receiving IV generated in SKE protocol.  When this mode is
        used to encrypt sending traffic the Sending IV is used, when used
        to decrypt receiving traffic the Receiving IV is used.  This assures
        that two parties of the protocol use different IV for sending



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        traffic.  Each rekey MUST produce a new value.

        If the IV Included flag is negotiated in SKE or CTR mode is used
        where the IV is included in the data payload, this field is the
        Nonce field from the IV received in the packet, defined below.

      o Packet Counter (4 bytes) - This is MSB first ordered monotonically
        increasing packet counter.  It is set value 1 for first packet and
        increases for subsequent packets.  After rekey the counter MUST
        restart from 1.

        If the IV Included flag is negotiated in SKE or CTR mode is used
        where the IV is included in the data payload, this field is the
        Packet Counter field from the IV received in the packet, defined
        below.

      o Block Counter (4 bytes) - This is an MSB first ordered block
        counter starting from 1 for first block and increasing for
        subsequent blocks.  The counter is always set to value 1 for
        a new packet.

   CTR mode MUST NOT be used with "none" MAC.  Implementations also MUST
   assure that the same counter block is not used to encrypt more than
   one block.  None of the counters must be allowed to wrap without rekey.
   Also, the key material used with CTR mode MUST be fresh key material.
   Static keys (pre-shared keys) MUST NOT be used with CTR mode.  For this
   reason using CTR mode to encrypt for example channel messages or private
   messages with a pre-shared key is inappropriate.  For private messages,
   the Key Agreement [SILC2] could be performed to produce fresh key material.

   If the IV Included flag was negotiated in SKE, or CTR mode is used to
   protect channel messages where the IV will be included in the Message
   Payload, the Initialization Vector (IV) to be used is a 64-bit block
   containing randomness and packet counter.  Also note, that in this case
   the decryption process is not stateful and receiver cannot precompute
   the key stream.  Hence, the Initialization Vector (IV) when CTR mode is
   used is as follows.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Nonce                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Packet Counter                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7:  CTR Mode Initialization Vector




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   o Nonce (4 bytes) - This field should be random or otherwise not
     easily determinable and SHOULD change for each packet.

   o Packet Counter (4 bytes) - This is MSB first ordered monotonically
     increasing packet counter.  It is set value 1 for first packet and
     increases for subsequent packets.  After rekey the counter MUST
     restart from 1.

   When decrypting the packet the Counter Block is assembled by concatenating
   the truncated hash, with the received nonce and packet counter, and with
   the block counter.  The Counter Block is then used to compute the key
   stream to perform the decryption.


3.10.1.3 Randomized CBC Mode

   The "rcbc" encryption mode is CBC mode with randomized IV.  This means
   that each IV for each packet MUST be chosen randomly.  When encrypting
   more than one block the normal IV chaining is used, but for the first
   block new random IV is selected in each packet.  In this mode the IV
   is appended to the ciphertext.  If this mode is used to secure the SILC
   session, the IV Included flag must be negotiated in SILC Key Exchange
   protocol.  It may also be used to secure Message Payloads which can
   deliver the IV to the recipient.


3.10.2 Public Key Algorithms

   Public keys are used in SILC to authenticate entities in SILC network
   and to perform other tasks related to public key cryptography.  The
   public keys are also used in the SILC Key Exchange protocol [SILC3].

   The following public key algorithms are defined in SILC protocol:

      rsa        RSA  (REQUIRED)
      dss        DSS  (OPTIONAL)

   DSS is described in [Menezes].  The RSA MUST be implemented according
   PKCS #1 [PKCS1].  When using SILC Public Key version 2 the PKCS #1
   implementation MUST be compliant with PKCS #1 version 1.5.  The signatures
   are computed with appendix; the hash OID is included in the signature.
   The user may always select the hash algorithm for the signatures.  When
   using SILC Public Key version 1 the PKCS #1 implementation MUST be
   compliant with PKCS #1 version 1.5 where signatures are computed without
   appendix; the hash OID is not present in the signature.  The hash
   algorithm used is specified separately or the default hash algorithm is
   used, as defined below.




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   Additional public key algorithms MAY be defined to be used in SILC.

   When signatures are computed in SILC the computing of the signature is
   denoted as sign().  The signature computing procedure is dependent of
   the public key algorithm, and the public key or certificate encoding.
   When using SILC public key the signature is computed as described in
   previous paragraph for RSA and DSS keys.  If the hash function is not
   specified separately for signing process SHA-1 MUST be used, except with
   SILC public key version 2 and RSA algorithm when the user MAY always
   select the hash algorithm.  In this case the hash algorithm is included
   in the signature and can be retrieved during verification.  When using
   SSH2 public keys the signature is computed as described in [SSH-TRANS].
   When using X.509 version 3 certificates the signature is computed as
   described in [PKCS7].  When using OpenPGP certificates the signature is
   computed as described in [PGP] and the PGP signature type used is 0x00.


3.10.2.1 Multi-Precision Integers

   Multi-Precision (MP) integers in SILC are encoded and decoded as defined
   in PKCS #1 [PKCS1].  MP integers are unsigned, encoded with the exact
   octet length of the integer.  No extra leading zero octets may appear.
   The actual length of the integer is the bit size of the integer not
   counting any leading zero bits.  The octet length is derived by calculating
   (bit_length + 7) / 8.


3.10.3 Hash Functions

   Hash functions are used as part of MAC algorithms defined in the next
   section.  They are also used in the SILC Key Exchange protocol defined
   in the [SILC3].

   The following Hash algorithm are defined in SILC protocol:

      sha1             SHA-1, length = 20 bytes       (REQUIRED)
      sha256           SHA-256, length = 32 bytes     (RECOMMENDED)
      md5              MD5, length = 16 bytes         (RECOMMENDED)


3.10.4 MAC Algorithms

   Data integrity is protected by computing a message authentication code
   (MAC) of the packet data.  See [SILC2] for details how to compute the
   MAC for a packet.

   The following MAC algorithms are defined in SILC protocol:




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      hmac-sha1-96     HMAC-SHA1, length = 12 bytes   (REQUIRED)
      hmac-sha256-96   HMAC-SHA256, length = 12 bytes (RECOMMENDED)
      hmac-md5-96      HMAC-MD5, length = 12 bytes    (OPTIONAL)
      hmac-sha1        HMAC-SHA1, length = 20 bytes   (OPTIONAL)
      hmac-sha256      HMAC-SHA256, length = 32 bytes (OPTIONAL)
      hmac-md5         HMAC-MD5, length = 16 bytes    (OPTIONAL)
      none             No MAC                         (OPTIONAL)

   The "none" MAC is not recommended to be used as the packet is not
   authenticated when MAC is not computed.  It is recommended that no
   client or server would accept none MAC except in special debugging
   mode.

   The HMAC algorithm is described in [HMAC].  The hash algorithms used
   in HMACs, the SHA-1 is described in [RFC3174] and MD5 is described
   in [RFC1321].  The SHA-256 algorithm and its used with HMAC is described
   in [SHA256].

   Additional MAC algorithms MAY be defined to be used in SILC.


3.10.5 Compression Algorithms

   SILC protocol supports compression that may be applied to unencrypted
   data.  It is recommended to use compression on slow links as it may
   significantly speed up the data transmission.  By default, SILC does not
   use compression which is the mode that must be supported by all SILC
   implementations.

   The following compression algorithms are defined:

      none        No compression               (REQUIRED)
      zlib        GNU ZLIB (LZ77) compression  (OPTIONAL)

   Additional compression algorithms MAY be defined to be used in SILC.


3.11 SILC Public Key

   This section defines the type and format of the SILC public key.  All
   implementations MUST support this public key type.  See [SILC3] for
   other optional public key and certificate types allowed in the SILC
   protocol.  Public keys in SILC may be used to authenticate entities
   and to perform other tasks related to public key cryptography.

   The format of the SILC Public Key is as follows:





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                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Public Key Length                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Algorithm Name Length     |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                         Algorithm Name                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Identifier Length       |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                           Identifier                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                           Public Data                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5:  SILC Public Key


      o Public Key Length (4 bytes) - Indicates the full length
        of the SILC Public Key, not including this field.

      o Algorithm Name Length (2 bytes) - Indicates the length
        of the Algorithm Length field, not including this field.

      o Algorithm name (variable length) - Indicates the name
        of the public key algorithm that the key is.  See the
        section 3.10.2 Public Key Algorithms for defined names.

      o Identifier Length (2 bytes) - Indicates the length of
        the Identifier field, not including this field.

      o Identifier (variable length) - Indicates the identifier
        of the public key.  This data can be used to identify the
        owner of the key.  The identifier may be of the following
        format:

           UN     User name
           HN     Host name or IP address
           RN     Real name
           E      EMail address
           O      Organization



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           C      Country
           V      Version

        Examples of an identifier:

          `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'

          `UN=sam, HN=dummy.fi, RN=Sammy Sam, C=Finland, V=2'

        At least user name (UN) and host name (HN) MUST be provided as
        identifier.  The fields are separated by commas (`,').  If
        comma is in the identifier string it must be escaped as `\,',
        for example, `O=Company XYZ\, Inc.'.  Other characters that
        require escaping are listed in [RFC2253] and are to be escaped
        as defined therein.  The Version (V) may only be a decimal digit.

      o Public Data (variable length) - Includes the actual
        public data of the public key.

        The format of this field for RSA algorithm is
        as follows:

           4 bytes            Length of e
           variable length    e
           4 bytes            Length of n
           variable length    n


        The format of this field for DSS algorithm is
        as follows:

           4 bytes            Length of p
           variable length    p
           4 bytes            Length of q
           variable length    q
           4 bytes            Length of g
           variable length    g
           4 bytes            Length of y
           variable length    y

        The variable length fields are multiple precession
        integers encoded as strings in both examples.

        Other algorithms must define their own type of this
        field if they are used.

   The SILC Public Key is version is 2.  If the Version (V) identifier is
   not present the SILC Public Key version is expected to be 1.  All new



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   implementations SHOULD support version 1 but SHOULD only generate version 2.
   In this case the Version (V) identifier MUST be present.

   All fields in the public key are in MSB (most significant byte first)
   order.  All strings in the public key MUST be UTF-8 encoded.

   If an external protocol needs to refer to SILC Public Key by name, the
   names "silc-rsa" and "silc-dss" for SILC Public Key based on RSA algorithm
   and SILC Public Key based on DSS algorithm, respectively, are to be used.
   However, this SILC specification does not use these names directly, and
   they are defined here for external protocols (protocols that may like
   to use SILC Public Key).

   A fingerprint from SILC Public Key is computed from the whole encoded
   public key data block.  All fields are included in computation.  Compliant
   implementations MUST support computing a 160-bit SHA-1 fingerprint.


3.12 SILC Version Detection

   The version detection of both client and server is performed at the
   connection phase while executing the SILC Key Exchange protocol.  The
   version identifier is exchanged between initiator and responder.  The
   version identifier is of the following format:

      SILC-<protocol version>-<software version>

   The version strings are of the following format:

      protocol version = <major>.<minor>
      software version = <major>[.<minor>[.<build or vendor string>]]

   Protocol version MUST provide both major and minor version.  Currently
   implementations MUST set the protocol version and accept at least the
   protocol version as SILC-1.2-<software version>.  If new protocol version
   causes incompatibilities with older version the <minor> version number
   MUST be incremented.  The <major> is incremented if new protocol version
   is fully incompatible.

   Software version MAY provide major, minor and build (vendor) version.
   The software version MAY be freely set and accepted.  The version string
   MUST consist of printable US-ASCII characters.

   Thus, the version strings could be, for example:

      SILC-1.1-2.0.2
      SILC-1.0-1.2
      SILC-1.2-1.0.VendorXYZ



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      SILC-1.2-2.4.5 Vendor Limited


3.13 UTF-8 Strings in SILC

   By default all strings that are sent in SILC protocol MUST be UTF-8
   [RFC3269] encoded, unless otherwise defined.  This means that any string
   sent inside for example, command, command reply, notify or any packet
   payload is UTF-8 encoded.  Also nicknames, channel names, server names,
   and hostnames are UTF-8 encoded.  This definition does not affect
   messages sent in SILC, as the Message Payload provides its own mechanism
   to indicate whether a message is UTF-8 text message, data message, which
   may use its own character encoding, or pure binary message [SILC2].

   Certain limitations are imposed on the UTF-8 encoded strings in SILC.
   The UTF-8 encoded strings MUST NOT include any characters that are
   marked in the Unicode standard as control codes, noncharacters,
   reserved or private range characters, or any other illegal Unicode
   characters.  Also the BOM (Byte-Order Mark) MUST NOT be used as byte
   order signature in UTF-8 encoded strings.  A string containing these
   characters MUST be treated as malformed UTF-8 encoding.

   The Unicode standard defines that malformed sequences shall be signalled
   by replacing the sequence with a replacement character.  Even though,
   in case of SILC these strings may not be malformed UTF-8 encodings
   they MUST be treated as malformed strings.  Implementation MAY use
   a replacement character, however, the character Unicode standard defines
   MUST NOT be used, but another character must be chosen.  It is, however,
   RECOMMENDED that an error is returned instead of using replacement
   character if it is possible.  For example, when setting a nickname
   with SILC_COMMAND_NICK command, implementation is able to send error
   indication back to the command sender.  It must be noted that on server
   implementation if a character sequence is merely outside of current
   character subset, but is otherwise valid character, it MUST NOT be
   replaced by a replacement character.

   On user interface where UTF-8 strings are displayed the implementation
   is RECOMMENDED to escape any character that it is unable to render
   properly.  The escaping may be done for example as described in
   [RFC2253].  The escaping makes it possible to retrieve the original
   UTF-8 encoding.  Alternatively, a replacement character may be used
   if it does not cause practical problems to the implementation.


3.13.1 UTF-8 Identifier Strings

   Identifier strings are special strings in SILC protocol that require
   more careful processing, than the general UTF-8 strings described in the



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   previous section.  These strings include the nicknames, server names,
   hostnames and some other identifier strings.  These strings are prepared
   using the stringprep [RFC3454] standard.  The Appendix A defines the
   stringprep profile for SILC identifier strings and conforming
   implementation MUST use the profile to prepare any identifier string.

   The stringprep profile describes how identifier strings are prepared,
   what characters they may include, and which characters are prohibited.
   Identifier strings with prohibited characters MUST be treated as
   malformed strings.

   The channel name is also special identifier strings with some slight
   differences to other identifier strings.  The Appendix B defines the
   stringprep profile for the channel name strings and conforming
   implementation MUST use the profile to prepare any channel name string.

   Because of the profile the identifier strings in SILC may generally
   include only letters, numbers, most punctuation characters, and some
   other characters.  For practical reasons most symbol characters and
   many other special characters are prohibited.  All identifier strings
   are case folded and comparing the identifier strings MUST be done as
   caseless matching.

   In general, the identifier strings does not have a maximum length.
   However, the length of a nickname string MUST NOT exceed 128 bytes, and
   the length of a channel name string MUST NOT exceed 256 bytes.  Since
   these strings are UTF-8 encoded the length of one character may be
   longer than one byte.  This means that the character length of these
   strings may be shorter than the maximum length of the string in bytes.
   The minimum length of an identifier string MUST be at least one character,
   which may be one byte or more in length.  Implementation MAY limit the
   maximum length of an identifier string, with exception of the nickname
   and channel name strings which has the explicit length definition.


3.14 Backup Routers

   Backup routers may exist in the cell in addition to the primary router.
   However, they must not be active routers or act as routers in the cell.
   Only one router may be acting as primary router in the cell.  In the case
   of failure of the primary router one of the backup routers becomes active.
   The purpose of backup routers are in case of failure of the primary router
   to maintain working connections inside the cell and outside the cell and
   to avoid netsplits.

   Backup routers are normal servers in the cell that are prepared to take
   over the tasks of the primary router if needed.  They need to have at
   least one direct and active connection to the primary router of the cell.



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   This communication channel is used to send the router information to
   the backup router.  When the backup router connects to the primary router
   of the cell it MUST present itself as router server in the Connection
   Authentication protocol, even though it is normal server as long as the
   primary router is available.  Reason for this is that the configuration
   needed in the responder end requires usually router connection level
   configuration.  The responder, however must understand and treat the
   connection as normal server (except when feeding router level data to
   the backup router).

   Backup router must know everything that the primary router knows to be
   able to take over the tasks of the primary router.  It is the primary
   router's responsibility to feed the data to the backup router.  If the
   backup router does not know all the data in the case of failure some
   connections may be lost.  The primary router of the cell must consider
   the backup router being an actual router server when it feeds the data
   to it.

   In addition to having direct connection to the primary router of the
   cell, the backup router must also have connection to the same router
   to which the primary router of the cell is connected.  However, it must
   not be the active router connection meaning that the backup router must
   not use that channel as its primary route and it must not notify the
   router about having connected servers, channels and clients behind it.
   It merely connects to the router.  This sort of connection is later
   referred to as being a passive connection.  Some keepalive actions may
   be needed by the router to keep the connection alive.

   It is required that other normal servers have passive connections to
   the backup router(s) in the cell.  Some keepalive actions may be needed
   by the server to keep the connection alive.  After they notice the
   failure of the primary router they must start using the connection to
   the first backup router as their primary route.

   Also, if any other router in the network is using the cell's primary
   router as its own primary router, it must also have passive connection
   to the cell's backup router.  It too is prepared to switch to use the
   backup router as its new primary router as soon as the original primary
   router becomes unresponsive.

   All of the parties of this protocol know which one is the backup router
   of the cell from their local configuration.  Each of the entities must
   be configured accordingly and care must be taken when configuring the
   backup routers, servers and other routers in the network.

   It must be noted that some of the channel messages and private messages
   may be lost during the switch to the backup router, unless the message
   flag SILC_MESSAGE_FLAG_ACK is set in the message.  The announcements



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   assure that the state of the network is not lost during the switch.

   It is RECOMMENDED that there would be at least one backup router in
   the cell.  It is NOT RECOMMENDED to have all servers in the cell acting
   as backup routers as it requires establishing several connections to
   several servers in the cell.  Large cells can easily have several
   backup routers in the cell.

   The order of the backup routers are decided at the local configuration
   phase.  All the parties of this protocol must be configured accordingly to
   understand the order of the backup routers.  It is not required that the
   backup server is actually an active server in the cell.  The backup router
   may be a redundant server in the cell that does not accept normal client
   connections at all.  It may be reserved purely for the backup purposes.

   If also the first backup router is down as well and there is another
   backup router in the cell then it will start acting as the primary
   router as described above.


3.14.1 Switching to Backup Router

   When the primary router of the cell becomes unresponsive, for example
   by sending EOF to the connection, all the parties of this protocol MUST
   replace the old connection to the primary router with first configured
   backup router.  The backup router usually needs to do local modifications
   to its database in order to update all the information needed to maintain
   working routes.  The backup router must understand that clients that
   were originated from the primary router are now originated from some of
   the existing server connections and must update them accordingly.  It
   must also remove those clients that were owned by the primary router
   since those connections were lost when the primary router became
   unresponsive.

   All the other parties of the protocol must also update their local
   database to understand that the route to the primary router will now go
   to the backup router.

   Servers connected to the backup router MUST send SILC_PACKET_RESUME_ROUTER
   packet with type value 21, to indicate that the server will start using
   the backup router as primary router.  The backup router MUST NOT allow
   this action if it detects that primary is still up and running.  If
   backup router knows that primary is up and running it MUST send
   SILC_PACKET_FAILURE with type value 21 (4 bytes, MSB first order) back
   to the server.  The server then MUST NOT use the backup as primary
   router, but must try to establish connection back to the primary router.
   If the action is allowed type value 21 is sent back to the server from
   the backup router.  It is RECOMMENDED that implementations use the



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   SILC_COMMAND_PING command to detect whether primary router is responsive.
   If the backup router notices that the primary router is unresponsive
   it SHOULD NOT start sending data to server links before the server has
   sent the SILC_PACKET_RESUME_ROUTER with type value 21.

   The servers connected to the backup router must then announce their
   clients, channels, channel users, channel user modes, channel modes,
   topics and other information to the backup router.  This is to assure
   that none of the important notify packets were lost during the switch
   to the backup router.  The backup router must check which of these
   announced entities it already has and distribute the new ones to the
   primary router.

   The backup router too must announce its servers, clients, channels
   and other information to the new primary router.  The primary router
   of the backup router too must announce its information to the backup
   router.  Both must process only the ones they do not know about.  If
   any of the announced modes do not match then they are enforced in
   normal manner as defined in section 4.2.1 Announcing Clients, Channels
   and Servers.


3.14.2 Resuming Primary Router

   Usually the primary router is unresponsive only a short period of time
   and it is intended that the original router of the cell will resume
   its position as primary router when it comes back online.  The backup
   router that is now acting as primary router of the cell must constantly
   try to connect to the original primary router of the cell.  It is
   RECOMMENDED that it would try to reconnect in 30 second intervals to
   the primary router.

   When the connection is established to the primary router the backup
   resuming protocol is executed.  The protocol is advanced as follows:

     1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
        value 1 to the primary router that came back online.  The packet
        will indicate the primary router has been replaced by the backup
        router.  After sending the packet the backup router will announce
        all of its channels, channel users, modes etc. to the primary
        router.

        If the primary knows that it has not been replaced (for example
        the backup itself disconnected from the primary router and thinks
        that it is now primary in the cell) the primary router send
        SILC_PACKET_FAILURE with the type value 1 (4 bytes, MSB first
        order) back to the backup router.  If backup receives this it
        MUST NOT continue with the backup resuming protocol.



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     2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
        value 1 to its current primary router to indicate that it will
        resign as being primary router.  Then, backup router sends the
        SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
        connected servers to also indicate that it will resign as being
        primary router.

     3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
        type value 1 to the router that is using the backup router
        currently as its primary router.

     4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
        with type value 1 must reconnect immediately to the primary
        router of the cell that came back online.  After they have created
        the connection they MUST NOT use that connection as active primary
        route but still route all packets to the backup router.  After
        the connection is created they MUST send SILC_PACKET_RESUME_ROUTER
        with type value 2 back to the backup router.  The session ID value
        found in the first packet MUST be set in this packet.

     5. Backup router MUST wait for all packets with type value 2 before
        it continues with the protocol.  It knows from the session ID values
        set in the packet when it has received all packets.  The session
        value should be different in all packets it has sent earlier.
        After the packets are received the backup router sends the
        SILC_PACKET_RESUME_ROUTER packet with type value 3 to the
        primary router that came back online.  This packet will indicate
        that the backup router is now ready to resign as being primary
        router.  The session ID value in this packet MUST be the same as
        in the first packet sent to the primary router.  During this time
        the backup router must still route all packets it is receiving
        from server connections.

     6. The primary router receives the packet and send the packet
        SILC_PACKET_RESUME_ROUTER with type value 4 to all connected servers
        including the backup router.  It also sends the packet with type
        value 4 to its primary router, and to the router that is using
        it as its primary router.  The Session ID value in these packets
        SHOULD be zero (0).

     7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
        packet with type value 4 must switch their primary route to the new
        primary router and remove the route for the backup router, since
        it is no longer the primary router of the cell.  They must also
        update their local database to understand that the clients are
        not originated from the backup router but from the locally connected
        servers.  After that they MUST announce their channels, channel
        users, modes etc. to the primary router.  They MUST NOT use the



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        backup router connection after this and the connection is considered
        to be a passive connection.  The implementation SHOULD be able
        to disable the connection without closing the actual link.

   After this protocol is executed the backup router is now again a normal
   server in the cell that has the backup link to the primary router.  The
   primary router feeds the router specific data again to the backup router.
   All server connections to the backup router are considered passive
   connections.

   When the primary router of the cell comes back online and connects
   to its remote primary router, the remote primary router MUST send the
   SILC_PACKET_RESUME_ROUTER packet with type value 20 indicating that the
   connection is not allowed since the router has been replaced by an
   backup router in the cell.  The session ID value in this packet SHOULD be
   zero (0).  When the primary router receives this packet it MUST NOT use
   the connection as active connection but must understand that it cannot
   act as primary router in the cell, until the backup resuming protocol has
   been executed.

   The following type values has been defined for SILC_PACKET_RESUME_ROUTER
   packet:

     1    SILC_SERVER_BACKUP_START
     2    SILC_SERVER_BACKUP_START_CONNECTED
     3    SILC_SERVER_BACKUP_START_ENDING
     4    SILC_SERVER_BACKUP_START_RESUMED
     20   SILC_SERVER_BACKUP_START_REPLACED
     21   SILC_SERVER_BACKUP_START_USE

   If any other value is found in the type field the packet MUST be
   discarded.  The SILC_PACKET_RESUME_ROUTER packet and its payload
   is defined in [SILC2].


4 SILC Procedures

   This section describes various SILC procedures such as how the
   connections are created and registered, how channels are created and
   so on.  The references [SILC2], [SILC3] and [SILC4] permeate this
   section's definitions.


4.1 Creating Client Connection

   This section describes the procedure when a client connects to SILC
   server.  When client connects to server the server MUST perform IP
   address lookup and reverse IP address lookup to assure that the origin



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   host really is who it claims to be.  Client, a host, connecting to server
   SHOULD have both valid IP address and fully qualified domain name (FQDN).

   After that the client and server performs SILC Key Exchange protocol
   which will provide the key material used later in the communication.
   The key exchange protocol MUST be completed successfully before the
   connection registration may continue.  The SILC Key Exchange protocol
   is described in [SILC3].

   Typical server implementation would keep a list of connections that it
   allows to connect to the server.  The implementation would check, for
   example, the connecting client's IP address from the connection list
   before the SILC Key Exchange protocol has been started.  The reason for
   this is that if the host is not allowed to connect to the server there
   is no reason to perform the key exchange protocol.

   After successful key exchange protocol the client and server perform
   connection authentication protocol.  The purpose of the protocol is to
   authenticate the client connecting to the server.  Flexible
   implementation could also accept the client to connect to the server
   without explicit authentication.  However, if authentication is
   desired for a specific client it may be based on passphrase or
   public key authentication.  If authentication fails the connection
   MUST be terminated.  The connection authentication protocol is described
   in [SILC3].

   After successful key exchange and authentication protocol the client
   MUST register itself by sending SILC_PACKET_NEW_CLIENT packet to the
   server.  This packet includes various information about the client
   that the server uses to register the client.  Server registers the
   client and sends SILC_PACKET_NEW_ID to the client which includes the
   created Client ID that the client MUST start using after that.  After
   that all SILC packets from the client MUST have the Client ID as the
   Source ID in the SILC Packet Header, described in [SILC2].

   Client MUST also get the server's Server ID that is to be used as
   Destination ID in the SILC Packet Header when communicating with
   the server (for example when sending commands to the server).  The
   ID may be resolved in two ways.  Client can take the ID from an
   previously received packet from server that MUST include the ID,
   or to send SILC_COMMAND_INFO command and receive the Server ID as
   command reply.

   Server MAY choose not to use the information received in the
   SILC_PACKET_NEW_CLIENT packet.  For example, if public key or
   certificate were used in the authentication, server MAY use that
   information rather than what it received from client.  This is a suitable
   way to get the true information about client if it is available.



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   The nickname of client is initially set to the username sent in the
   SILC_PACKET_NEW_CLIENT packet.  User may set the nickname to something
   more desirable by sending SILC_COMMAND_NICK command.  However, this is
   not required as part of registration process.

   Server MUST also distribute the information about newly registered
   client to its router (or if the server is router, to all routers in
   the SILC network).  More information about this in [SILC2].

   Router server MUST also check whether some client in the local cell
   is watching for the nickname this new client has, and send the
   SILC_NOTIFY_TYPE_WATCH to the watcher.


4.2 Creating Server Connection

   This section describes the procedure when server connects to its
   router (or when router connects to other router, the cases are
   equivalent).  The procedure is very much alike to when a client
   connects to the server thus it is not repeated here.

   One difference is that server MUST perform connection authentication
   protocol with proper authentication.  A proper authentication is based
   on passphrase authentication or public key authentication based on
   digital signatures.

   After server and router have successfully performed the key exchange
   and connection authentication protocol, the server MUST register itself
   to the router by sending SILC_PACKET_NEW_SERVER packet.  This packet
   includes the server's Server ID that it has created by itself and
   other relevant information about the server.  The router receiving the
   ID MUST verify that the IP address in the Server ID is same as the
   server's real IP address.

   After router has received the SILC_PACKET_NEW_SERVER packet it
   distributes the information about newly registered server to all routers
   in the SILC network.  More information about this is in [SILC2].

   As the client needed to resolve the destination ID this MUST be done by
   the server that connected to the router, as well.  The way to resolve it
   is to get the ID from previously received packet.  The server MAY also
   use SILC_COMMAND_INFO command to resolve the ID.  Server MUST also start
   using its own Server ID as Source ID in SILC Packet Header and the
   router's Server ID as Destination when communicating with the router.







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4.2.1 Announcing Clients, Channels and Servers

   After server or router has connected to the remote router, and it already
   has connected clients and channels it MUST announce them to the router.
   If the server is router server, also all the local servers in the cell
   MUST be announced.

   All clients are announced by compiling a list of ID Payloads into the
   SILC_PACKET_NEW_ID packet.  All channels are announced by compiling a
   list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet.
   Channels' mode, founder public key, channel public keys, and other
   channel mode specific data is announced by sending the
   SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.

   The channel public keys that are announced are compiled in Argument
   List Payload where the argument type is 0x03, and each argument is
   Public Key Payload containing one public key or certificate.

   Also, the channel users on the channels must be announced by compiling
   a list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type
   into the SILC_PACKET_NOTIFY packet.  The users' modes on the channel
   must also be announced by compiling list of Notify Payloads with the
   SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
   packet.

   The router MUST also announce the local servers by compiling list of
   ID Payloads into the SILC_PACKET_NEW_ID packet.

   Also, clients' modes (user modes in SILC) MUST be announced.  This is
   done by compiling a list of Notify Payloads with SILC_NOTIFY_UMODE_CHANGE
   notify type into the SILC_PACKET_NOTIFY packet.  Also, channels' topics
   MUST be announced by compiling a list of Notify Payloads with the
   SILC_NOTIFY_TOPIC_SET notify type into the SILC_PACKET_NOTIFY packet.
   Also, channel's invite and ban lists MUST be announced by compiling list
   of Notify Payloads with the SILC_NOTIFY_TYPE_INVITE and
   SILC_NOTIFY_TYPE_BAN notify types, respectively, into the
   SILC_PACKET_NOTIFY packet.

   The router which receives these lists MUST process them and broadcast
   the packets to its primary router.  When processing the announced channels
   and channel users the router MUST check whether a channel exists already
   with the same name.  If channel exists with the same name it MUST check
   whether the Channel ID is different.  If the Channel ID is different the
   router MUST send the notify type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the
   server to force the channel ID change to the ID the router has.  If the
   mode of the channel is different the router MUST send the notify type
   SILC_NOTIFY_TYPE_CMODE_CHANGE to the server to force the mode change
   to the mode that the router has.



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   The router MUST also generate new channel key and distribute it to the
   channel.  The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
   is set.

   If the channel has a channel founder already on the router, the router
   MUST send the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server
   to force the mode change for the channel founder on the server.  The
   channel founder privileges MUST be removed on the server.

   If the channel public keys are already set on the on router, the router
   MUST ignore the received channel public key list and send the notify
   type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server which includes the
   channel public key list that is on router.  The server MUST change the
   list to the one it receives from router.

   The router processing the channels MUST also compile a list of
   Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
   SILC_PACKET_NOTIFY and send the packet to the server.  This way the
   server (or router) will receive the clients on the channel that
   the router has.


4.3 Joining to a Channel

   This section describes the procedure when client joins to a channel.
   Client joins to channel by sending command SILC_COMMAND_JOIN to the
   server.  If the receiver receiving join command is normal server the
   server MUST check its local list whether this channel already exists
   locally.  This would indicate that some client connected to the server
   has already joined to the channel.  If this is the case, the client is
   joined to the channel, new channel key is created and information about
   newly joined channel is sent to the router.  The router is informed
   by sending SILC_NOTIFY_TYPE_JOIN notify type.  The notify type MUST
   also be sent to the local clients on the channel.  The new channel key
   is also sent to the router and to local clients on the channel.

   If the channel does not exist in the local list the client's command
   MUST be sent to the router which will then perform the actual joining
   procedure.  When server receives the reply to the command from the
   router it MUST be sent to the client which sent the command originally.
   Server will also receive the channel key from the server that it MUST
   send to the client which originally requested the join command.  The
   server MUST also save the channel key.

   If the receiver of the join command is router it MUST first check its
   local list whether anyone in the cell has already joined to the channel.
   If this is the case, the client is joined to the channel and reply is
   sent to the client.  If the command was sent by server the command reply



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   is sent to the server which sent it.  Then the router MUST also create
   new channel key and distribute it to all clients on the channel and
   all servers that have clients on the channel.  Router MUST also send
   the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
   and to local servers that have clients on the channel.

   If the channel does not exist on the router's local list it MUST
   check the global list whether the channel exists at all.  If it does
   the client is joined to the channel as described previously.  If
   the channel does not exist the channel is created and the client
   is joined to the channel.  The channel key is also created and
   distributed as previously described.  The client joining to the created
   channel is made automatically channel founder and both channel founder
   and channel operator privileges are set for the client.

   If the router created the channel in the process, information about the
   new channel MUST be broadcast to all routers.  This is done by
   broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
   route.  When the router joins the client to the channel it MUST also
   send information about newly joined client to all routers in the SILC
   network.  This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
   type to the router's primary route.

   It is important to note that new channel key is created always when
   new client joins to channel, whether the channel has existed previously
   or not.  This way the new client on the channel is not able to decrypt
   any of the old traffic on the channel.  Client which receives the reply to
   the join command MUST start using the received Channel ID in the channel
   message communication thereafter.  Client also receives the key for the
   channel in the command reply.  Note that the channel key is never
   generated or distributed if the SILC_CMODE_PRIVKEY mode is set.


4.4 Channel Key Generation

   Channel keys are created by router which creates the channel by taking
   enough randomness from cryptographically strong random number generator.
   The key is generated always when channel is created, when new client
   joins a channel and after the key has expired.  Key could expire for
   example in an hour.

   The key MUST also be re-generated whenever some client leaves a channel.
   In this case the key is created from scratch by taking enough randomness
   from the random number generator.  After that the key is distributed to
   all clients on the channel.  However, channel keys are cell specific thus
   the key is created only on the cell where the client, which left the
   channel, exists.  While the server or router is creating the new channel
   key, no other client may join to the channel.  Messages that are sent



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   while creating the new key are still processed with the old key.  After
   server has sent the SILC_PACKET_CHANNEL_KEY packet client MUST start
   using the new key.  If server creates the new key the server MUST also
   send the new key to its router.  See [SILC2] for more information about
   how channel messages must be encrypted and decrypted when router is
   processing them.

   If the key changes very often due to joining traffic on the channel it
   is RECOMMENDED that client implementation would cache some of the old
   channel keys for short period of time so that it is able to decrypt all
   channel messages it receives.  It is possible that on a heavy traffic
   channel a message encrypted with channel key that was just changed
   is received by client after the new key was set into use.  This is
   possible because not all clients may receive the new key at the same
   time, and may still be sending messages encrypted with the old key.

   When client receives the SILC_PACKET_CHANNEL_KEY packet with the
   Channel Key Payload it MUST process the key data to create encryption
   and decryption key, and to create the MAC key that is used to compute
   the MACs of the channel messages.  The processing is as follows:

     channel_key  = raw key data
     MAC key      = hash(raw key data)

   The raw key data is the key data received in the Channel Key Payload.
   It is used for both encryption and decryption.  The hash() is the hash
   function used with the HMAC of the channel.  Note that the server also
   MUST save the channel key.


4.5 Private Message Sending and Reception

   Private messages are sent point to point.  Client explicitly destine
   a private message to specific client that is delivered to only to that
   client.  No other client may receive the private message.  The receiver
   of the private message is destined in the SILC Packet Header as in any
   other packet as well.  The Source ID in the SILC Packet Header MUST be
   the ID of the sender of the message.

   If the sender of a private message does not know the receiver's Client
   ID, it MUST resolve it from server.  There are two ways to resolve the
   client ID from server; it is RECOMMENDED that client implementations
   send SILC_COMMAND_IDENTIFY command to receive the Client ID.  Client
   MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
   If the sender has received earlier a private message from the receiver
   it should have cached the Client ID from the SILC Packet Header.

   If server receives a private message packet which includes invalid



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   destination Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
   notify to the client with error status indicating that such Client ID
   does not exist.

   See [SILC2] for description of private message encryption and decryption
   process.


4.6 Private Message Key Generation

   Private message MAY be protected with a key generated by the client.
   One way to generate private message key is to use static or pre-shared
   keys in the client implementation.  Client that wants to indicate other
   client on the network that a private message key should be set, the
   client MAY send SILC_PACKET_PRIVATE_MESSAGE_KEY packet to indicate this.
   The actual key material has to be transferred outside the SILC network,
   or it has to be pre-shared key.  The client receiving this packet knows
   that the sender wishes to use private message key in private message
   communication.  In case of static or pre-shared keys the IV used in
   the encryption SHOULD be chosen randomly.  Sending the
   SILC_PACKET_PRIVATE_MESSAGE_KEY is not mandatory, and clients may
   naturally agree to use a key without sending the packet.

   Another choice to use private message keys is to negotiate fresh key
   material by performing the Key Agreement.  The SILC_PACKET_KEY_AGREEMENT
   packet MAY be used to negotiate the fresh key material.  In this case
   the resulting key material is used to secure the private messages.
   Also, the IV used in encryption is used as defined in [SILC3], unless
   otherwise stated by the encryption mode used.  By performing Key
   Agreement the clients can also negotiate the cipher and HMAC to be used
   in the private message encryption and to negotiate additional security
   parameters.  The actual Key Agreement [SILC2] is performed by executing
   the SILC Key Exchange protocol [SILC3], peer to peer.  Because of NAT
   devices in the network, it might be impossible to perform the Key
   Agreement.  In this case using static or pre-shared key and sending the
   SILC_PACKET_PRIVATE_MESSAGE_KEY to indicate the use of a private message
   key is a working alternative.

   If the key is pre-shared key or other key material not generated by
   Key Agreement, then the key material SHOULD be processed as defined
   in [SILC3].  In the processing, however, the HASH, as defined in [SILC3]
   MUST be ignored.  After processing the key material it is employed as
   defined in [SILC3].  If the SILC_PACKET_PRIVATE_MESSAGE_KEY was sent,
   then it defines the cipher and HMAC to be used.  The hash algorithm to be
   used in the key material processing is the one that HMAC algorithm is
   defined to use.  If the SILC_PACKET_PRIVATE_MESSAGE_KEY was not sent at
   all, then the hash algorithm to be used SHOULD be SHA1.  In this case
   also, implementations SHOULD use the SILC protocol's mandatory cipher



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   and HMAC in private message encryption.


4.7 Channel Message Sending and Reception

   Channel messages are delivered to a group of users.  The group forms a
   channel and all clients on the channel receives messages sent to the
   channel.  The Source ID in the SILC Packet Header MUST be the ID
   of the sender of the message.

   Channel messages are destined to a channel by specifying the Channel ID
   as Destination ID in the SILC Packet Header.  The server MUST then
   distribute the message to all clients, except to the original sender,
   on the channel by sending the channel message destined explicitly to a
   client on the channel.  However, the Destination ID MUST still remain
   as the Channel ID.

   If server receives a channel message packet which includes invalid
   destination Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
   notify to the sender with error status indicating that such Channel ID
   does not exist.

   See the [SILC2] for description of channel message routing for router
   servers, and channel message encryption and decryption process.


4.8 Session Key Regeneration

   Session keys MUST be regenerated periodically, say, once in an hour.
   The re-key process is started by sending SILC_PACKET_REKEY packet to
   other end, to indicate that re-key must be performed.  The initiator
   of the connection SHOULD initiate the re-key.

   If perfect forward secrecy (PFS) flag was selected in the SILC Key
   Exchange protocol [SILC3] the re-key MUST cause new key exchange with
   SKE protocol.  In this case the protocol is secured with the old key
   and the protocol results to new key material.  See [SILC3] for more
   information.  After the SILC_PACKET_REKEY packet is sent the sender
   will perform the SKE protocol.

   If PFS flag was set the resulted key material is processed as described
   in the section Processing the Key Material in [SILC3].  The difference
   with re-key in the processing is that the initial data for the hash
   function is just the resulted key material and not the HASH as it
   is not computed at all with re-key.  Other than that, the key processing
   it equivalent to normal SKE negotiation.

   If PFS flag was not set, which is the default case, then re-key is done



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   without executing SKE protocol.  In this case, the new key is created by
   providing the current sending encryption key to the SKE protocol's key
   processing function.  The process is described in the section Processing
   the Key Material in [SILC3].  The difference in the processing is that
   the initial data for the hash function is the current sending encryption
   key and not the SKE's KEY and HASH values.  Other than that, the key
   processing is equivalent to normal SKE negotiation.

   After both parties have regenerated the session key, both MUST send
   SILC_PACKET_REKEY_DONE packet to each other.  These packets are still
   secured with the old key.  After these packets, the subsequent packets
   MUST be protected with the new key.  Note that, in case SKE was performed
   again the SILC_PACKET_SUCCESS is not sent.  The SILC_PACKET_REKEY_DONE
   is sent in its stead.


4.9 Command Sending and Reception

   Client usually sends the commands in the SILC network.  In this case
   the client simply sends the command packet to server and the server
   processes it and replies with command reply packet.  See the [SILC4]
   for detailed description of all commands.

   However, if the server is not able to process the command, it is sent to
   the server's router.  This is case for example with commands such as
   SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands.  However, there are
   other commands as well [SILC4].  For example, if client sends the WHOIS
   command requesting specific information about some client the server must
   send the WHOIS command to router so that all clients in SILC network are
   searched.  The router, on the other hand, sends the WHOIS command further
   to receive the exact information about the requested client.  The WHOIS
   command travels all the way to the server which owns the client and it
   replies with command reply packet.  Finally, the server which sent the
   command receives the command reply and it must be able to determine which
   client sent the original command.  The server then sends command reply to
   the client.  Implementations should have some kind of cache to handle, for
   example, WHOIS information.  Servers and routers along the route could all
   cache the information for faster referencing in the future.

   The commands sent by server may be sent hop by hop until someone is able
   to process the command.  However, it is preferred to destine the command
   as precisely as it is possible.  In this case, other routers en route
   MUST route the command packet by checking the true sender and true
   destination of the packet.  However, servers and routers MUST NOT route
   command reply packets to clients coming from other servers.  Client
   MUST NOT accept command reply packet originated from anyone else but
   from its own server.




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4.10 Closing Connection

   When remote client connection is closed the server MUST send the notify
   type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
   the client was joined.  The server MUST also save the client's information
   for a period of time for history purposes.

   When remote server or router connection is closed the server or router
   MUST also remove all the clients that was behind the server or router
   from the SILC Network.  The server or router MUST also send the notify
   type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
   local clients that are joined on the same channels with the remote
   server's or router's clients.

   Router server MUST also check whether some client in the local cell
   is watching for the nickname this client has, and send the
   SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
   the network has the SILC_UMODE_REJECT_WATCHING user mode set.


4.11 Detaching and Resuming a Session

   SILC protocol provides a possibility for a client to detach itself from
   the network without actually signing off from the network.  The client
   connection to the server is closed but the client remains as valid client
   in the network.  The client may then later resume its session back from
   any server in the network.

   When client wishes to detach from the network it MUST send the
   SILC_COMMAND_DETACH command to its server.  The server then MUST set
   SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
   notify to its primary router, which then MUST broadcast it further
   to other routers in the network.  This user mode indicates that the
   client is detached from the network.  Implementations MUST NOT use
   the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
   to the client.  All packets MUST still be sent to the client even if
   client is detached from the network.  Only the server that originally
   had the active client connection is able to make the decision after it
   notices that the network connection is not active.  In this case the
   default case is to discard the packet.

   The SILC_UMODE_DETACHED flag cannot be set by client itself directly
   with SILC_COMMAND_UMODE command, but only implicitly by sending the
   SILC_COMMAND_DETACH command.  The flag also cannot be unset by the
   client, server or router with SILC_COMMAND_UMODE command, but only
   implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
   packet.




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   When the client wishes to resume its session in the SILC Network it
   connects to a server in the network, which MAY also be a different
   from the original server, and performs normal procedures regarding
   creating a connection as described in section 4.1.  After the SKE
   and the Connection Authentication protocols has been successfully
   completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
   MUST send SILC_PACKET_RESUME_CLIENT packet.  This packet is used to
   perform the resuming procedure.  The packet MUST include the detached
   client's Client ID, which the client must know.  It also includes
   Authentication Payload which includes signature computed with the
   client's private key.  The signature is computed as defined in the
   section 3.9.1.  Thus, the authentication method MUST be based in
   public key authentication.

   When server receive the SILC_PACKET_RESUME_CLIENT packet it MUST
   do the following:  Server checks that the Client ID is valid client
   and that it has the SILC_UMODE_DETACHED mode set.  Then it verifies
   the Authentication Payload with the detached client's public key.
   If it does not have the public key it retrieves it by sending
   SILC_COMMAND_GETKEY command to the server that has the public key from
   the original client connection.  The server MUST NOT use the public
   key received in the SKE protocol for this connection.  If the
   signature is valid the server unsets the SILC_UMODE_DETACHED flag,
   and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
   The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
   flag when the packet is received.  If the server is router server it
   also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
   server whom owned the detached client.

   The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
   packet MUST know whether the packet already has been received for
   the client.  It is a protocol error to attempt to resume the client
   session from more than one server.  The implementations could set
   internal flag that indicates that the client is resumed.  If router
   receive SILC_PACKET_RESUME_CLIENT packet for client that is already
   resumed the client MUST be killed from the network.  This would
   indicate that the client is attempting to resume the session more
   than once which is a protocol error.  In this case the router sends
   SILC_NOTIFY_TYPE_KILLED to the client.  All routers that detect
   the same situation MUST also send the notify for the client.

   The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
   must also understand that the client may not be found behind the
   same server that it originally came from.  They must update their
   caches according to this.  The server that now owns the client session
   MUST check whether the Client ID of the resumed client is based
   on the server's Server ID.  If it is not it creates a new Client
   ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network.  It MUST



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   also send the channel keys of all channels that the client has
   joined to the client since it does not have them.  Whether the
   Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
   packet to the client.  Only after this is the client resumed back
   to the network and may start sending packets and messages.

   It is also possible that the server did not know about the global
   channels before the client resumed.  In this case it joins the client
   to the channels, generates new channel keys and distributes the keys
   to the channels as described in section 4.4.

   It is an implementation issue for how long servers keep detached client
   sessions.  It is RECOMMENDED that the detached sessions would be
   persistent as long as the server is running.



4.12 UDP/IP Connections

   SILC protocol allows the use of UDP/IP instead of TCP/IP.  There may be
   many reasons to use UDP, such as video and audio conferencing might
   be more efficient with UDP.

   When UDP/IP is used, in the SILC Key Exchange protocol the IV Included
   flag MUST be set and the first 16-bits of the Cookie field in the Key
   Exchange Start Payload MUST include the port that the other end will use
   as the SILC session port.  The port is in MSB first order.  Both initiator
   and responder will set the port they are going to use and all packets
   after the SKE has been completed with the SILC_PACKET_SUCCESS packet MUST
   be sent to the specified port.  Initiator will send them to the port
   responder specified and vice versa.  When verifying the cookie for
   modifications the first two bytes are to be ignored in case IV Included
   flag has been set.

   The default SILC port or port where the SILC server is listenning for
   incoming packets is used only during initial key exchange protocol.  After
   SKE has been completed all packets are sent to the specified ports,
   including connection authentication packets and rekey packets even when
   PFS is used in rekey.

   Changing the ports during SILC session is possible only by first detaching
   from the server (with client-server connections) and then performing the
   SILC Key Exchange protocol from the beginning and resuming the detached
   session.

   Since the UDP is unreliable transport the SKE packets may not arrive to
   the recipient.  Implementation should support retransmission of SKE
   packets by using exponential backoff algorithm.  Also other SILC packets



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   such as messages may drop en route.  With message packets only way to
   assure reliable delivery is to use message acking and retransmit the
   message by using for example exponential backoff algorithm.  With SKE
   packets the initial timeout value should be no more than 1000
   milliseconds.  With message packets the initial timeout value should be
   around 5000 milliseconds.


5 Security Considerations

   Security is central to the design of this protocol, and these security
   considerations permeate the specification.  Common security considerations
   such as keeping private keys truly private and using adequate lengths for
   symmetric and asymmetric keys must be followed in order to maintain the
   security of this protocol.

   Special attention must also be paid to the servers and routers that are
   running the SILC service.  The SILC protocol's security depends greatly
   on the security and the integrity of the servers and administrators that
   are running the service.  It is recommended that some form of registration
   is required by the server and router administrator prior to acceptance to
   the SILC Network.  Even though the SILC protocol is secure in a network
   of mutual distrust between clients, servers, routers and administrators
   of the servers, the client should be able to trust the servers they are
   using if they wish to do so.

   It however must be noted that if the client requires absolute security
   by not trusting any of the servers or routers in the SILC Network, it can
   be accomplished by negotiating private secret keys outside the SILC
   Network, either using SKE or some other key exchange protocol, or to use
   some other external means for distributing the keys.  This applies for
   all messages, private messages and channel messages.

   It is important to note that SILC, like any other security protocol, is
   not a foolproof system; the SILC servers and routers could very well be
   compromised.  However, to provide an acceptable level of security and
   usability for end users, the protocol uses many times session keys or
   other keys generated by the servers to secure the messages.  This is an
   intentional design feature to allow ease of use for end users.  This way
   the network is still usable, and remains encrypted even if the external
   means of distributing the keys is not working.  The implementation,
   however, may like to not follow this design feature, and may always
   negotiate the keys outside SILC network.  This is an acceptable solution
   and many times recommended.  The implementation still must be able to
   work with the server generated keys.

   If this is unacceptable for the client or end user, the private keys
   negotiated outside the SILC Network should always be used.  In the end



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   it is the implementor's choice whether to negotiate private keys by
   default or whether to use the keys generated by the servers.

   It is also recommended that router operators in the SILC Network would
   form a joint forum to discuss the router and SILC Network management
   issues.  Also, router operators along with the cell's server operators
   should have a forum to discuss the cell management issues.


6 References

   [SILC2]      Riikonen, P., "SILC Packet Protocol", Internet Draft,
                January 2007.

   [SILC3]      Riikonen, P., "SILC Key Exchange and Authentication
                Protocols", Internet Draft, January 2007.

   [SILC4]      Riikonen, P., "SILC Commands", Internet Draft, January 2007.

   [IRC]        Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
                RFC 1459, May 1993.

   [IRC-ARCH]   Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
                April 2000.

   [IRC-CHAN]   Kalt, C., "Internet Relay Chat: Channel Management", RFC
                2811, April 2000.

   [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
                2812, April 2000.

   [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
                2813, April 2000.

   [SSH-TRANS]  Ylonen, T., et al, "SSH Transport Layer Protocol",
                Internet Draft.

   [PGP]        Callas, J., et al, "OpenPGP Message Format", RFC 2440,
                November 1998.

   [SPKI]       Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
                September 1999.

   [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
                Infrastructure, Certificate and CRL Profile", RFC 2459,
                January 1999.

   [Schneier]   Schneier, B., "Applied Cryptography Second Edition",



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                John Wiley & Sons, New York, NY, 1996.

   [Menezes]    Menezes, A., et al, "Handbook of Applied Cryptography",
                CRC Press 1997.

   [OAKLEY]     Orman, H., "The OAKLEY Key Determination Protocol",
                RFC 2412, November 1998.

   [ISAKMP]     Maughan D., et al, "Internet Security Association and
                Key Management Protocol (ISAKMP)", RFC 2408, November
                1998.

   [IKE]        Harkins D., and Carrel D., "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.

   [HMAC]       Krawczyk, H., "HMAC: Keyed-Hashing for Message
                Authentication", RFC 2104, February 1997.

   [PKCS1]      Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
                Specifications, Version 2.0", RFC 2437, October 1998.

   [RFC2119]    Bradner, S., "Key Words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3629, November 2003.

   [RFC1321]    Rivest R., "The MD5 Message-Digest Algorithm", RFC 1321,
                April 1992.

   [RFC3174]    Eastlake, F., et al., "US Secure Hash Algorithm 1 (SHA1)",
                RFC 3174, September 2001.

   [PKCS7]      Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
                Version 1.5", RFC 2315, March 1998.

   [RFC2253]    Wahl, M., et al., "Lightweight Directory Access Protocol
                (v3): UTF-8 String Representation of Distinguished Names",
                RFC 2253, December 1997.

   [RFC3454]    Hoffman, P., et al., "Preparation of Internationalized
                Strings ("stringprep")", RFC 3454, December 2002.

   [SHA256]     Eastlake 3rd, D., et al., "US Secure Hash Algorithms (SHA
                and HMAC-SHA)", RFC 4634, July 2006.






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7 Author's Address

   Pekka Riikonen
   Helsinki
   Finland

   EMail: priikone@iki.fi


Appendix A

   This appendix defines the stringprep [RFC3454] profile for string
   identifiers in SILC protocol.  Compliant implementation MUST use this
   profile to prepare the identifier strings in the SILC protocol.  The
   profile defines the following as required by [RFC3454].

   - Intended applicability of the profile:  the following identifiers in
     the SILC Protocol;  nicknames, usernames, server names, hostnames,
     service names, algorithm names and other security property names [SILC3],
     and SILC Public Key name.

   - The character repertoire that is the input and output to
     stringprep:  Unicode 3.2 with the list of unassigned code points
     being the Table A.1, as defined in [RFC3454].

   - The mapping tables used:  the following tables are used, in order,
     as defined in [RFC3454].

       Table B.1
       Table B.2

     The mandatory case folding is done using the Table B.2 which includes
     the characters for the normalization form KC.

   - The Unicode normalization used:  the Unicode normalization form
     KC is used, as defined in [RFC3454].

   - The prohibited characters as output:  the following tables are used
     to prohibit characters, as defined in [RFC3454];

       Table C.1.1
       Table C.1.2
       Table C.2.1
       Table C.2.2
       Table C.3
       Table C.4
       Table C.5
       Table C.6



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       Table C.7
       Table C.8
       Table C.9

   - Additional prohibited characters as output:  in addition, the following
     tables are used to prohibit characters, as defined in this document;

       Appendix C
       Appendix D

   - The bidirectional string testing used:  bidirectional string testing
     is ignored in this profile.

   This profile is to be maintained in the IANA registry for stringprep
   profiles.  The name of this profile is "silc-identifier-prep" and this
   document defines the profile.  This document defines the first version of
   this profile.


Appendix B

   This appendix defines the stringprep [RFC3454] profile for channel name
   strings in SILC protocol.  Compliant implementation MUST use this profile
   to prepare the channel name strings in the SILC protocol.  The profile
   defines the following as required by [RFC3454].

   - Intended applicability of the profile:  channel names.

   - The character repertoire that is the input and output to
     stringprep:  Unicode 3.2 with the list of unassigned code points
     being the Table A.1, as defined in [RFC3454].

   - The mapping tables used:  the following tables are used, in order,
     as defined in [RFC3454].

       Table B.1
       Table B.2

     The mandatory case folding is done using the Table B.2 which includes
     the characters for the normalization form KC.

   - The Unicode normalization used:  the Unicode normalization form
     KC is used, as defined in [RFC3454].

   - The prohibited characters as output:  the following tables are used
     to prohibit characters, as defined in [RFC3454];

       Table C.1.1



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       Table C.1.2
       Table C.2.1
       Table C.2.2
       Table C.3
       Table C.4
       Table C.5
       Table C.6
       Table C.7
       Table C.8
       Table C.9

   - Additional prohibited characters as output:  in addition, the following
     tables are used to prohibit characters, as defined in this document;

       Appendix D

   - The bidirectional string testing used:  bidirectional string testing
     is ignored in this profile.

   This profile is to be maintained in the IANA registry for stringprep
   profiles.  The name of this profile is "silc-identifier-ch-prep" and this
   document defines the profile.  This document defines the first version of
   this profile.


Appendix C

   This appendix defines additional prohibited characters in the identifier
   strings as defined in the stringprep profile in Appendix A.

   Reserved US-ASCII characters
   0021 002A 002C 003F 0040


Appendix D

   This appendix defines additional prohibited characters in the identifier
   strings as defined in the stringprep profile in Appendix A and Appendix B.
   Note that the prohibited character tables listed in the Appendix A and
   Appendix B may include some of the same characters listed in this
   appendix as well.

   Symbol characters and other symbol like characters
   00A2-00A9 00AC 00AE 00AF 00B0 00B1 00B4 00B6 00B8 00D7 00F7
   02C2-02C5 02D2-02FF 0374 0375 0384 0385 03F6 0482 060E 060F
   06E9 06FD 06FE 09F2 09F3 09FA 0AF1 0B70 0BF3-0BFA 0E3F
   0F01-0F03 0F13-0F17 0F1A-0F1F 0F34 0F36 0F38 0FBE 0FBF
   0FC0-0FC5 0FC7-0FCF 17DB 1940 19E0-19FF 1FBD 1FBF-1FC1



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   1FCD-1FCF 1FDD-1FDF 1FED-1FEF 1FFD 1FFE 2044 2052 207A-207C
   208A-208C 20A0-20B1 2100-214F 2150-218F 2190-21FF 2200-22FF
   2300-23FF 2400-243F 2440-245F 2460-24FF 2500-257F 2580-259F
   25A0-25FF 2600-26FF 2700-27BF 27C0-27EF 27F0-27FF 2800-28FF
   2900-297F 2980-29FF 2A00-2AFF 2B00-2BFF 2E9A 2EF4-2EFF
   2FF0-2FFF 303B-303D 3040 3095-3098 309F-30A0 30FF-3104
   312D-3130 318F 31B8-31FF 321D-321F 3244-325F 327C-327E
   32B1-32BF 32CC-32CF 32FF 3377-337A 33DE-33DF 33FF 4DB6-4DFF
   9FA6-9FFF A48D-A48F A4A2-A4A3 A4B4 A4C1 A4C5 A4C7-ABFF
   D7A4-D7FF FA2E-FAFF FFE0-FFEE FFFC 10000-1007F 10080-100FF
   10100-1013F 1D000-1D0FF 1D100-1D1FF 1D300-1D35F 1D400-1D7FF

   Other characters
   E0100-E01EF


Full Copyright Statement

   Copyright (C) The Internet Society (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.




















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