--- /dev/null
+.pl 10.0i
+.po 0
+.ll 7.2i
+.lt 7.2i
+.nr LL 7.2i
+.nr LT 7.2i
+.ds LF Riikonen
+.ds RF FORMFEED[Page %]
+.ds CF
+.ds LH Internet Draft
+.ds RH XXX
+.ds CH
+.na
+.hy 0
+.in 0
+.nf
+Network Working Group P. Riikonen
+Internet-Draft
+draft-riikonen-silc-spec-06.txt XXX
+Expires: XXX
+
+.in 3
+
+.ce 3
+Secure Internet Live Conferencing (SILC),
+Protocol Specification
+<draft-riikonen-silc-spec-07.txt>
+
+.ti 0
+Status of this Memo
+
+This document is an Internet-Draft and is in full conformance with
+all provisions of Section 10 of RFC 2026. Internet-Drafts are
+working documents of the Internet Engineering Task Force (IETF), its
+areas, and its working groups. Note that other groups may also
+distribute working documents as Internet-Drafts.
+
+Internet-Drafts are draft documents valid for a maximum of six months
+and may be updated, replaced, or obsoleted by other documents at any
+time. It is inappropriate to use Internet-Drafts as reference
+material or to cite them other than as "work in progress."
+
+The list of current Internet-Drafts can be accessed at
+http://www.ietf.org/ietf/1id-abstracts.txt
+
+The list of Internet-Draft Shadow Directories can be accessed at
+http://www.ietf.org/shadow.html
+
+The distribution of this memo is unlimited.
+
+
+.ti 0
+Abstract
+
+This memo describes a Secure Internet Live Conferencing (SILC)
+protocol which provides secure conferencing services over insecure
+network channel. SILC is IRC [IRC] like protocol, however, it is
+not equivalent to IRC and does not support IRC. Strong cryptographic
+methods are used to protect SILC packets inside the SILC network.
+Three other Internet Drafts relates very closely to this memo;
+SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
+Protocols [SILC3] and SILC Commands [SILC4].
+
+
+
+
+
+
+.ti 0
+Table of Contents
+
+.nf
+1 Introduction .................................................. 3
+ 1.1 Requirements Terminology .................................. 4
+2 SILC Concepts ................................................. 4
+ 2.1 SILC Network Topology ..................................... 4
+ 2.2 Communication Inside a Cell ............................... 6
+ 2.3 Communication in the Network .............................. 6
+ 2.4 Channel Communication ..................................... 7
+ 2.5 Router Connections ........................................ 8
+3 SILC Specification ............................................ 8
+ 3.1 Client .................................................... 9
+ 3.1.1 Client ID ........................................... 9
+ 3.2 Server .................................................... 10
+ 3.2.1 Server's Local ID List .............................. 10
+ 3.2.2 Server ID ........................................... 11
+ 3.2.3 SILC Server Ports ................................... 12
+ 3.3 Router .................................................... 12
+ 3.3.1 Router's Local ID List .............................. 13
+ 3.3.2 Router's Global ID List ............................. 13
+ 3.3.3 Router's Server ID .................................. 14
+ 3.4 Channels .................................................. 14
+ 3.4.1 Channel ID .......................................... 15
+ 3.5 Operators ................................................. 16
+ 3.6 SILC Commands ............................................. 16
+ 3.7 SILC Packets .............................................. 17
+ 3.8 Packet Encryption ......................................... 17
+ 3.8.1 Determination of the Source and the Destination ..... 18
+ 3.8.2 Client To Client .................................... 18
+ 3.8.3 Client To Channel ................................... 20
+ 3.8.4 Server To Server .................................... 20
+ 3.9 Key Exchange And Authentication ........................... 20
+ 3.9.1 Authentication Payload .............................. 21
+ 3.10 Algorithms ............................................... 23
+ 3.10.1 Ciphers ............................................ 23
+ 3.10.1.1 CBC Mode .................................. 24
+ 3.10.1.2 CTR Mode .................................. 24
+ 3.10.1.3 Randomized CBC Mode ....................... 25
+ 3.10.2 Public Key Algorithms .............................. 26
+ 3.10.3 Hash Functions ..................................... 26
+ 3.10.4 MAC Algorithms ..................................... 27
+ 3.10.5 Compression Algorithms ............................. 27
+ 3.11 SILC Public Key .......................................... 28
+ 3.12 SILC Version Detection ................................... 30
+ 3.13 Backup Routers ........................................... 31
+ 3.13.1 Switching to Backup Router ......................... 32
+ 3.13.2 Resuming Primary Router ............................ 33
+ 3.13.3 Discussion on Backup Router Scheme ................. 36
+4 SILC Procedures ............................................... 36
+ 4.1 Creating Client Connection ................................ 36
+ 4.2 Creating Server Connection ................................ 38
+ 4.2.1 Announcing Clients, Channels and Servers ............ 38
+ 4.3 Joining to a Channel ...................................... 39
+ 4.4 Channel Key Generation .................................... 41
+ 4.5 Private Message Sending and Reception ..................... 41
+ 4.6 Private Message Key Generation ............................ 42
+ 4.7 Channel Message Sending and Reception ..................... 43
+ 4.8 Session Key Regeneration .................................. 43
+ 4.9 Command Sending and Reception ............................. 44
+ 4.10 Closing Connection ....................................... 45
+ 4.11 Detaching and Resuming a Session ......................... 45
+5 Security Considerations ....................................... 47
+6 References .................................................... 48
+7 Author's Address .............................................. 49
+
+
+.ti 0
+List of Figures
+
+.nf
+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
+
+
+.ti 0
+1. Introduction
+
+This document describes a Secure Internet Live Conferencing (SILC)
+protocol which provides secure conferencing services over insecure
+network channel. SILC is IRC [IRC] like protocol, however, it is
+not equivalent to IRC and does not support IRC. Some of the SILC's
+features are not found in IRC but in traditional Instant Message (IM)
+protocols. SILC combines features from both of these chat protocol
+styles, and SILC can be implemented as either IRC-like system or
+IM-like system.
+
+Strong cryptographic methods are used to protect SILC packets inside
+the SILC network. Three other Internet Drafts 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, and for any security protocol for that
+matter, is 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 TCP/IP network
+protocol, although it could be made to work on other network protocols
+with only minor changes. However, it is recommended that TCP/IP
+protocol is used under SILC protocol. Typical implementation would
+be made in client-server model.
+
+
+.ti 0
+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].
+
+
+.ti 0
+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 routers and servers 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.
+
+
+.ti 0
+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 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 routers.
+
+A difference between normal server and router server is that routers
+knows everything about everything in the network. They also do the
+actual routing of the messages to the correct receiver. Normal servers
+knows only about local information and nothing about global information.
+This makes the network faster as there are less servers that needs to
+keep global information up to date at all time.
+
+This, on the other hand, leads to kind of 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.
+
+.in 8
+.nf
+ ---- ---- ---- ---- ---- ----
+ | 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.
+.in 3
+
+.ce
+Figure 1: SILC Network Topology
+
+
+A cell is formed when a server or servers connect to one router. In
+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 normal SILC
+server; clients may connect to it the same way as to normal SILC
+server. 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.
+
+There are many issues in this network topology that needs to be careful
+about. Issues like 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.
+
+
+.ti 0
+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:
+
+
+.in 25
+.nf
+1 --- S1 S4 --- 5
+ S/R
+ 2 -- S2 S3
+ / |
+ 4 3
+.in 3
+
+
+.ce
+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
+ 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.
+
+
+.ti 0
+2.3 Communication in the Network
+
+If the message is destined to server that does not belong to local cell
+the message is routed to the router server to which the destination
+server 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.
+
+
+
+.in 16
+.nf
+1 --- S1 S4 --- 5 S2 --- 1
+ S/R - - - - - - - - S/R
+ 2 -- S2 S3 S1
+ / | \\
+ 4 3 2
+
+ Cell 1. Cell 2.
+.in 3
+
+
+.ce
+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.
+
+
+.ti 0
+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.
+
+
+
+.ti 0
+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:
+
+
+.in 16
+.nf
+ S/R1 - < - < - < - < - < - < - S/R2
+ \\ /
+ v ^
+ \\ - > - > - S/R3 - > - > - /
+.in 3
+
+
+.ce
+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. 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
+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 cause interesting
+routing problems. The [SILC2] discusses the routing of packets in this
+sort of network in more detail.
+
+
+.ti 0
+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.
+
+
+.ti 0
+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 nickname selected by the user. User uses logical nicknames
+in communication which are then mapped to the corresponding Client ID.
+Client ID's 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. The maximum
+length of nickname is 128 bytes.
+
+
+.ti 0
+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 ID's,
+and ID's based on IPv6 addresses extends this to 2^224 different Client
+ID's. Collisions are not expected to happen. The Client ID is defined
+as follows.
+
+.in 6
+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 to the client has 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 lowercase 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 in
+ lowercase format.
+
+.in 3
+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 problems if it would occur. Nicknames are usually logical and
+it is unlikely to have two distinct logical nicknames produce same
+truncated hash value.
+
+
+.ti 0
+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.
+
+
+.ti 0
+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 ID's, 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 don't have to worry about global clients. Server is also responsible
+of creating the Client ID's for their clients.
+
+Normal server also keeps information about locally created channels and
+their Channel ID's.
+
+Hence, local list for normal server includes:
+
+.in 6
+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
+ 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 ID's on channel
+ o Client ID modes on channel
+ o Channel key
+.in 3
+
+
+.ti 0
+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 ID's also provide
+information for Client ID's, see section 3.1.1 Client ID. Server ID is
+defined as follows.
+
+.in 6
+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.
+
+.in 3
+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.
+
+
+.ti 0
+3.2.3 SILC Server Ports
+
+The following ports has been assigned by IANA for the SILC protocol:
+
+.in 10
+silc 706/tcp SILC
+silc 706/udp SILC
+.in 3
+
+
+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.
+
+
+.ti 0
+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
+is also a normal server thus clients may connect to it as it would be
+just normal SILC server.
+
+However, router servers has a lot of important tasks that normal servers
+do not have. Router server knows everything about everything in the SILC.
+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. And, this
+is what they must do.
+
+
+.ti 0
+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:
+
+.in 6
+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
+
+channel list - All channels in the cell
+ o Channel ID
+ o Client ID's on channel
+ o Client ID modes on channel
+ o Channel key
+.in 3
+
+
+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.
+
+
+.ti 0
+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 ID's, all created channels
+and their Channel ID's and all servers and routers on SILC and their
+Server ID's. 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:
+
+.in 6
+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 ID's on channel
+ o Client ID modes on channel
+.in 3
+
+
+
+.ti 0
+3.3.3 Router's Server ID
+
+Router's Server ID's are equivalent to normal Server ID's. As routers
+are normal servers as well same types of ID's applies for routers as well.
+Thus, see section 3.2.2 Server ID.
+
+
+.ti 0
+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 name 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. The channel name is a string of
+maximum length of 256 bytes. Channel names MUST NOT contain any
+whitespaces (` '), any non-printable ASCII characters, commas (`,')
+and wildcard characters.
+
+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.
+
+.in 6
+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 administrate the
+ 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.
+.in 3
+
+
+.ti 0
+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.
+
+.in 6
+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
+
+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
+
+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 router knows
+ 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 - To further randomize the Channel ID. This makes
+ sure that there are no collisions. This also means that
+ in a cell there can be 2^16 channels.
+.in 3
+
+
+.ti 0
+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
+channel, to gain access to the contents of a 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 everyone else except they are able to administrate
+their server or router.
+
+
+.ti 0
+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 miss-used 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 miss-use of commands.
+
+SILC commands are described in [SILC4].
+
+
+.ti 0
+3.7 SILC Packets
+
+Packets are naturally the most important part of the protocol and the
+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 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.
+
+
+.ti 0
+3.8 Packet Encryption
+
+All packets passed in SILC network MUST be encrypted. This section
+defines 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 has 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 has 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 before hand to be able to talk on the
+channel. In this case, no server or router know the key for 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 route. However, every packet passed
+from router to other router, including packets for channels, are
+encrypted with the shared session key. Every router connection has
+their own session keys.
+
+
+.ti 0
+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 who is next 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 ID's attached to the header. The ID's 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.
+
+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. However, the header and the packet may be encrypted
+with same key. This is the case, for example, with command packets.
+
+
+.ti 0
+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
+ 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 has established 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 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.
+
+o Router determines the destination of the packet and sends the
+ packet to the server.
+
+o Server determines the client to which the packet is destined
+ to and sends the packet to the client.
+
+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 packet.
+
+The process for clients on same server is much simpler as there are
+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.
+
+
+.ti 0
+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.
+
+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.
+
+
+.ti 0
+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].
+
+
+.ti 0
+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 router connects 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
+key based on digital signatures. All passphrases sent in SILC protocol
+MUST be UTF-8 [RFC2279] encoded. The connection authentication protocol
+is described in detail in [SILC3].
+
+
+.ti 0
+3.9.1 Authentication Payload
+
+Authentication payload is used separately from the SKE and the Connection
+Authentication protocol. It can be used during the session to authenticate
+with the remote. For example, the client can authenticate itself to the
+server to become server operator. In this case, Authentication Payload is
+used.
+
+The format of the Authentication Payload is as follows:
+
+.in 5
+.nf
+ 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 ~
+| |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+.in 3
+
+.ce
+Figure 5: Authentication Payload
+
+
+.in 6
+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
+ this field MAY include a 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.
+.in 3
+
+
+If the authentication method is password based, the Authentication
+Data field includes the plaintext UTF-8 encoded password. It is safe
+to send plaintext password 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 password based
+authentication. This way it is not possible to approximate the length
+of the password 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. In case of public key authentication
+also this payload is encrypted.
+
+
+.ti 0
+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.
+
+
+.ti 0
+3.10.1 Ciphers
+
+Cipher is the encryption algorithm that is used to protect the data
+in the SILC packets. See [SILC2] of 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 as to be used in SILC using the same 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.
+
+
+.ti 0
+3.10.1.1 CBC Mode
+
+The "cbc" encryption mode is CBC mode with inter-packet chaining. This
+means that the Initial Vector (IV) for the next encryption block is
+the previous ciphertext block. The very first IV MUST be random and is
+generated as described in [SILC3].
+
+
+.ti 0
+3.10.1.2 CTR Mode
+
+The "ctr" encryption mode is CTR mode. 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. When
+SILC specifications refer to Initial Vector (IV) in general cases, in
+case of CTR mode it refers to the counter block. The format of the
+128 bit counter block is as follows:
+
+.in 5
+.nf
+ 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 |
+| |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+| Block Counter |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+.in 3
+
+.ce
+Figure 6: Counter Block
+
+.in 6
+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 (8 bytes) - This value is the
+ first 8 bytes from the Sending IV or Receiving IV generated in
+ the 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 traffic. Each rekey
+ MUST produce a new value.
+
+o Block Counter (4 bytes) - This is the counter value for the
+ counter block and is MSB ordered number starting from one (1)
+ value for first block and incrementing for subsequent blocks.
+ The same value MUST NOT be used twice. The rekey MUST be
+ performed before this counter value wraps.
+.in 3
+
+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. 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 could be
+performed to produce fresh key material.
+
+If the IV Included flag was negotiated in SKE, implementations SHOULD
+still use the same counter block format as defined above. However,
+implementations are RECOMMENDED to replace the Truncated HASH field
+with a 32 bit random value for each IV (counter block) per encrypted
+SILC packet. Also note, that in this case the decryption process is
+not stateful and receiver cannot precompute the key stream.
+
+
+.ti 0
+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 inter-packet chaining is used, but for
+the first block new random IV is selected in each packet. In this mode
+the IV is appended at the end of the last ciphertext block and thus
+delivered to the recipient. This mode increases the ciphertext size by
+one ciphertext block. Note also that some data payloads in SILC are
+capable of delivering the IV to the recipient. When explicitly
+encrypting these payloads with randomized CBC the IV MUST NOT be appended
+at the end of the ciphertext. When encrypting these payloads with
+"cbc" mode they implicitly become randomized CBC since the IV is
+usually selected random and included in the ciphertext. In these
+cases using either CBC or randomized CBC is actually equivalent.
+
+
+.ti 0
+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:
+
+.in 6
+rsa RSA (REQUIRED)
+dss DSS (OPTIONAL)
+.in 3
+
+DSS is described in [Menezes]. The RSA MUST be implemented according
+PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
+compliant to either PKCS #1 version 1.5 or newer with the following
+notes: The signature encoding is always in same format as the encryption
+encoding regardless of the PKCS #1 version. The signature with appendix
+(with hash algorithm OID in the data) MUST NOT be used in the SILC. The
+rationale for this is that there is no binding between the PKCS #1 OIDs
+and the hash algorithms used in the SILC protocol. Hence, the encoding
+is always in PKCS #1 version 1.5 format.
+
+Additional public key algorithms MAY be defined to be used in SILC.
+
+When signatures are computed in SILC the computing of the signature is
+represented 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 sha1 MUST be used. 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].
+
+
+.ti 0
+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:
+
+.in 6
+sha1 SHA-1, length = 20 (REQUIRED)
+md5 MD5, length = 16 (RECOMMENDED)
+.in 3
+
+
+
+.ti 0
+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.
+
+The following MAC algorithms are defined in SILC protocol:
+
+.in 6
+hmac-sha1-96 HMAC-SHA1, length = 12 bytes (REQUIRED)
+hmac-md5-96 HMAC-MD5, length = 12 bytes (OPTIONAL)
+hmac-sha1 HMAC-SHA1, length = 20 bytes (OPTIONAL)
+hmac-md5 HMAC-MD5, length = 16 bytes (OPTIONAL)
+none No MAC (OPTIONAL)
+.in 3
+
+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] and hash algorithms that
+are used as part of the HMACs are described in [Scheneir] and in
+[Menezes].
+
+Additional MAC algorithms MAY be defined to be used in SILC.
+
+
+.ti 0
+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:
+
+.in 6
+none No compression (REQUIRED)
+zlib GNU ZLIB (LZ77) compression (OPTIONAL)
+.in 3
+
+Additional compression algorithms MAY be defined to be used in SILC.
+
+
+
+.ti 0
+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:
+
+
+.in 5
+.nf
+ 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 ~
+| |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+.in 3
+
+.ce
+Figure 5: SILC Public Key
+
+
+.in 6
+o Public Key Length (4 bytes) - Indicates the full length
+ of the 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 is of the following
+ format:
+
+ UN User name
+ HN Host name or IP address
+ RN Real name
+ E EMail address
+ O Organization
+ C Country
+
+
+ Examples of an identifier:
+
+ `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
+
+ `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
+
+ 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 written as `\\,',
+ for example, `O=Company XYZ\\, Inc.'.
+
+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.
+.in 3
+
+All fields in the public key are in MSB (most significant byte first)
+order. All strings in the public key are UTF-8 encoded.
+
+If an external protocol need to refer to SILC Public Key by name, the
+name "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).
+
+
+.ti 0
+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:
+
+.in 6
+SILC-<protocol version>-<software version>
+.in 3
+
+The version strings are of the following format:
+
+.in 6
+protocol version = <major>.<minor>
+software version = <major>[.<minor>[.<build or vendor string>]]
+.in 3
+
+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:
+
+.in 6
+SILC-1.1-2.0.2
+SILC-1.0-1.2
+SILC-1.2-1.0.VendorXYZ
+SILC-1.2-2.4.5 Vendor Limited
+.in 3
+
+
+.ti 0
+3.13 Backup Routers
+
+Backup routers may exist in the cell in addition of the primary router.
+However, they must not be active routers and 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 may one of the backup routers become
+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.
+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 actual router server when it feeds the data to
+it.
+
+In addition of having direct connection to the primary router of the
+cell, the backup router must also have connection to the same router
+the primary router of the cell is connected. However, it must not be
+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 as being 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 knows which one is the backup router
+of the cell from their local configuration. Each of the entity 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. The announcements
+assures 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 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 active server in the cell. Backup router
+may be a spare server in the cell that does not accept normal client
+connections at all. It may be reserved purely for the backup purposes.
+These, however, are cell management issues.
+
+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.
+
+
+.ti 0
+3.13.1 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 number 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 type
+number 22 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 number 21 is sent back
+to the server from the backup router.
+
+The servers connected to the backup router must then announce their
+clients, channels, channel users, channel user modes and channel modes
+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 have
+and distribute the new ones to the primary route.
+
+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 informations to the backup
+router. Both must process only the ones they do not know about. If
+any of the announced modes does not match then they are enforced in
+normal manner defined later in this specification.
+
+
+.ti 0
+3.13.2 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 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 back to the backup
+ router. If backup receives this it MUST NOT continue with the
+ backup resuming protocol.
+
+ 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
+ value 2 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 2 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 or 2 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 3 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 3 before
+ it continues with the protocol. It knows from the session ID values
+ set in the packet when it have received all packets. The session
+ value should be different in all packets it have sent earlier.
+ After the packets is received the backup router sends the
+ SILC_PACKET_RESUME_ROUTER packet with type value 4 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 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
+ SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
+ including the backup router. It also sends the packet with type
+ value 6 to its primary router, and to the router that is using
+ it as its primary router. The Session ID value in this packet
+ SHOULD be zero (0).
+
+ 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
+ with type value 5 or 6 must switch their primary route to the
+ new primary router and remove the route for the backup router, since
+ it is not anymore 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
+ backup router connection after this and the connection is considered
+ to be passive connection. The implementations SHOULD be able
+ to disable the connection without closing the actual link.
+
+After this protocol is executed the backup router is now again 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 in the backup router are considered passive
+connections.
+
+When the primary router of the cell comes back online and connects
+to its primary router, the remote primary router must send the
+SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
+connection is not allowed since the router has been replaced by an
+backup router. The session ID value in this packet SHOULD be zero (0).
+When the router receives this packet it must not use the connection
+as active connection but to understand that it cannot act as primary
+router in the cell. It must wait that the backup router connects to
+it, and the backup resuming protocol is executed.
+
+The following type values has been defined for SILC_PACKET_RESUME_ROUTER
+packet:
+
+ 1 SILC_SERVER_BACKUP_START
+ 2 SILC_SERVER_BACKUP_START_GLOBAL
+ 3 SILC_SERVER_BACKUP_START_CONNECTED
+ 4 SILC_SERVER_BACKUP_START_ENDING
+ 5 SILC_SERVER_BACKUP_START_RESUMED
+ 6 SILC_SERVER_BACKUP_START_RESUMED_GLOBAL
+ 20 SILC_SERVER_BACKUP_START_REPLACED
+ 21 SILC_SERVER_BACKUP_START_USE
+ 22 SILC_SERVER_BACKUP_START_USE_DENIED
+
+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].
+
+
+
+
+.ti 0
+3.13.3 Discussion on Backup Router Scheme
+
+It is clear that this backup router support is not able to handle all
+possible situations arising in unreliable network environment. This
+scheme for example does not handle situation when the router actually
+does not go offline but the network link goes down temporarily. It would
+require some intelligence to figure out when it is best time to switch
+to the backup router. To make it even more complicated it is possible
+that the backup router may have not lost the network link to the primary
+router.
+
+Other possible situation is when the network link is lost temporarily
+between two primary routers in the SILC network. Unless the routers
+notice the link going down they cannot perhaps find alternative routes.
+Worst situation is when the link goes down only for a short period of
+time, thus causing lag. Should the routers or servers find alternative
+routes if they cannot get response from the router during the lag?
+When alternative routes are being found it must be careful not to
+mess up existing primary routes between routers in the network.
+
+It is suggested that the current backup router scheme is only temporary
+solution and existing backup router protocols are studied further. It
+is also suggested that the backup router specification will be separated
+from this SILC specification Internet-Draft and additional specification
+is written on the subject.
+
+
+.ti 0
+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 section describes the procedures only generally as details
+are described in [SILC2] and [SILC3].
+
+
+.ti 0
+4.1 Creating Client Connection
+
+This section describes the procedure when 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 host really is
+who it claims to be. Client, 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. 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 performs
+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
+registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
+server. This packet includes various information about the client
+that the server uses to create the client. Server creates 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 those
+informations rather than what it received from client. This is suitable
+way to get the true information about client if it is available.
+
+The nickname of client is initially set to the username sent in the
+SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
+suitable 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.
+
+
+.ti 0
+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 when 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 has successfully performed the key exchange
+and connection authentication protocol, the server 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.
+
+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 in [SILC2].
+
+As 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.
+
+
+.ti 0
+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 and founder public key and other channel mode specific
+data is announced by sending SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.
+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, channel's 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.
+
+The router which receives these lists MUST process them and broadcast
+the packets to its primary route. 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.
+
+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 channel founder 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.
+
+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.
+
+
+.ti 0
+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 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
+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 has 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 has 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 is set for the client.
+
+If the router created the channel in the process, information about the
+new channel MUST be broadcasted 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 if the SILC_CMODE_PRIVKEY mode is set.
+
+
+.ti 0
+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
+while creating the new key are still processed with the old key. After
+server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
+using the new key. If server creates the new key the server MUST also
+send the new key to its router. See [SILC2] on more information about
+how channel messages must be encrypted and decrypted when router is
+processing them.
+
+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 HMAC key that is used to compute
+the MACs of the channel messages. The processing is as follows:
+
+ channel_key = raw key data
+ HMAC key = hash(raw key data)
+
+The raw key data is the key data received in the Channel Key Payload.
+The hash() function is the hash function used in the HMAC of the channel.
+Note that the server also MUST save the channel key.
+
+
+.ti 0
+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 any
+other packet as well.
+
+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
+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.
+
+
+.ti 0
+4.6 Private Message Key Generation
+
+Private message MAY be protected with a key generated by the client.
+The key may be generated and sent to the other client by sending packet
+SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
+and is secured by session keys. After that the private message key
+is used in the private message communication between those clients.
+The key sent inside the payload SHOULD be randomly generated. This
+packet MUST NOT be used to send pre-shared keys.
+
+Other choice is to entirely use keys that are not sent through
+the SILC network at all. This significantly adds security. This key
+could be a pre-shared-key that is known by both of the clients. Both
+agree about using the key and starts sending packets that indicate
+that the private message is secured using private message key. In
+case of pre-shared keys (static keys) the IV used in encryption SHOULD
+be chosen randomly.
+
+It is also possible to negotiate fresh key material by performing
+Key Agreement. The SILC_PACKET_KEY_AGREEMENT packet MAY be used to
+negotiate the fresh key material. In this case the resulted 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
+may negotiate the cipher and HMAC to be used in the private message
+encryption and to negotiate additional security parameters.
+
+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]. The hash function to be used SHOULD be SHA1. 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].
+In this case also, implementations SHOULD use the SILC protocol's
+mandatory cipher and HMAC in private message encryption.
+
+
+.ti 0
+4.7 Channel Message Sending and Reception
+
+Channel messages are delivered to group of users. The group forms a
+channel and all clients on the channel receives messages sent to the
+channel.
+
+Channel messages are destined to channel by specifying the Channel ID
+as Destination ID in the SILC Packet Header. The server MUST then
+distribute the message to all clients on the channel by sending the
+channel message destined explicitly to a client on the channel.
+
+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.
+
+
+.ti 0
+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
+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 has 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.
+
+
+.ti 0
+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 [SILC3]
+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. 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 server. Client
+MUST NOT accept command reply packet originated from anyone else but
+from its own server.
+
+
+
+.ti 0
+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.
+
+
+.ti 0
+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 will 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.
+
+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 made 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 receives 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 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 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 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
+also send the channel keys of all channels that the client is
+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 the client is resumed back
+to the network and may start sending packets and messages.
+
+It is also possible that the server does not know about the channels
+that the client has joined. In this case it join the client internally
+to the channels, generate new channel keys and distribute the keys
+to the channels as described in section 4.4.
+
+It is 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.
+
+
+.ti 0
+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 on 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 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 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 full proof system; the SILC servers and routers could very well be
+compromised. However, to provide acceptable level of security and
+usability for end user the protocol use many times session keys or other
+keys generated by the servers to secure the messages. This is
+intentional design feature to allow ease of use for end user. 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 always negotiate
+the keys outside SILC network. This is 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
+it is always 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.
+
+
+.ti 0
+6 References
+
+[SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
+ May 2002.
+
+[SILC3] Riikonen, P., "SILC Key Exchange and Authentication
+ Protocols", Internet Draft, May 2002.
+
+[SILC4] Riikonen, P., "SILC Commands", Internet Draft, May 2002.
+
+[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",
+ 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.
+
+[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
+ 10646", RFC 2279, January 1998.
+
+[PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
+ Version 1.5", RFC 2315, March 1998.
+
+
+.ti 0
+7 Author's Address
+
+.nf
+Pekka Riikonen
+Snellmaninkatu 34 A 15
+70100 Kuopio
+Finland
+
+EMail: priikone@iki.fi
+
+This Internet-Draft expires XXX
projects / silc.git / commitdiff