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+
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+
+
+
+
+Network Working Group P. Riikonen
+Internet-Draft
+draft-riikonen-silc-spec-09.txt 15 January 2007
+Expires: 15 July 2007
+
+
+ Secure Internet Live Conferencing (SILC),
+ Protocol Specification
+ <draft-riikonen-silc-spec-09.txt>
+
+Status of this Draft
+
+ By submitting this Internet-Draft, each author represents that any
+ applicable patent or other IPR claims of which he or she is aware
+ have been or will be disclosed, and any of which he or she becomes
+ aware will be disclosed, in accordance with Section 6 of BCP 79.
+
+ 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/1id-abstracts.html
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+
+
+Abstract
+
+ This memo describes a Secure Internet Live Conferencing (SILC)
+ protocol which provides secure conferencing services over insecure
+ network channel. SILC provides advanced and feature rich conferencing
+ services with security as main design principal. Strong cryptographic
+ methods are used to protect SILC packets inside the SILC network.
+ Three other specifications relates very closely to this memo;
+ SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
+ Protocols [SILC3] and SILC Commands [SILC4].
+
+
+
+
+
+
+
+
+
+Riikonen [Page 1]
+\f
+Internet Draft 15 January 2007
+
+
+Table of Contents
+
+ 1 Introduction .................................................. 3
+ 1.1 Requirements Terminology .................................. 4
+ 2 SILC Concepts ................................................. 4
+ 2.1 SILC Network Topology ..................................... 5
+ 2.2 Communication Inside a Cell ............................... 6
+ 2.3 Communication in the Network .............................. 7
+ 2.4 Channel Communication ..................................... 7
+ 2.5 Router Connections ........................................ 8
+ 3 SILC Specification ............................................ 9
+ 3.1 Client .................................................... 9
+ 3.1.1 Client ID ........................................... 10
+ 3.2 Server .................................................... 11
+ 3.2.1 Server's Local ID List .............................. 11
+ 3.2.2 Server ID ........................................... 12
+ 3.2.3 SILC Server Ports ................................... 12
+ 3.3 Router .................................................... 13
+ 3.3.1 Router's Local ID List .............................. 13
+ 3.3.2 Router's Global ID List ............................. 14
+ 3.3.3 Router's Server ID .................................. 15
+ 3.4 Channels .................................................. 15
+ 3.4.1 Channel ID .......................................... 16
+ 3.5 Operators ................................................. 17
+ 3.6 SILC Commands ............................................. 17
+ 3.7 SILC Packets .............................................. 17
+ 3.8 Packet Encryption ......................................... 18
+ 3.8.1 Determination of the Source and the Destination ..... 18
+ 3.8.2 Client To Client .................................... 19
+ 3.8.3 Client To Channel ................................... 20
+ 3.8.4 Server To Server .................................... 21
+ 3.9 Key Exchange And Authentication ........................... 21
+ 3.9.1 Authentication Payload .............................. 22
+ 3.10 Algorithms ............................................... 24
+ 3.10.1 Ciphers ............................................ 24
+ 3.10.1.1 CBC Mode .................................. 24
+ 3.10.1.2 CTR Mode .................................. 25
+ 3.10.1.3 Randomized CBC Mode ....................... 27
+ 3.10.2 Public Key Algorithms .............................. 27
+ 3.10.2.1 Multi-Precision Integers .................. 28
+ 3.10.3 Hash Functions ..................................... 28
+ 3.10.4 MAC Algorithms ..................................... 28
+ 3.10.5 Compression Algorithms ............................. 29
+ 3.11 SILC Public Key .......................................... 29
+ 3.12 SILC Version Detection ................................... 32
+ 3.13 UTF-8 Strings in SILC .................................... 33
+ 3.13.1 UTF-8 Identifier Strings ........................... 33
+ 3.14 Backup Routers ........................................... 34
+
+
+
+Riikonen [Page 2]
+\f
+Internet Draft 15 January 2007
+
+
+ 3.14.1 Switching to Backup Router ......................... 36
+ 3.14.2 Resuming Primary Router ............................ 37
+ 4 SILC Procedures ............................................... 39
+ 4.1 Creating Client Connection ................................ 39
+ 4.2 Creating Server Connection ................................ 41
+ 4.2.1 Announcing Clients, Channels and Servers ............ 42
+ 4.3 Joining to a Channel ...................................... 43
+ 4.4 Channel Key Generation .................................... 44
+ 4.5 Private Message Sending and Reception ..................... 45
+ 4.6 Private Message Key Generation ............................ 46
+ 4.7 Channel Message Sending and Reception ..................... 47
+ 4.8 Session Key Regeneration .................................. 47
+ 4.9 Command Sending and Reception ............................. 48
+ 4.10 Closing Connection ....................................... 49
+ 4.11 Detaching and Resuming a Session ......................... 49
+ 4.12 UDP/IP Connections ...................................... 51
+ 5 Security Considerations ....................................... 52
+ 6 References .................................................... 53
+ 7 Author's Address .............................................. 55
+ Appendix A ...................................................... 55
+ Appendix B ...................................................... 56
+ Appendix C ...................................................... 57
+ Appendix D ...................................................... 57
+ Full Copyright Statement ........................................ 58
+
+List of Figures
+
+ Figure 1: SILC Network Topology
+ Figure 2: Communication Inside cell
+ Figure 3: Communication Between Cells
+ Figure 4: Router Connections
+ Figure 5: SILC Public Key
+ Figure 6: Counter Block
+ Figure 7: CTR Mode Initialization Vector
+
+
+1. Introduction
+
+ This document describes a Secure Internet Live Conferencing (SILC)
+ protocol which provides secure conferencing services over insecure
+ network channel. SILC can be used as a secure conferencing service
+ that provides rich conferencing features. Some of the SILC features
+ are found in traditional chat protocols such as IRC [IRC] but many
+ of the SILC features can also be found in Instant Message (IM) style
+ protocols. SILC combines features from both of these chat protocol
+ styles, and can be implemented as either IRC-like system or IM-like
+ system. Some of the more advanced and secure features of the
+ protocol are new to all conferencing protocols. SILC also supports
+
+
+
+Riikonen [Page 3]
+\f
+Internet Draft 15 January 2007
+
+
+ multimedia messages and can also be implemented as a video and audio
+ conferencing system.
+
+ Strong cryptographic methods are used to protect SILC packets inside
+ the SILC network. Three other specifications relates very closely
+ to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
+ Authentication Protocols [SILC3] and SILC Commands [SILC4].
+
+ The protocol uses extensively packets as conferencing protocol
+ requires message and command sending. The SILC Packet Protocol is
+ described in [SILC2] and should be read to fully comprehend this
+ document and protocol. [SILC2] also describes the packet encryption
+ and decryption in detail. The SILC Packet Protocol provides secured
+ and authenticated packets, and the protocol is designed to be compact.
+ This makes SILC also suitable in environment of low bandwidth
+ requirements such as mobile networks. All packet payloads in SILC
+ can be also compressed.
+
+ The security of SILC protocol sessions are based on strong and secure
+ key exchange protocol. The SILC Key Exchange protocol is described
+ in [SILC3] along with connection authentication protocol and should
+ be read to fully comprehend this document and protocol.
+
+ The SILC protocol has been developed to work on both TCP/IP and UDP/IP
+ network protocols. However, typical implementation would use only TCP/IP
+ with SILC protocol. Typical implementation would be made in client-server
+ model.
+
+
+1.1 Requirements Terminology
+
+ The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
+ MAY, and OPTIONAL, when they appear in this document, are to be
+ interpreted as described in [RFC2119].
+
+
+2. SILC Concepts
+
+ This section describes various SILC protocol concepts that forms the
+ actual protocol, and in the end, the actual SILC network. The mission
+ of the protocol is to deliver messages from clients to other clients
+ through servers and routers in secure manner. The messages may also
+ be delivered from one client to many clients forming a group, also
+ known as a channel.
+
+ This section does not focus to security issues. Instead, basic network
+ concepts are introduced to make the topology of the SILC network
+ clear.
+
+
+
+Riikonen [Page 4]
+\f
+Internet Draft 15 January 2007
+
+
+2.1 SILC Network Topology
+
+ SILC network forms a ring as opposed to tree style network topology that
+ conferencing protocols usually have. The network has a cells which are
+ constructed from a router and zero or more servers. The servers are
+ connected to the router in a star like network topology. Routers in the
+ network are connected to each other forming a ring. The rationale for
+ this is to have servers that can perform specific kind of tasks what
+ other servers cannot perform. This leads to two kinds of servers; normal
+ SILC servers and SILC router servers.
+
+ A difference between normal server and router server is that routers
+ knows all global information and keep the global network state up to date.
+ They also do the actual routing of the messages to the correct receiver
+ within the cell and between other cells. Normal servers knows only local
+ information and receive global information only when it is needed. They do
+ not need to keep the global network state up to date. This makes the
+ network faster and scalable as there are less servers that needs to
+ maintain global network state.
+
+ This, on the other hand, leads into a cellular like network, where
+ routers are in the center of the cell and servers are connected to the
+ router.
+
+ The following diagram represents SILC network topology.
+
+ ---- ---- ---- ---- ---- ----
+ | S8 | S5 | S4 | | S7 | S5 | S6 |
+ ----- ---- ----- ----- ---- -----
+ | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
+ ---- ------ ---- ---- ------ ----
+ | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
+ ---- ---- ---- ---- ---- ---- | S3 | S1 |
+ Cell 1. \ Cell 2. | \____ ----- -----
+ | | | S4 | S/R4 |
+ ---- ---- ---- ---- ---- ---- ---- ------
+ | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
+ ----- ---- ----- ----- ---- ----- ---- ----
+ | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
+ ---- ------ ---- ---- ------ ----
+ | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
+ ---- ---- ---- ---- ---- ----
+ Cell 3. Cell 5.
+
+ Figure 1: SILC Network Topology
+
+
+ A cell is formed when a server or servers connect to one router. In
+
+
+
+Riikonen [Page 5]
+\f
+Internet Draft 15 January 2007
+
+
+ SILC network normal server cannot directly connect to other normal
+ server. Normal server may only connect to SILC router which then
+ routes the messages to the other servers in the cell. Router servers
+ on the other hand may connect to other routers to form the actual SILC
+ network, as seen in above figure. However, router is also able to act
+ as normal SILC server; clients may connect to it the same way as to
+ normal SILC server. This, however is not a requirement and if needed
+ router servers may be hidden from users by not allowing direct client
+ connections. Normal server also cannot have active connections to more
+ than one router. Normal server cannot be connected to two different
+ cells. Router servers, on the other hand, may have as many router to
+ router connections as needed. Other direct routes between other routers
+ is also possible in addition of the mandatory ring connections. This
+ leads into a hybrid ring-mesh network topology.
+
+ There are many issues in this network topology that needs to be careful
+ about. Issues like routing, the size of the cells, the number of the
+ routers in the SILC network and the capacity requirements of the
+ routers. These issues should be discussed in the Internet Community
+ and additional documents on the issue may be written.
+
+
+2.2 Communication Inside a Cell
+
+ It is always guaranteed that inside a cell message is delivered to the
+ recipient with at most two server hops. A client which is connected to
+ server in the cell and is talking on channel to other client connected
+ to other server in the same cell, will have its messages delivered from
+ its local server first to the router of the cell, and from the router
+ to the other server in the cell.
+
+ The following diagram represents this scenario:
+
+
+ 1 --- S1 S4 --- 5
+ S/R
+ 2 -- S2 S3
+ / |
+ 4 3
+
+
+ Figure 2: Communication Inside cell
+
+
+ Example: Client 1. connected to Server 1. send message to
+ Client 4. connected to Server 2. travels from Server 1.
+ first to Router which routes the message to Server 2.
+ which then sends it to the Client 4. All the other
+
+
+
+Riikonen [Page 6]
+\f
+Internet Draft 15 January 2007
+
+
+ servers in the cell will not see the routed message.
+
+
+ If the client is connected directly to the router, as router is also normal
+ SILC server, the messages inside the cell are always delivered only with
+ one server hop. If clients communicating with each other are connected
+ to the same server, no router interaction is needed. This is the optimal
+ situation of message delivery in the SILC network.
+
+
+2.3 Communication in the Network
+
+ If the message is destined to client that does not belong to local cell
+ the message is routed to the router server to which the destination
+ client belongs, if the local router is connected to destination router.
+ If there is no direct connection to the destination router, the local
+ router routes the message to its primary route. The following diagram
+ represents message sending between cells.
+
+
+
+ 1 --- S1 S4 --- 5 S2 --- 1
+ S/R - - - - - - - - S/R
+ 2 -- S2 S3 S1
+ / | \
+ 4 3 2
+
+ Cell 1. Cell 2.
+
+
+ Figure 3: Communication Between Cells
+
+
+ Example: Client 5. connected to Server 4. in Cell 1. sends message
+ to Client 2. connected to Server 1. in Cell 2. travels
+ from Server 4. to Router which routes the message to
+ Router in Cell 2, which then routes the message to
+ Server 1. All the other servers and routers in the
+ network will not see the routed message.
+
+
+ The optimal case of message delivery from the client point of view is
+ when clients are connected directly to the routers and the messages
+ are delivered from one router to the other.
+
+
+
+
+
+
+
+Riikonen [Page 7]
+\f
+Internet Draft 15 January 2007
+
+
+2.4 Channel Communication
+
+ Messages may be sent to group of clients as well. Sending messages to
+ many clients works the same way as sending messages point to point, from
+ message delivery point of view. Security issues are another matter
+ which are not discussed in this section.
+
+ Router server handles the message routing to multiple recipients. If
+ any recipient is not in the same cell as the sender the messages are
+ routed further.
+
+ Server distributes the channel message to its local clients which are
+ joined to the channel. Router also distributes the message to its
+ local clients on the channel.
+
+
+2.5 Router Connections
+
+ Router connections play very important role in making the SILC like
+ network topology to work. For example, sending broadcast packets in
+ SILC network require special connections between routers; routers must
+ be connected in a specific way.
+
+ Every router has their primary route which is a connection to another
+ router in the network. Unless there is only two routers in the network
+ must not routers use each other as their primary routes. The router
+ connections in the network must form a ring.
+
+ Example with three routers in the network:
+
+
+ S/R1 - < - < - < - < - < - < - S/R2
+ \ /
+ v ^
+ \ - > - > - S/R3 - > - > - /
+
+
+ Figure 4: Router Connections
+
+
+ Example: Network with three routers. Router 1. uses Router 2. as its
+ primary router. Router 2. uses Router 3. as its primary router,
+ and Router 3. uses Router 1. as its primary router. When there
+ are four or more routers in th enetwork, there may be other
+ direct connections between the routers but they must not be used
+ as primary routes.
+
+ The above example is applicable to any amount of routers in the network
+
+
+
+Riikonen [Page 8]
+\f
+Internet Draft 15 January 2007
+
+
+ except for two routers. If there are only two routers in the network both
+ routers must be able to handle situation where they use each other as their
+ primary routes.
+
+ The issue of router connections are very important especially with SILC
+ broadcast packets. Usually all router wide information in the network is
+ distributed by SILC broadcast packets. This sort of ring network, with
+ ability to have other direct routes in the network can cause interesting
+ routing problems. The [SILC2] discusses the routing of packets in this
+ sort of network in more detail.
+
+
+3. SILC Specification
+
+ This section describes the SILC protocol. However, [SILC2] and
+ [SILC3] describes other important protocols that are part of this SILC
+ specification and must be read.
+
+
+3.1 Client
+
+ A client is a piece of software connecting to SILC server. SILC client
+ cannot be SILC server. Purpose of clients is to provide the user
+ interface of the SILC services for end user. Clients are distinguished
+ from other clients by unique Client ID. Client ID is a 128 bit ID that
+ is used in the communication in the SILC network. The client ID is
+ based on the user's IP address and nickname. User use logical nicknames
+ in communication which are then mapped to the corresponding Client ID.
+ Client IDs are low level identifications and should not be seen by the
+ end user.
+
+ Clients provide other information about the end user as well. Information
+ such as the nickname of the user, username and the host name of the end
+ user and user's real name. See section 3.2 Server for information of
+ the requirements of keeping this information.
+
+ The nickname selected by the user is not unique in the SILC network.
+ There can be 2^8 same nicknames for one IP address. As for comparison to
+ IRC [IRC] where nicknames are unique this is a fundamental difference
+ between SILC and IRC. This typically causes the server names or client's
+ host names to be used along with the nicknames on user interface to
+ identify specific users when sending messages. This feature of SILC
+ makes IRC style nickname-wars obsolete as no one owns their nickname;
+ there can always be someone else with the same nickname. Also, any kind
+ of nickname registering service becomes obsolete. See the section 3.13.1
+ for more information about nicknames.
+
+
+
+
+
+Riikonen [Page 9]
+\f
+Internet Draft 15 January 2007
+
+
+3.1.1 Client ID
+
+ Client ID is used to identify users in the SILC network. The Client ID
+ is unique to the extent that there can be 2^128 different Client IDs,
+ and IDs based on IPv6 addresses extends this to 2^224 different Client
+ IDs. Collisions are not expected to happen. The Client ID is defined
+ as follows.
+
+ 128 bit Client ID based on IPv4 addresses:
+
+ 32 bit Server ID IP address (bits 1-32)
+ 8 bit Random number or counter
+ 88 bit Truncated MD5 hash value of the nickname
+
+ 224 bit Client ID based on IPv6 addresses:
+
+ 128 bit Server ID IP address (bits 1-128)
+ 8 bit Random number or counter
+ 88 bit Truncated MD5 hash value of the nickname
+
+ o Server ID IP address - Indicates the server where this
+ client is coming from. The IP address hence equals the
+ server IP address where the client is connected.
+
+ o Random number or counter - Random number to further
+ randomize the Client ID. Another choice is to use
+ a counter starting from the zero (0). This makes it
+ possible to have 2^8 same nicknames from the same
+ server IP address.
+
+ o MD5 hash - MD5 hash value of the case folded nickname is
+ truncated taking 88 bits from the start of the hash value.
+ This hash value is used to search the user's Client ID
+ from the ID lists. Note that the nickname MUST be prepared
+ using the stringprep [RFC3454] profile described in the
+ Appendix A before computing the MD5 hash. See also the
+ section 3.13.1 for more information.
+
+ Collisions could occur when more than 2^8 clients using same nickname
+ from the same server IP address is connected to the SILC network.
+ Server MUST be able to handle this situation by refusing to accept
+ anymore of that nickname.
+
+ Another possible collision may happen with the truncated hash value of
+ the nickname. It could be possible to have same truncated hash value
+ for two different nicknames. However, this is not expected to happen
+ nor cause any serious problems if it would occur. Nicknames are usually
+ logical and it is unlikely to have two distinct logical nicknames
+
+
+
+Riikonen [Page 10]
+\f
+Internet Draft 15 January 2007
+
+
+ produce same truncated hash value. Use of MD5 in nickname hash is not
+ a security feature.
+
+
+3.2 Server
+
+ Servers are the most important parts of the SILC network. They form the
+ basis of the SILC, providing a point to which clients may connect to.
+ There are two kinds of servers in SILC; normal servers and router servers.
+ This section focus on the normal server and router server is described
+ in the section 3.3 Router.
+
+ Normal servers MUST NOT directly connect to other normal server. Normal
+ servers may only directly connect to router server. If the message sent
+ by the client is destined outside the local server it is always sent to
+ the router server for further routing. Server may only have one active
+ connection to router on same port. Normal server MUST NOT connect to other
+ cell's router except in situations where its cell's router is unavailable.
+
+
+3.2.1 Server's Local ID List
+
+ Normal server keeps various information about the clients and their end
+ users connected to it. Every normal server MUST keep list of all locally
+ connected clients, Client IDs, nicknames, usernames and host names and
+ user's real name. Normal servers only keeps local information and it
+ does not keep any global information. Hence, normal servers knows only
+ about their locally connected clients. This makes servers efficient as
+ they do not have to worry about global clients. Server is also responsible
+ of creating the Client IDs for their clients.
+
+ Normal server also keeps information about locally created channels and
+ their Channel IDs.
+
+ Hence, local list for normal server includes:
+
+ server list - Router connection
+ o Server name
+ o Server IP address
+ o Server ID
+ o Sending key
+ o Receiving key
+ o Public key
+
+ client list - All clients in server
+ o Nickname
+ o Username@host
+ o Real name
+
+
+
+Riikonen [Page 11]
+\f
+Internet Draft 15 January 2007
+
+
+ o Client ID
+ o Sending key
+ o Receiving key
+ o Public key
+
+ channel list - All channels in server
+ o Channel name
+ o Channel ID
+ o Client IDs on channel
+ o Client ID modes on channel
+ o Channel key
+
+
+3.2.2 Server ID
+
+ Servers are distinguished from other servers by unique 64 bit Server ID
+ (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
+ the SILC to route messages to correct servers. Server IDs also provide
+ information for Client IDs, see section 3.1.1 Client ID. Server ID is
+ defined as follows.
+
+ 64 bit Server ID based on IPv4 addresses:
+
+ 32 bit IP address of the server
+ 16 bit Port
+ 16 bit Random number
+
+ 160 bit Server ID based on IPv6 addresses:
+
+ 128 bit IP address of the server
+ 16 bit Port
+ 16 bit Random number
+
+ o IP address of the server - This is the real IP address of
+ the server.
+
+ o Port - This is the port the server is bound to.
+
+ o Random number - This is used to further randomize the Server ID.
+
+ Collisions are not expected to happen in any conditions. The Server ID
+ is always created by the server itself and server is responsible of
+ distributing it to the router.
+
+
+3.2.3 SILC Server Ports
+
+ The following ports has been assigned by IANA for the SILC protocol:
+
+
+
+Riikonen [Page 12]
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+Internet Draft 15 January 2007
+
+
+ silc 706/tcp SILC
+ silc 706/udp SILC
+
+
+ If there are needs to create new SILC networks in the future the port
+ numbers must be officially assigned by the IANA.
+
+ Server on network above privileged ports (>1023) SHOULD NOT be trusted
+ as they could have been set up by untrusted party.
+
+
+3.3 Router
+
+ Router server in SILC network is responsible for keeping the cell together
+ and routing messages to other servers and to other routers. Router server
+ may also act as normal server when clients may connect to it. This is not
+ requirement and router servers may be hidden from clients.
+
+ However, router servers have a lot of important tasks that normal servers
+ do not have. Router server knows everything and keeps the global state.
+ They know all clients currently on SILC, all servers and routers and all
+ channels in SILC. Routers are the only servers in SILC that care about
+ global information and keeping them up to date at all time.
+
+
+3.3.1 Router's Local ID List
+
+ Router server as well MUST keep local list of connected clients and
+ locally created channels. However, this list is extended to include all
+ the informations of the entire cell, not just the server itself as for
+ normal servers.
+
+ However, on router this list is a lot smaller since routers do not need
+ to keep information about user's nickname, username and host name and real
+ name since these are not needed by the router. The router keeps only
+ information that it needs.
+
+ Hence, local list for router includes:
+
+ server list - All servers in the cell
+ o Server name
+ o Server ID
+ o Router's Server ID
+ o Sending key
+ o Receiving key
+
+ client list - All clients in the cell
+ o Client ID
+
+
+
+Riikonen [Page 13]
+\f
+Internet Draft 15 January 2007
+
+
+ channel list - All channels in the cell
+ o Channel ID
+ o Client IDs on channel
+ o Client ID modes on channel
+ o Channel key
+
+
+ Note that locally connected clients and other information include all the
+ same information as defined in section section 3.2.1 Server's Local ID
+ List. Router MAY also cache same detailed information for other clients
+ if needed.
+
+
+3.3.2 Router's Global ID List
+
+ Router server MUST also keep global list. Normal servers do not have
+ global list as they know only about local information. Global list
+ includes all the clients on SILC, their Client IDs, all created channels
+ and their Channel IDs and all servers and routers on SILC and their
+ Server IDs. That is said, global list is for global information and the
+ list must not include the local information already on the router's local
+ list.
+
+ Note that the global list does not include information like nicknames,
+ usernames and host names or user's real names. Router does not need to
+ keep these informations as they are not needed by the router. This
+ information is available from the client's server which maybe queried
+ when needed.
+
+ Hence, global list includes:
+
+ server list - All servers in SILC
+ o Server name
+ o Server ID
+ o Router's Server ID
+
+ client list - All clients in SILC
+ o Client ID
+
+ channel list - All channels in SILC
+ o Channel ID
+ o Client IDs on channel
+ o Client ID modes on channel
+
+
+
+
+
+
+
+
+Riikonen [Page 14]
+\f
+Internet Draft 15 January 2007
+
+
+3.3.3 Router's Server ID
+
+ Router's Server ID is equivalent to normal Server ID. As routers are
+ normal servers same types of IDs applies for routers as well. See
+ section 3.2.2 Server ID.
+
+
+
+
+3.4 Channels
+
+ A channel is a named group of one or more clients which will all receive
+ messages addressed to that channel. The channel is created when first
+ client requests JOIN command to the channel, and the channel ceases to
+ exist when the last client has left it. When channel exists, any client
+ can reference it using the Channel ID of the channel. If the channel has
+ a founder mode set and last client leaves the channel the channel does
+ not cease to exist. The founder mode can be used to make permanent
+ channels in the network. The founder of the channel can regain the
+ channel founder privileges on the channel later when he joins the
+ channel.
+
+ Channel names are unique although the real uniqueness comes from 64 bit
+ Channel ID. However, channel names are still unique and no two global
+ channels with same name may exist. See the section 3.13.1 for more
+ information about channel names.
+
+ Channels can have operators that can administrate the channel and operate
+ all of its modes. The following operators on channel exist on the
+ SILC network.
+
+ o Channel founder - When channel is created the joining client becomes
+ channel founder. Channel founder is channel operator with some more
+ privileges. Basically, channel founder can fully operate the channel
+ and all of its modes. The privileges are limited only to the
+ particular channel. There can be only one channel founder per
+ channel. Channel founder supersedes channel operator's privileges.
+
+ Channel founder privileges cannot be removed by any other operator on
+ channel. When channel founder leaves the channel there is no channel
+ founder on the channel. However, it is possible to set a mode for
+ the channel which allows the original channel founder to regain the
+ founder privileges even after leaving the channel. Channel founder
+ also cannot be removed by force from the channel.
+
+ o Channel operator - When client joins to channel that has not existed
+ previously it will become automatically channel operator (and channel
+ founder discussed above). Channel operator is able to administrate the
+
+
+
+Riikonen [Page 15]
+\f
+Internet Draft 15 January 2007
+
+
+ channel, set some modes on channel, remove a badly behaving client
+ from the channel and promote other clients to become channel
+ operator. The privileges are limited only to the particular channel.
+
+ Normal channel user may be promoted (opped) to channel operator
+ gaining channel operator privileges. Channel founder or other
+ channel operator may also demote (deop) channel operator to normal
+ channel user.
+
+
+
+
+3.4.1 Channel ID
+
+ Channels are distinguished from other channels by unique Channel ID.
+ The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
+ collisions are not expected to happen in any conditions. Channel names
+ are just for logical use of channels. The Channel ID is created by the
+ server where the channel is created. The Channel ID is defined as
+ follows.
+
+ 64 bit Channel ID based on IPv4 addresses:
+
+ 32 bit Router's Server ID IP address (bits 1-32)
+ 16 bit Router's Server ID port (bits 33-48)
+ 16 bit Random number or counter
+
+ 160 bit Channel ID based on IPv6 addresses:
+
+ 128 bit Router's Server ID IP address (bits 1-128)
+ 16 bit Router's Server ID port (bits 129-144)
+ 16 bit Random number or counter
+
+ o Router's Server ID IP address - Indicates the IP address of
+ the router of the cell where this channel is created. This is
+ taken from the router's Server ID. This way SILC routers know
+ where this channel resides in the SILC network.
+
+ o Router's Server ID port - Indicates the port of the channel on
+ the server. This is taken from the router's Server ID.
+
+ o Random number or counter - To further randomize the Channel ID.
+ Another choice is to use a counter starting from zero (0).
+ This makes sure that there are no collisions. This also means
+ that in a cell there can be 2^16 different channels.
+
+
+
+
+
+
+Riikonen [Page 16]
+\f
+Internet Draft 15 January 2007
+
+
+3.5 Operators
+
+ Operators are normal users with extra privileges to their server or
+ router. Usually these people are SILC server and router administrators
+ that take care of their own server and clients on them. The purpose of
+ operators is to administrate the SILC server or router. However, even
+ an operator with highest privileges is not able to enter invite-only
+ channels, to gain access to the contents of encrypted and authenticated
+ packets traveling in the SILC network or to gain channel operator
+ privileges on public channels without being promoted. They have the
+ same privileges as any normal user except they are able to administrate
+ their server or router.
+
+
+3.6 SILC Commands
+
+ Commands are very important part on SILC network especially for client
+ which uses commands to operate on the SILC network. Commands are used
+ to set nickname, join to channel, change modes and many other things.
+
+ Client usually sends the commands and server replies by sending a reply
+ packet to the command. Server MAY also send commands usually to serve
+ the original client's request. Usually server cannot send commands to
+ clients, however there MAY be commands that allow the server to send
+ commands to client. By default servers MAY send commands only to other
+ servers and routers.
+
+ Note that the command reply is usually sent only after client has sent
+ the command request but server is allowed to send command reply packet
+ to client even if client has not requested the command. Client MAY
+ choose to ignore the command reply.
+
+ It is expected that some of the commands may be misused by clients
+ resulting various problems on the server side. Every implementation
+ SHOULD assure that commands may not be executed more than once, say,
+ in two (2) seconds. However, to keep response rate up, allowing for
+ example five (5) commands before limiting is allowed. It is RECOMMENDED
+ that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
+ SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
+ as they require heavy operations. This should be sufficient to prevent
+ the misuse of commands.
+
+ SILC commands are described in [SILC4].
+
+
+3.7 SILC Packets
+
+ Packets are naturally the most important part of the protocol and the
+
+
+
+Riikonen [Page 17]
+\f
+Internet Draft 15 January 2007
+
+
+ packets are what actually makes the protocol. Packets in SILC network
+ are always encrypted using, usually the shared secret session key
+ or some other key, for example, channel key, when encrypting channel
+ messages. It is not possible to send a packet in SILC network without
+ encryption. The SILC Packet Protocol is a wide protocol and is described
+ in [SILC2]. This document does not define or describe details of
+ SILC packets.
+
+
+3.8 Packet Encryption
+
+ All packets passed in SILC network MUST be encrypted. This section
+ gives generic description of how packets must be encrypted in the SILC
+ network. The detailed description of the actual encryption process
+ of the packets are described in [SILC2].
+
+ Client and its server shares secret symmetric session key which is
+ established by the SILC Key Exchange Protocol, described in [SILC3].
+ Every packet sent from client to server, with exception of packets for
+ channels, are encrypted with this session key.
+
+ Channels have a channel key that are shared by every client on the channel.
+ However, the channel keys are cell specific thus one cell does not know
+ the channel key of the other cell, even if that key is for same channel.
+ Channel key is also known by the routers and all servers that have clients
+ on the channel. However, channels MAY have channel private keys that are
+ entirely local setting for the client. All clients on the channel MUST
+ know the channel private key beforehand to be able to talk on the
+ channel. In this case, no server or router knows the key for the channel.
+
+ Server shares secret symmetric session key with router which is
+ established by the SILC Key Exchange Protocol. Every packet passed from
+ server to router, with exception of packets for channels, are encrypted
+ with the shared session key. Same way, router server shares secret
+ symmetric key with its primary router. However, every packet passed
+ from router to other router, including packets for channels, are
+ encrypted with the shared session key. Every router connection MUST
+ have their own session keys.
+
+
+3.8.1 Determination of the Source and the Destination
+
+ The source and the destination of the packet needs to be determined
+ to be able to route the packets to correct receiver. This information
+ is available in the SILC Packet Header which is included in all packets
+ sent in SILC network. The SILC Packet Header is described in [SILC2].
+
+ The header MUST be encrypted with the session key of who is the next
+
+
+
+Riikonen [Page 18]
+\f
+Internet Draft 15 January 2007
+
+
+ receiver of the packet along the route. The receiver of the packet, for
+ example a router along the route, is able to determine the sender and the
+ destination of the packet by decrypting the SILC Packet Header and
+ checking the IDs attached to the header. The IDs in the header will
+ tell to where the packet needs to be sent and where it is coming from.
+
+ The header in the packet MUST NOT change during the routing of the
+ packet. The original sender, for example client, assembles the packet
+ and the packet header and server or router between the sender and the
+ receiver MUST NOT change the packet header. Note however, that some
+ packets such as commands may be resent by a server to serve the client's
+ original command. In this case the command packet sent by the server
+ includes the server's IDs as it is a different packet. When server
+ or router receives a packet it MUST verify that the Source ID is
+ valid and correct ID for that sender.
+
+ Note that the packet and the packet header may be encrypted with
+ different keys. For example, packets to channels are encrypted with
+ the channel key, however, the header is encrypted with the session key
+ as described above. Most other packets have both header and packet
+ payload encrypted with the same key, such as command packets.
+
+
+3.8.2 Client To Client
+
+ The process of message delivery and encryption from client to another
+ client is as follows.
+
+ Example: Private message from client to another client on different
+ servers. Clients do not share private message delivery
+ keys; normal session keys are used.
+
+ o Client 1 sends encrypted packet to its server. The packet is
+ encrypted with the session key shared between client and its
+ server.
+
+ o Server determines the destination of the packet and decrypts
+ the packet. Server encrypts the packet with session key shared
+ between the server and its router, and sends the packet to the
+ router.
+
+ o Router determines the destination of the packet and decrypts
+ the packet. Router encrypts the packet with session key
+ shared between the router and the destination server, and sends
+ the packet to the server.
+
+ o Server determines the client to which the packet is destined
+ to and decrypts the packet. Server encrypts the packet with
+
+
+
+Riikonen [Page 19]
+\f
+Internet Draft 15 January 2007
+
+
+ session key shared between the server and the destination client,
+ and sends the packet to the client.
+
+ o Client 2 decrypts the packet.
+
+
+ Example: Private message from client to another client on different
+ servers. Clients have established a secret shared private
+ message delivery key with each other and that is used in
+ the message encryption.
+
+ o Client 1 sends encrypted packet to its server. The packet header
+ is encrypted with the session key shared between the client and
+ server, and the private message payload is encrypted with the
+ private message delivery key shared between clients.
+
+ o Server determines the destination of the packet and sends the
+ packet to the router. Header is encrypted with the session key.
+
+ o Router determines the destination of the packet and sends the
+ packet to the server. Header is encrypted with the session key.
+
+ o Server determines the client to which the packet is destined
+ to and sends the packet to the client. Header is encrypted with
+ the session key.
+
+ o Client 2 decrypts the packet with the secret shared key.
+
+ If clients share secret key with each other the private message
+ delivery is much simpler since servers and routers between the
+ clients do not need to decrypt and re-encrypt the entire packet.
+ The packet header however is always encrypted with session key and
+ is decrypted and re-encrypted with the session key of next recipient.
+
+ The process for clients on same server is much simpler as there is
+ no need to send the packet to the router. The process for clients
+ on different cells is same as above except that the packet is routed
+ outside the cell. The router of the destination cell routes the
+ packet to the destination same way as described above.
+
+
+3.8.3 Client To Channel
+
+ Process of message delivery from client on channel to all the clients
+ on the channel.
+
+ Example: Channel of four users; two on same server, other two on
+ different cells. Client sends message to the channel.
+
+
+
+Riikonen [Page 20]
+\f
+Internet Draft 15 January 2007
+
+
+ Packet header is encrypted with the session key, message
+ data is encrypted with channel key.
+
+ o Client 1 encrypts the packet with channel key and sends the
+ packet to its server.
+
+ o Server determines local clients on the channel and sends the
+ packet to the Client on the same server. Server then sends
+ the packet to its router for further routing.
+
+ o Router determines local clients on the channel, if found
+ sends packet to the local clients. Router determines global
+ clients on the channel and sends the packet to its primary
+ router or fastest route.
+
+ o (Other router(s) do the same thing and sends the packet to
+ the server(s).)
+
+ o Server determines local clients on the channel and sends the
+ packet to the client.
+
+ o All clients receiving the packet decrypts it.
+
+
+3.8.4 Server To Server
+
+ Server to server packet delivery and encryption is described in above
+ examples. Router to router packet delivery is analogous to server to
+ server. However, some packets, such as channel packets, are processed
+ differently. These cases are described later in this document and
+ more in detail in [SILC2].
+
+
+3.9 Key Exchange And Authentication
+
+ Key exchange is done always when for example client connects to server
+ but also when server and router, and router and another router connect
+ to each other. The purpose of key exchange protocol is to provide secure
+ key material to be used in the communication. The key material is used
+ to derive various security parameters used to secure SILC packets. The
+ SILC Key Exchange protocol is described in detail in [SILC3].
+
+ Authentication is done after key exchange protocol has been successfully
+ completed. The purpose of authentication is to authenticate for example
+ client connecting to the server. However, clients MAY be accepted
+ to connect to server without explicit authentication. Servers are
+ REQUIRED to use authentication protocol when connecting. The
+ authentication may be based on passphrase (pre-shared secret) or public
+
+
+
+Riikonen [Page 21]
+\f
+Internet Draft 15 January 2007
+
+
+ key based on digital signatures. All passphrases sent in SILC protocol
+ MUST be UTF-8 [RFC3629] encoded. The connection authentication protocol
+ is described in detail in [SILC3].
+
+
+3.9.1 Authentication Payload
+
+ Authentication Payload is used separately from the SKE and the Connection
+ Authentication protocols. It can be used during the session to
+ authenticate with a remote. For example, a client can authenticate
+ itself to a server to become server operator. In this case,
+ Authentication Payload is used.
+
+ The format of the Authentication Payload is as follows:
+
+ 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Payload Length | Authentication Method |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Public Data Length | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
+ | |
+ ~ Public Data ~
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Authentication Data Length | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
+ | |
+ ~ Authentication Data ~
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 5: Authentication Payload
+
+
+ o Payload Length (2 bytes) - Length of the entire payload.
+
+ o Authentication Method (2 bytes) - The method of the
+ authentication. The authentication methods are defined
+ in [SILC2] in the Connection Auth Request Payload. The NONE
+ authentication method SHOULD NOT be used.
+
+ o Public Data Length (2 bytes) - Indicates the length of
+ the Public Data field.
+
+ o Public Data (variable length) - This is defined only if
+ the authentication method is public key. If it is any other
+
+
+
+Riikonen [Page 22]
+\f
+Internet Draft 15 January 2007
+
+
+ this field MAY include random data for padding purposes.
+ However, in this case the field MUST be ignored by the
+ receiver.
+
+ When the authentication method is public key this includes
+ 128 to 4096 bytes of non-zero random data that is used in
+ the signature process, described subsequently.
+
+ o Authentication Data Length (2 bytes) - Indicates the
+ length of the Authentication Data field. If zero (0)
+ value is found in this field the payload MUST be
+ discarded.
+
+ o Authentication Data (variable length) - Authentication
+ method dependent authentication data.
+
+
+ If the authentication method is passphrase-based, the Authentication
+ Data field includes the plaintext UTF-8 encoded passphrase. It is safe
+ to send plaintext passphrase since the entire payload is encrypted. In
+ this case the Public Data Length is set to zero (0), but MAY also include
+ random data for padding purposes. It is also RECOMMENDED that maximum
+ amount of padding is applied to SILC packet when using passphrase-based
+ authentication. This way it is not possible to approximate the length
+ of the passphrase from the encrypted packet.
+
+ If the authentication method is public key based (or certificate)
+ the Authentication Data is computed as follows:
+
+ HASH = hash(random bytes | ID | public key (or certificate));
+ Authentication Data = sign(HASH);
+
+ The hash() and the sign() are the hash function and the public key
+ cryptography function selected in the SKE protocol, unless otherwise
+ stated in the context where this payload is used. The public key
+ is SILC style public key unless certificates are used. The ID is the
+ entity's ID (Client or Server ID) which is authenticating itself. The
+ ID encoding is described in [SILC2]. The random bytes are non-zero
+ random bytes of length between 128 and 4096 bytes, and will be included
+ into the Public Data field as is.
+
+ The receiver will compute the signature using the random data received
+ in the payload, the ID associated to the connection and the public key
+ (or certificate) received in the SKE protocol. After computing the
+ receiver MUST verify the signature. Also in case of public key
+ authentication this payload is always encrypted. This payload is
+ always sent as part of some other payload.
+
+
+
+
+Riikonen [Page 23]
+\f
+Internet Draft 15 January 2007
+
+
+3.10 Algorithms
+
+ This section defines all the allowed algorithms that can be used in
+ the SILC protocol. This includes mandatory cipher, mandatory public
+ key algorithm and MAC algorithms.
+
+
+3.10.1 Ciphers
+
+ Cipher is the encryption algorithm that is used to protect the data
+ in the SILC packets. See [SILC2] for the actual encryption process and
+ definition of how it must be done. SILC has a mandatory algorithm that
+ must be supported in order to be compliant with this protocol.
+
+ The following ciphers are defined in SILC protocol:
+
+ aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
+ aes-256-ctr AES in CTR mode, 256 bit key (RECOMMENDED)
+ aes-256-rcbc AES in randomized CBC mode, 256 bit key (OPTIONAL)
+ aes-192-<mode> AES in <mode> mode, 192 bit key (OPTIONAL)
+ aes-128-<mode> AES in <mode> mode, 128 bit key (RECOMMENDED)
+ twofish-256-<mode> Twofish in <mode> mode, 256 bit key (OPTIONAL)
+ twofish-192-<mode> Twofish in <mode> mode, 192 bit key (OPTIONAL)
+ twofish-128-<mode> Twofish in <mode> mode, 128 bit key (OPTIONAL)
+ cast-256-<mode> CAST-256 in <mode> mode, 256 bit key (OPTIONAL)
+ cast-192-<mode> CAST-256 in <mode> mode, 192 bit key (OPTIONAL)
+ cast-128-<mode> CAST-256 in <mode> mode, 128 bit key (OPTIONAL)
+ serpent-<len>-<mode> Serpent in <mode> mode, <len> bit key (OPTIONAL)
+ rc6-<len>-<mode> RC6 in <mode> mode, <len> bit key (OPTIONAL)
+ mars-<len>-<mode> MARS in <mode> mode, <len> bit key (OPTIONAL)
+ none No encryption (OPTIONAL)
+
+ The <mode> is either "cbc", "ctr" or "rcbc". Other encryption modes MAY
+ be defined to be used in SILC using the same name format. The <len> is
+ either 256, 192 or 128 bit key length. Also, additional ciphers MAY be
+ defined to be used in SILC by using the same name format as above.
+
+ Algorithm "none" does not perform any encryption process at all and
+ thus is not recommended to be used. It is recommended that no client
+ or server implementation would accept "none" algorithm except in special
+ debugging mode.
+
+
+3.10.1.1 CBC Mode
+
+ The "cbc" encryption mode is the standard cipher-block chaining mode.
+ The very first IV is derived from the SILC Key Exchange protocol.
+ Subsequent IVs for encryption is the previous ciphertext block. The very
+
+
+
+Riikonen [Page 24]
+\f
+Internet Draft 15 January 2007
+
+
+ first IV MUST be random and is generated as described in [SILC3].
+
+
+3.10.1.2 CTR Mode
+
+ The "ctr" encryption mode is Counter Mode (CTR). The CTR mode in SILC is
+ stateful in encryption and decryption. Both sender and receiver maintain
+ the counter for the CTR mode and thus can precompute the key stream for
+ encryption and decryption. By default, CTR mode does not require
+ plaintext padding, however implementations MAY apply padding to the
+ packets. If the last key block is larger than the last plaintext block
+ the resulted value is truncated to the size of the plaintext block and
+ the most significant bits are used. When sending authentication data
+ inside packets the maximum amount of padding SHOULD be applied with
+ CTR mode as well.
+
+ In CTR mode only the encryption operation of the cipher is used. The
+ decryption operation is not needed since both encryption and decryption
+ process is simple XOR with the plaintext block and the key stream block.
+
+ The counter block is used to create the key for the CTR mode. The format
+ of the 128 bit counter block is as follows:
+
+ 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Truncated HASH from SKE |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Sending/Receiving IV from SKE |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Packet Counter |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Block Counter |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 6: Counter Block
+
+ o Truncated HASH from SKE (4 bytes) - This value is the first 4
+ bytes from the HASH value that was computed as a result of SKE
+ protocol. This acts as session identifier and each rekey MUST
+ produce a new HASH value.
+
+ o Sending/Receiving IV from SKE (4 bytes) - If the CTR mode is fully
+ stateful this field MUST include the first 4 bytes from the Sending
+ IV or Receiving IV generated in SKE protocol. When this mode is
+ used to encrypt sending traffic the Sending IV is used, when used
+ to decrypt receiving traffic the Receiving IV is used. This assures
+ that two parties of the protocol use different IV for sending
+
+
+
+Riikonen [Page 25]
+\f
+Internet Draft 15 January 2007
+
+
+ traffic. Each rekey MUST produce a new value.
+
+ If the IV Included flag is negotiated in SKE or CTR mode is used
+ where the IV is included in the data payload, this field is the
+ Nonce field from the IV received in the packet, defined below.
+
+ o Packet Counter (4 bytes) - This is MSB first ordered monotonically
+ increasing packet counter. It is set value 1 for first packet and
+ increases for subsequent packets. After rekey the counter MUST
+ restart from 1.
+
+ If the IV Included flag is negotiated in SKE or CTR mode is used
+ where the IV is included in the data payload, this field is the
+ Packet Counter field from the IV received in the packet, defined
+ below.
+
+ o Block Counter (4 bytes) - This is an MSB first ordered block
+ counter starting from 1 for first block and increasing for
+ subsequent blocks. The counter is always set to value 1 for
+ a new packet.
+
+ CTR mode MUST NOT be used with "none" MAC. Implementations also MUST
+ assure that the same counter block is not used to encrypt more than
+ one block. None of the counters must be allowed to wrap without rekey.
+ Also, the key material used with CTR mode MUST be fresh key material.
+ Static keys (pre-shared keys) MUST NOT be used with CTR mode. For this
+ reason using CTR mode to encrypt for example channel messages or private
+ messages with a pre-shared key is inappropriate. For private messages,
+ the Key Agreement [SILC2] could be performed to produce fresh key material.
+
+ If the IV Included flag was negotiated in SKE, or CTR mode is used to
+ protect channel messages where the IV will be included in the Message
+ Payload, the Initialization Vector (IV) to be used is a 64-bit block
+ containing randomness and packet counter. Also note, that in this case
+ the decryption process is not stateful and receiver cannot precompute
+ the key stream. Hence, the Initialization Vector (IV) when CTR mode is
+ used is as follows.
+
+ 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Nonce |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Packet Counter |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 7: CTR Mode Initialization Vector
+
+
+
+
+Riikonen [Page 26]
+\f
+Internet Draft 15 January 2007
+
+
+ o Nonce (4 bytes) - This field should be random or otherwise not
+ easily determinable and SHOULD change for each packet.
+
+ o Packet Counter (4 bytes) - This is MSB first ordered monotonically
+ increasing packet counter. It is set value 1 for first packet and
+ increases for subsequent packets. After rekey the counter MUST
+ restart from 1.
+
+ When decrypting the packet the Counter Block is assembled by concatenating
+ the truncated hash, with the received nonce and packet counter, and with
+ the block counter. The Counter Block is then used to compute the key
+ stream to perform the decryption.
+
+
+3.10.1.3 Randomized CBC Mode
+
+ The "rcbc" encryption mode is CBC mode with randomized IV. This means
+ that each IV for each packet MUST be chosen randomly. When encrypting
+ more than one block the normal IV chaining is used, but for the first
+ block new random IV is selected in each packet. In this mode the IV
+ is appended to the ciphertext. If this mode is used to secure the SILC
+ session, the IV Included flag must be negotiated in SILC Key Exchange
+ protocol. It may also be used to secure Message Payloads which can
+ deliver the IV to the recipient.
+
+
+3.10.2 Public Key Algorithms
+
+ Public keys are used in SILC to authenticate entities in SILC network
+ and to perform other tasks related to public key cryptography. The
+ public keys are also used in the SILC Key Exchange protocol [SILC3].
+
+ The following public key algorithms are defined in SILC protocol:
+
+ rsa RSA (REQUIRED)
+ dss DSS (OPTIONAL)
+
+ DSS is described in [Menezes]. The RSA MUST be implemented according
+ PKCS #1 [PKCS1]. When using SILC Public Key version 2 the PKCS #1
+ implementation MUST be compliant with PKCS #1 version 1.5. The signatures
+ are computed with appendix; the hash OID is included in the signature.
+ The user may always select the hash algorithm for the signatures. When
+ using SILC Public Key version 1 the PKCS #1 implementation MUST be
+ compliant with PKCS #1 version 1.5 where signatures are computed without
+ appendix; the hash OID is not present in the signature. The hash
+ algorithm used is specified separately or the default hash algorithm is
+ used, as defined below.
+
+
+
+
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+
+
+ Additional public key algorithms MAY be defined to be used in SILC.
+
+ When signatures are computed in SILC the computing of the signature is
+ denoted as sign(). The signature computing procedure is dependent of
+ the public key algorithm, and the public key or certificate encoding.
+ When using SILC public key the signature is computed as described in
+ previous paragraph for RSA and DSS keys. If the hash function is not
+ specified separately for signing process SHA-1 MUST be used, except with
+ SILC public key version 2 and RSA algorithm when the user MAY always
+ select the hash algorithm. In this case the hash algorithm is included
+ in the signature and can be retrieved during verification. When using
+ SSH2 public keys the signature is computed as described in [SSH-TRANS].
+ When using X.509 version 3 certificates the signature is computed as
+ described in [PKCS7]. When using OpenPGP certificates the signature is
+ computed as described in [PGP] and the PGP signature type used is 0x00.
+
+
+3.10.2.1 Multi-Precision Integers
+
+ Multi-Precision (MP) integers in SILC are encoded and decoded as defined
+ in PKCS #1 [PKCS1]. MP integers are unsigned, encoded with the exact
+ octet length of the integer. No extra leading zero octets may appear.
+ The actual length of the integer is the bit size of the integer not
+ counting any leading zero bits. The octet length is derived by calculating
+ (bit_length + 7) / 8.
+
+
+3.10.3 Hash Functions
+
+ Hash functions are used as part of MAC algorithms defined in the next
+ section. They are also used in the SILC Key Exchange protocol defined
+ in the [SILC3].
+
+ The following Hash algorithm are defined in SILC protocol:
+
+ sha1 SHA-1, length = 20 bytes (REQUIRED)
+ sha256 SHA-256, length = 32 bytes (RECOMMENDED)
+ md5 MD5, length = 16 bytes (RECOMMENDED)
+
+
+3.10.4 MAC Algorithms
+
+ Data integrity is protected by computing a message authentication code
+ (MAC) of the packet data. See [SILC2] for details how to compute the
+ MAC for a packet.
+
+ The following MAC algorithms are defined in SILC protocol:
+
+
+
+
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+
+
+ hmac-sha1-96 HMAC-SHA1, length = 12 bytes (REQUIRED)
+ hmac-sha256-96 HMAC-SHA256, length = 12 bytes (RECOMMENDED)
+ hmac-md5-96 HMAC-MD5, length = 12 bytes (OPTIONAL)
+ hmac-sha1 HMAC-SHA1, length = 20 bytes (OPTIONAL)
+ hmac-sha256 HMAC-SHA256, length = 32 bytes (OPTIONAL)
+ hmac-md5 HMAC-MD5, length = 16 bytes (OPTIONAL)
+ none No MAC (OPTIONAL)
+
+ The "none" MAC is not recommended to be used as the packet is not
+ authenticated when MAC is not computed. It is recommended that no
+ client or server would accept none MAC except in special debugging
+ mode.
+
+ The HMAC algorithm is described in [HMAC]. The hash algorithms used
+ in HMACs, the SHA-1 is described in [RFC3174] and MD5 is described
+ in [RFC1321]. The SHA-256 algorithm and its used with HMAC is described
+ in [SHA256].
+
+ Additional MAC algorithms MAY be defined to be used in SILC.
+
+
+3.10.5 Compression Algorithms
+
+ SILC protocol supports compression that may be applied to unencrypted
+ data. It is recommended to use compression on slow links as it may
+ significantly speed up the data transmission. By default, SILC does not
+ use compression which is the mode that must be supported by all SILC
+ implementations.
+
+ The following compression algorithms are defined:
+
+ none No compression (REQUIRED)
+ zlib GNU ZLIB (LZ77) compression (OPTIONAL)
+
+ Additional compression algorithms MAY be defined to be used in SILC.
+
+
+3.11 SILC Public Key
+
+ This section defines the type and format of the SILC public key. All
+ implementations MUST support this public key type. See [SILC3] for
+ other optional public key and certificate types allowed in the SILC
+ protocol. Public keys in SILC may be used to authenticate entities
+ and to perform other tasks related to public key cryptography.
+
+ The format of the SILC Public Key is as follows:
+
+
+
+
+
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+Internet Draft 15 January 2007
+
+
+ 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Public Key Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Algorithm Name Length | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
+ | |
+ ~ Algorithm Name ~
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Identifier Length | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
+ | |
+ ~ Identifier ~
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | |
+ ~ Public Data ~
+ | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 5: SILC Public Key
+
+
+ o Public Key Length (4 bytes) - Indicates the full length
+ of the SILC Public Key, not including this field.
+
+ o Algorithm Name Length (2 bytes) - Indicates the length
+ of the Algorithm Length field, not including this field.
+
+ o Algorithm name (variable length) - Indicates the name
+ of the public key algorithm that the key is. See the
+ section 3.10.2 Public Key Algorithms for defined names.
+
+ o Identifier Length (2 bytes) - Indicates the length of
+ the Identifier field, not including this field.
+
+ o Identifier (variable length) - Indicates the identifier
+ of the public key. This data can be used to identify the
+ owner of the key. The identifier may be of the following
+ format:
+
+ UN User name
+ HN Host name or IP address
+ RN Real name
+ E EMail address
+ O Organization
+
+
+
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+
+
+ C Country
+ V Version
+
+ Examples of an identifier:
+
+ `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
+
+ `UN=sam, HN=dummy.fi, RN=Sammy Sam, C=Finland, V=2'
+
+ At least user name (UN) and host name (HN) MUST be provided as
+ identifier. The fields are separated by commas (`,'). If
+ comma is in the identifier string it must be escaped as `\,',
+ for example, `O=Company XYZ\, Inc.'. Other characters that
+ require escaping are listed in [RFC2253] and are to be escaped
+ as defined therein. The Version (V) may only be a decimal digit.
+
+ o Public Data (variable length) - Includes the actual
+ public data of the public key.
+
+ The format of this field for RSA algorithm is
+ as follows:
+
+ 4 bytes Length of e
+ variable length e
+ 4 bytes Length of n
+ variable length n
+
+
+ The format of this field for DSS algorithm is
+ as follows:
+
+ 4 bytes Length of p
+ variable length p
+ 4 bytes Length of q
+ variable length q
+ 4 bytes Length of g
+ variable length g
+ 4 bytes Length of y
+ variable length y
+
+ The variable length fields are multiple precession
+ integers encoded as strings in both examples.
+
+ Other algorithms must define their own type of this
+ field if they are used.
+
+ The SILC Public Key is version is 2. If the Version (V) identifier is
+ not present the SILC Public Key version is expected to be 1. All new
+
+
+
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+
+
+ implementations SHOULD support version 1 but SHOULD only generate version 2.
+ In this case the Version (V) identifier MUST be present.
+
+ All fields in the public key are in MSB (most significant byte first)
+ order. All strings in the public key MUST be UTF-8 encoded.
+
+ If an external protocol needs to refer to SILC Public Key by name, the
+ names "silc-rsa" and "silc-dss" for SILC Public Key based on RSA algorithm
+ and SILC Public Key based on DSS algorithm, respectively, are to be used.
+ However, this SILC specification does not use these names directly, and
+ they are defined here for external protocols (protocols that may like
+ to use SILC Public Key).
+
+ A fingerprint from SILC Public Key is computed from the whole encoded
+ public key data block. All fields are included in computation. Compliant
+ implementations MUST support computing a 160-bit SHA-1 fingerprint.
+
+
+3.12 SILC Version Detection
+
+ The version detection of both client and server is performed at the
+ connection phase while executing the SILC Key Exchange protocol. The
+ version identifier is exchanged between initiator and responder. The
+ version identifier is of the following format:
+
+ SILC-<protocol version>-<software version>
+
+ The version strings are of the following format:
+
+ protocol version = <major>.<minor>
+ software version = <major>[.<minor>[.<build or vendor string>]]
+
+ Protocol version MUST provide both major and minor version. Currently
+ implementations MUST set the protocol version and accept at least the
+ protocol version as SILC-1.2-<software version>. If new protocol version
+ causes incompatibilities with older version the <minor> version number
+ MUST be incremented. The <major> is incremented if new protocol version
+ is fully incompatible.
+
+ Software version MAY provide major, minor and build (vendor) version.
+ The software version MAY be freely set and accepted. The version string
+ MUST consist of printable US-ASCII characters.
+
+ Thus, the version strings could be, for example:
+
+ SILC-1.1-2.0.2
+ SILC-1.0-1.2
+ SILC-1.2-1.0.VendorXYZ
+
+
+
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+
+
+ SILC-1.2-2.4.5 Vendor Limited
+
+
+3.13 UTF-8 Strings in SILC
+
+ By default all strings that are sent in SILC protocol MUST be UTF-8
+ [RFC3269] encoded, unless otherwise defined. This means that any string
+ sent inside for example, command, command reply, notify or any packet
+ payload is UTF-8 encoded. Also nicknames, channel names, server names,
+ and hostnames are UTF-8 encoded. This definition does not affect
+ messages sent in SILC, as the Message Payload provides its own mechanism
+ to indicate whether a message is UTF-8 text message, data message, which
+ may use its own character encoding, or pure binary message [SILC2].
+
+ Certain limitations are imposed on the UTF-8 encoded strings in SILC.
+ The UTF-8 encoded strings MUST NOT include any characters that are
+ marked in the Unicode standard as control codes, noncharacters,
+ reserved or private range characters, or any other illegal Unicode
+ characters. Also the BOM (Byte-Order Mark) MUST NOT be used as byte
+ order signature in UTF-8 encoded strings. A string containing these
+ characters MUST be treated as malformed UTF-8 encoding.
+
+ The Unicode standard defines that malformed sequences shall be signalled
+ by replacing the sequence with a replacement character. Even though,
+ in case of SILC these strings may not be malformed UTF-8 encodings
+ they MUST be treated as malformed strings. Implementation MAY use
+ a replacement character, however, the character Unicode standard defines
+ MUST NOT be used, but another character must be chosen. It is, however,
+ RECOMMENDED that an error is returned instead of using replacement
+ character if it is possible. For example, when setting a nickname
+ with SILC_COMMAND_NICK command, implementation is able to send error
+ indication back to the command sender. It must be noted that on server
+ implementation if a character sequence is merely outside of current
+ character subset, but is otherwise valid character, it MUST NOT be
+ replaced by a replacement character.
+
+ On user interface where UTF-8 strings are displayed the implementation
+ is RECOMMENDED to escape any character that it is unable to render
+ properly. The escaping may be done for example as described in
+ [RFC2253]. The escaping makes it possible to retrieve the original
+ UTF-8 encoding. Alternatively, a replacement character may be used
+ if it does not cause practical problems to the implementation.
+
+
+3.13.1 UTF-8 Identifier Strings
+
+ Identifier strings are special strings in SILC protocol that require
+ more careful processing, than the general UTF-8 strings described in the
+
+
+
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+
+
+ previous section. These strings include the nicknames, server names,
+ hostnames and some other identifier strings. These strings are prepared
+ using the stringprep [RFC3454] standard. The Appendix A defines the
+ stringprep profile for SILC identifier strings and conforming
+ implementation MUST use the profile to prepare any identifier string.
+
+ The stringprep profile describes how identifier strings are prepared,
+ what characters they may include, and which characters are prohibited.
+ Identifier strings with prohibited characters MUST be treated as
+ malformed strings.
+
+ The channel name is also special identifier strings with some slight
+ differences to other identifier strings. The Appendix B defines the
+ stringprep profile for the channel name strings and conforming
+ implementation MUST use the profile to prepare any channel name string.
+
+ Because of the profile the identifier strings in SILC may generally
+ include only letters, numbers, most punctuation characters, and some
+ other characters. For practical reasons most symbol characters and
+ many other special characters are prohibited. All identifier strings
+ are case folded and comparing the identifier strings MUST be done as
+ caseless matching.
+
+ In general, the identifier strings does not have a maximum length.
+ However, the length of a nickname string MUST NOT exceed 128 bytes, and
+ the length of a channel name string MUST NOT exceed 256 bytes. Since
+ these strings are UTF-8 encoded the length of one character may be
+ longer than one byte. This means that the character length of these
+ strings may be shorter than the maximum length of the string in bytes.
+ The minimum length of an identifier string MUST be at least one character,
+ which may be one byte or more in length. Implementation MAY limit the
+ maximum length of an identifier string, with exception of the nickname
+ and channel name strings which has the explicit length definition.
+
+
+3.14 Backup Routers
+
+ Backup routers may exist in the cell in addition to the primary router.
+ However, they must not be active routers or act as routers in the cell.
+ Only one router may be acting as primary router in the cell. In the case
+ of failure of the primary router one of the backup routers becomes active.
+ The purpose of backup routers are in case of failure of the primary router
+ to maintain working connections inside the cell and outside the cell and
+ to avoid netsplits.
+
+ Backup routers are normal servers in the cell that are prepared to take
+ over the tasks of the primary router if needed. They need to have at
+ least one direct and active connection to the primary router of the cell.
+
+
+
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+
+
+ This communication channel is used to send the router information to
+ the backup router. When the backup router connects to the primary router
+ of the cell it MUST present itself as router server in the Connection
+ Authentication protocol, even though it is normal server as long as the
+ primary router is available. Reason for this is that the configuration
+ needed in the responder end requires usually router connection level
+ configuration. The responder, however must understand and treat the
+ connection as normal server (except when feeding router level data to
+ the backup router).
+
+ Backup router must know everything that the primary router knows to be
+ able to take over the tasks of the primary router. It is the primary
+ router's responsibility to feed the data to the backup router. If the
+ backup router does not know all the data in the case of failure some
+ connections may be lost. The primary router of the cell must consider
+ the backup router being an actual router server when it feeds the data
+ to it.
+
+ In addition to having direct connection to the primary router of the
+ cell, the backup router must also have connection to the same router
+ to which the primary router of the cell is connected. However, it must
+ not be the active router connection meaning that the backup router must
+ not use that channel as its primary route and it must not notify the
+ router about having connected servers, channels and clients behind it.
+ It merely connects to the router. This sort of connection is later
+ referred to as being a passive connection. Some keepalive actions may
+ be needed by the router to keep the connection alive.
+
+ It is required that other normal servers have passive connections to
+ the backup router(s) in the cell. Some keepalive actions may be needed
+ by the server to keep the connection alive. After they notice the
+ failure of the primary router they must start using the connection to
+ the first backup router as their primary route.
+
+ Also, if any other router in the network is using the cell's primary
+ router as its own primary router, it must also have passive connection
+ to the cell's backup router. It too is prepared to switch to use the
+ backup router as its new primary router as soon as the original primary
+ router becomes unresponsive.
+
+ All of the parties of this protocol know which one is the backup router
+ of the cell from their local configuration. Each of the entities must
+ be configured accordingly and care must be taken when configuring the
+ backup routers, servers and other routers in the network.
+
+ It must be noted that some of the channel messages and private messages
+ may be lost during the switch to the backup router, unless the message
+ flag SILC_MESSAGE_FLAG_ACK is set in the message. The announcements
+
+
+
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+
+
+ assure that the state of the network is not lost during the switch.
+
+ It is RECOMMENDED that there would be at least one backup router in
+ the cell. It is NOT RECOMMENDED to have all servers in the cell acting
+ as backup routers as it requires establishing several connections to
+ several servers in the cell. Large cells can easily have several
+ backup routers in the cell.
+
+ The order of the backup routers are decided at the local configuration
+ phase. All the parties of this protocol must be configured accordingly to
+ understand the order of the backup routers. It is not required that the
+ backup server is actually an active server in the cell. The backup router
+ may be a redundant server in the cell that does not accept normal client
+ connections at all. It may be reserved purely for the backup purposes.
+
+ If also the first backup router is down as well and there is another
+ backup router in the cell then it will start acting as the primary
+ router as described above.
+
+
+3.14.1 Switching to Backup Router
+
+ When the primary router of the cell becomes unresponsive, for example
+ by sending EOF to the connection, all the parties of this protocol MUST
+ replace the old connection to the primary router with first configured
+ backup router. The backup router usually needs to do local modifications
+ to its database in order to update all the information needed to maintain
+ working routes. The backup router must understand that clients that
+ were originated from the primary router are now originated from some of
+ the existing server connections and must update them accordingly. It
+ must also remove those clients that were owned by the primary router
+ since those connections were lost when the primary router became
+ unresponsive.
+
+ All the other parties of the protocol must also update their local
+ database to understand that the route to the primary router will now go
+ to the backup router.
+
+ Servers connected to the backup router MUST send SILC_PACKET_RESUME_ROUTER
+ packet with type value 21, to indicate that the server will start using
+ the backup router as primary router. The backup router MUST NOT allow
+ this action if it detects that primary is still up and running. If
+ backup router knows that primary is up and running it MUST send
+ SILC_PACKET_FAILURE with type value 21 (4 bytes, MSB first order) back
+ to the server. The server then MUST NOT use the backup as primary
+ router, but must try to establish connection back to the primary router.
+ If the action is allowed type value 21 is sent back to the server from
+ the backup router. It is RECOMMENDED that implementations use the
+
+
+
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+
+
+ SILC_COMMAND_PING command to detect whether primary router is responsive.
+ If the backup router notices that the primary router is unresponsive
+ it SHOULD NOT start sending data to server links before the server has
+ sent the SILC_PACKET_RESUME_ROUTER with type value 21.
+
+ The servers connected to the backup router must then announce their
+ clients, channels, channel users, channel user modes, channel modes,
+ topics and other information to the backup router. This is to assure
+ that none of the important notify packets were lost during the switch
+ to the backup router. The backup router must check which of these
+ announced entities it already has and distribute the new ones to the
+ primary router.
+
+ The backup router too must announce its servers, clients, channels
+ and other information to the new primary router. The primary router
+ of the backup router too must announce its information to the backup
+ router. Both must process only the ones they do not know about. If
+ any of the announced modes do not match then they are enforced in
+ normal manner as defined in section 4.2.1 Announcing Clients, Channels
+ and Servers.
+
+
+3.14.2 Resuming Primary Router
+
+ Usually the primary router is unresponsive only a short period of time
+ and it is intended that the original router of the cell will resume
+ its position as primary router when it comes back online. The backup
+ router that is now acting as primary router of the cell must constantly
+ try to connect to the original primary router of the cell. It is
+ RECOMMENDED that it would try to reconnect in 30 second intervals to
+ the primary router.
+
+ When the connection is established to the primary router the backup
+ resuming protocol is executed. The protocol is advanced as follows:
+
+ 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
+ value 1 to the primary router that came back online. The packet
+ will indicate the primary router has been replaced by the backup
+ router. After sending the packet the backup router will announce
+ all of its channels, channel users, modes etc. to the primary
+ router.
+
+ If the primary knows that it has not been replaced (for example
+ the backup itself disconnected from the primary router and thinks
+ that it is now primary in the cell) the primary router send
+ SILC_PACKET_FAILURE with the type value 1 (4 bytes, MSB first
+ order) back to the backup router. If backup receives this it
+ MUST NOT continue with the backup resuming protocol.
+
+
+
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+
+
+ 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
+ value 1 to its current primary router to indicate that it will
+ resign as being primary router. Then, backup router sends the
+ SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
+ connected servers to also indicate that it will resign as being
+ primary router.
+
+ 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
+ type value 1 to the router that is using the backup router
+ currently as its primary router.
+
+ 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
+ with type value 1 must reconnect immediately to the primary
+ router of the cell that came back online. After they have created
+ the connection they MUST NOT use that connection as active primary
+ route but still route all packets to the backup router. After
+ the connection is created they MUST send SILC_PACKET_RESUME_ROUTER
+ with type value 2 back to the backup router. The session ID value
+ found in the first packet MUST be set in this packet.
+
+ 5. Backup router MUST wait for all packets with type value 2 before
+ it continues with the protocol. It knows from the session ID values
+ set in the packet when it has received all packets. The session
+ value should be different in all packets it has sent earlier.
+ After the packets are received the backup router sends the
+ SILC_PACKET_RESUME_ROUTER packet with type value 3 to the
+ primary router that came back online. This packet will indicate
+ that the backup router is now ready to resign as being primary
+ router. The session ID value in this packet MUST be the same as
+ in the first packet sent to the primary router. During this time
+ the backup router must still route all packets it is receiving
+ from server connections.
+
+ 6. The primary router receives the packet and send the packet
+ SILC_PACKET_RESUME_ROUTER with type value 4 to all connected servers
+ including the backup router. It also sends the packet with type
+ value 4 to its primary router, and to the router that is using
+ it as its primary router. The Session ID value in these packets
+ SHOULD be zero (0).
+
+ 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
+ packet with type value 4 must switch their primary route to the new
+ primary router and remove the route for the backup router, since
+ it is no longer the primary router of the cell. They must also
+ update their local database to understand that the clients are
+ not originated from the backup router but from the locally connected
+ servers. After that they MUST announce their channels, channel
+ users, modes etc. to the primary router. They MUST NOT use the
+
+
+
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+
+
+ backup router connection after this and the connection is considered
+ to be a passive connection. The implementation SHOULD be able
+ to disable the connection without closing the actual link.
+
+ After this protocol is executed the backup router is now again a normal
+ server in the cell that has the backup link to the primary router. The
+ primary router feeds the router specific data again to the backup router.
+ All server connections to the backup router are considered passive
+ connections.
+
+ When the primary router of the cell comes back online and connects
+ to its remote primary router, the remote primary router MUST send the
+ SILC_PACKET_RESUME_ROUTER packet with type value 20 indicating that the
+ connection is not allowed since the router has been replaced by an
+ backup router in the cell. The session ID value in this packet SHOULD be
+ zero (0). When the primary router receives this packet it MUST NOT use
+ the connection as active connection but must understand that it cannot
+ act as primary router in the cell, until the backup resuming protocol has
+ been executed.
+
+ The following type values has been defined for SILC_PACKET_RESUME_ROUTER
+ packet:
+
+ 1 SILC_SERVER_BACKUP_START
+ 2 SILC_SERVER_BACKUP_START_CONNECTED
+ 3 SILC_SERVER_BACKUP_START_ENDING
+ 4 SILC_SERVER_BACKUP_START_RESUMED
+ 20 SILC_SERVER_BACKUP_START_REPLACED
+ 21 SILC_SERVER_BACKUP_START_USE
+
+ If any other value is found in the type field the packet MUST be
+ discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
+ is defined in [SILC2].
+
+
+4 SILC Procedures
+
+ This section describes various SILC procedures such as how the
+ connections are created and registered, how channels are created and
+ so on. The references [SILC2], [SILC3] and [SILC4] permeate this
+ section's definitions.
+
+
+4.1 Creating Client Connection
+
+ This section describes the procedure when a client connects to SILC
+ server. When client connects to server the server MUST perform IP
+ address lookup and reverse IP address lookup to assure that the origin
+
+
+
+Riikonen [Page 39]
+\f
+Internet Draft 15 January 2007
+
+
+ host really is who it claims to be. Client, a host, connecting to server
+ SHOULD have both valid IP address and fully qualified domain name (FQDN).
+
+ After that the client and server performs SILC Key Exchange protocol
+ which will provide the key material used later in the communication.
+ The key exchange protocol MUST be completed successfully before the
+ connection registration may continue. The SILC Key Exchange protocol
+ is described in [SILC3].
+
+ Typical server implementation would keep a list of connections that it
+ allows to connect to the server. The implementation would check, for
+ example, the connecting client's IP address from the connection list
+ before the SILC Key Exchange protocol has been started. The reason for
+ this is that if the host is not allowed to connect to the server there
+ is no reason to perform the key exchange protocol.
+
+ After successful key exchange protocol the client and server perform
+ connection authentication protocol. The purpose of the protocol is to
+ authenticate the client connecting to the server. Flexible
+ implementation could also accept the client to connect to the server
+ without explicit authentication. However, if authentication is
+ desired for a specific client it may be based on passphrase or
+ public key authentication. If authentication fails the connection
+ MUST be terminated. The connection authentication protocol is described
+ in [SILC3].
+
+ After successful key exchange and authentication protocol the client
+ MUST register itself by sending SILC_PACKET_NEW_CLIENT packet to the
+ server. This packet includes various information about the client
+ that the server uses to register the client. Server registers the
+ client and sends SILC_PACKET_NEW_ID to the client which includes the
+ created Client ID that the client MUST start using after that. After
+ that all SILC packets from the client MUST have the Client ID as the
+ Source ID in the SILC Packet Header, described in [SILC2].
+
+ Client MUST also get the server's Server ID that is to be used as
+ Destination ID in the SILC Packet Header when communicating with
+ the server (for example when sending commands to the server). The
+ ID may be resolved in two ways. Client can take the ID from an
+ previously received packet from server that MUST include the ID,
+ or to send SILC_COMMAND_INFO command and receive the Server ID as
+ command reply.
+
+ Server MAY choose not to use the information received in the
+ SILC_PACKET_NEW_CLIENT packet. For example, if public key or
+ certificate were used in the authentication, server MAY use that
+ information rather than what it received from client. This is a suitable
+ way to get the true information about client if it is available.
+
+
+
+Riikonen [Page 40]
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+Internet Draft 15 January 2007
+
+
+ The nickname of client is initially set to the username sent in the
+ SILC_PACKET_NEW_CLIENT packet. User may set the nickname to something
+ more desirable by sending SILC_COMMAND_NICK command. However, this is
+ not required as part of registration process.
+
+ Server MUST also distribute the information about newly registered
+ client to its router (or if the server is router, to all routers in
+ the SILC network). More information about this in [SILC2].
+
+ Router server MUST also check whether some client in the local cell
+ is watching for the nickname this new client has, and send the
+ SILC_NOTIFY_TYPE_WATCH to the watcher.
+
+
+4.2 Creating Server Connection
+
+ This section describes the procedure when server connects to its
+ router (or when router connects to other router, the cases are
+ equivalent). The procedure is very much alike to when a client
+ connects to the server thus it is not repeated here.
+
+ One difference is that server MUST perform connection authentication
+ protocol with proper authentication. A proper authentication is based
+ on passphrase authentication or public key authentication based on
+ digital signatures.
+
+ After server and router have successfully performed the key exchange
+ and connection authentication protocol, the server MUST register itself
+ to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
+ includes the server's Server ID that it has created by itself and
+ other relevant information about the server. The router receiving the
+ ID MUST verify that the IP address in the Server ID is same as the
+ server's real IP address.
+
+ After router has received the SILC_PACKET_NEW_SERVER packet it
+ distributes the information about newly registered server to all routers
+ in the SILC network. More information about this is in [SILC2].
+
+ As the client needed to resolve the destination ID this MUST be done by
+ the server that connected to the router, as well. The way to resolve it
+ is to get the ID from previously received packet. The server MAY also
+ use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
+ using its own Server ID as Source ID in SILC Packet Header and the
+ router's Server ID as Destination when communicating with the router.
+
+
+
+
+
+
+
+Riikonen [Page 41]
+\f
+Internet Draft 15 January 2007
+
+
+4.2.1 Announcing Clients, Channels and Servers
+
+ After server or router has connected to the remote router, and it already
+ has connected clients and channels it MUST announce them to the router.
+ If the server is router server, also all the local servers in the cell
+ MUST be announced.
+
+ All clients are announced by compiling a list of ID Payloads into the
+ SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
+ list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet.
+ Channels' mode, founder public key, channel public keys, and other
+ channel mode specific data is announced by sending the
+ SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.
+
+ The channel public keys that are announced are compiled in Argument
+ List Payload where the argument type is 0x03, and each argument is
+ Public Key Payload containing one public key or certificate.
+
+ Also, the channel users on the channels must be announced by compiling
+ a list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type
+ into the SILC_PACKET_NOTIFY packet. The users' modes on the channel
+ must also be announced by compiling list of Notify Payloads with the
+ SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
+ packet.
+
+ The router MUST also announce the local servers by compiling list of
+ ID Payloads into the SILC_PACKET_NEW_ID packet.
+
+ Also, clients' modes (user modes in SILC) MUST be announced. This is
+ done by compiling a list of Notify Payloads with SILC_NOTIFY_UMODE_CHANGE
+ notify type into the SILC_PACKET_NOTIFY packet. Also, channels' topics
+ MUST be announced by compiling a list of Notify Payloads with the
+ SILC_NOTIFY_TOPIC_SET notify type into the SILC_PACKET_NOTIFY packet.
+ Also, channel's invite and ban lists MUST be announced by compiling list
+ of Notify Payloads with the SILC_NOTIFY_TYPE_INVITE and
+ SILC_NOTIFY_TYPE_BAN notify types, respectively, into the
+ SILC_PACKET_NOTIFY packet.
+
+ The router which receives these lists MUST process them and broadcast
+ the packets to its primary router. When processing the announced channels
+ and channel users the router MUST check whether a channel exists already
+ with the same name. If channel exists with the same name it MUST check
+ whether the Channel ID is different. If the Channel ID is different the
+ router MUST send the notify type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the
+ server to force the channel ID change to the ID the router has. If the
+ mode of the channel is different the router MUST send the notify type
+ SILC_NOTIFY_TYPE_CMODE_CHANGE to the server to force the mode change
+ to the mode that the router has.
+
+
+
+Riikonen [Page 42]
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+Internet Draft 15 January 2007
+
+
+ The router MUST also generate new channel key and distribute it to the
+ channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
+ is set.
+
+ If the channel has a channel founder already on the router, the router
+ MUST send the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server
+ to force the mode change for the channel founder on the server. The
+ channel founder privileges MUST be removed on the server.
+
+ If the channel public keys are already set on the on router, the router
+ MUST ignore the received channel public key list and send the notify
+ type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server which includes the
+ channel public key list that is on router. The server MUST change the
+ list to the one it receives from router.
+
+ The router processing the channels MUST also compile a list of
+ Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
+ SILC_PACKET_NOTIFY and send the packet to the server. This way the
+ server (or router) will receive the clients on the channel that
+ the router has.
+
+
+4.3 Joining to a Channel
+
+ This section describes the procedure when client joins to a channel.
+ Client joins to channel by sending command SILC_COMMAND_JOIN to the
+ server. If the receiver receiving join command is normal server the
+ server MUST check its local list whether this channel already exists
+ locally. This would indicate that some client connected to the server
+ has already joined to the channel. If this is the case, the client is
+ joined to the channel, new channel key is created and information about
+ newly joined channel is sent to the router. The router is informed
+ by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
+ also be sent to the local clients on the channel. The new channel key
+ is also sent to the router and to local clients on the channel.
+
+ If the channel does not exist in the local list the client's command
+ MUST be sent to the router which will then perform the actual joining
+ procedure. When server receives the reply to the command from the
+ router it MUST be sent to the client which sent the command originally.
+ Server will also receive the channel key from the server that it MUST
+ send to the client which originally requested the join command. The
+ server MUST also save the channel key.
+
+ If the receiver of the join command is router it MUST first check its
+ local list whether anyone in the cell has already joined to the channel.
+ If this is the case, the client is joined to the channel and reply is
+ sent to the client. If the command was sent by server the command reply
+
+
+
+Riikonen [Page 43]
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+Internet Draft 15 January 2007
+
+
+ is sent to the server which sent it. Then the router MUST also create
+ new channel key and distribute it to all clients on the channel and
+ all servers that have clients on the channel. Router MUST also send
+ the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
+ and to local servers that have clients on the channel.
+
+ If the channel does not exist on the router's local list it MUST
+ check the global list whether the channel exists at all. If it does
+ the client is joined to the channel as described previously. If
+ the channel does not exist the channel is created and the client
+ is joined to the channel. The channel key is also created and
+ distributed as previously described. The client joining to the created
+ channel is made automatically channel founder and both channel founder
+ and channel operator privileges are set for the client.
+
+ If the router created the channel in the process, information about the
+ new channel MUST be broadcast to all routers. This is done by
+ broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
+ route. When the router joins the client to the channel it MUST also
+ send information about newly joined client to all routers in the SILC
+ network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
+ type to the router's primary route.
+
+ It is important to note that new channel key is created always when
+ new client joins to channel, whether the channel has existed previously
+ or not. This way the new client on the channel is not able to decrypt
+ any of the old traffic on the channel. Client which receives the reply to
+ the join command MUST start using the received Channel ID in the channel
+ message communication thereafter. Client also receives the key for the
+ channel in the command reply. Note that the channel key is never
+ generated or distributed if the SILC_CMODE_PRIVKEY mode is set.
+
+
+4.4 Channel Key Generation
+
+ Channel keys are created by router which creates the channel by taking
+ enough randomness from cryptographically strong random number generator.
+ The key is generated always when channel is created, when new client
+ joins a channel and after the key has expired. Key could expire for
+ example in an hour.
+
+ The key MUST also be re-generated whenever some client leaves a channel.
+ In this case the key is created from scratch by taking enough randomness
+ from the random number generator. After that the key is distributed to
+ all clients on the channel. However, channel keys are cell specific thus
+ the key is created only on the cell where the client, which left the
+ channel, exists. While the server or router is creating the new channel
+ key, no other client may join to the channel. Messages that are sent
+
+
+
+Riikonen [Page 44]
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+Internet Draft 15 January 2007
+
+
+ while creating the new key are still processed with the old key. After
+ server has sent the SILC_PACKET_CHANNEL_KEY packet client MUST start
+ using the new key. If server creates the new key the server MUST also
+ send the new key to its router. See [SILC2] for more information about
+ how channel messages must be encrypted and decrypted when router is
+ processing them.
+
+ If the key changes very often due to joining traffic on the channel it
+ is RECOMMENDED that client implementation would cache some of the old
+ channel keys for short period of time so that it is able to decrypt all
+ channel messages it receives. It is possible that on a heavy traffic
+ channel a message encrypted with channel key that was just changed
+ is received by client after the new key was set into use. This is
+ possible because not all clients may receive the new key at the same
+ time, and may still be sending messages encrypted with the old key.
+
+ When client receives the SILC_PACKET_CHANNEL_KEY packet with the
+ Channel Key Payload it MUST process the key data to create encryption
+ and decryption key, and to create the MAC key that is used to compute
+ the MACs of the channel messages. The processing is as follows:
+
+ channel_key = raw key data
+ MAC key = hash(raw key data)
+
+ The raw key data is the key data received in the Channel Key Payload.
+ It is used for both encryption and decryption. The hash() is the hash
+ function used with the HMAC of the channel. Note that the server also
+ MUST save the channel key.
+
+
+4.5 Private Message Sending and Reception
+
+ Private messages are sent point to point. Client explicitly destine
+ a private message to specific client that is delivered to only to that
+ client. No other client may receive the private message. The receiver
+ of the private message is destined in the SILC Packet Header as in any
+ other packet as well. The Source ID in the SILC Packet Header MUST be
+ the ID of the sender of the message.
+
+ If the sender of a private message does not know the receiver's Client
+ ID, it MUST resolve it from server. There are two ways to resolve the
+ client ID from server; it is RECOMMENDED that client implementations
+ send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
+ MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
+ If the sender has received earlier a private message from the receiver
+ it should have cached the Client ID from the SILC Packet Header.
+
+ If server receives a private message packet which includes invalid
+
+
+
+Riikonen [Page 45]
+\f
+Internet Draft 15 January 2007
+
+
+ destination Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
+ notify to the client with error status indicating that such Client ID
+ does not exist.
+
+ See [SILC2] for description of private message encryption and decryption
+ process.
+
+
+4.6 Private Message Key Generation
+
+ Private message MAY be protected with a key generated by the client.
+ One way to generate private message key is to use static or pre-shared
+ keys in the client implementation. Client that wants to indicate other
+ client on the network that a private message key should be set, the
+ client MAY send SILC_PACKET_PRIVATE_MESSAGE_KEY packet to indicate this.
+ The actual key material has to be transferred outside the SILC network,
+ or it has to be pre-shared key. The client receiving this packet knows
+ that the sender wishes to use private message key in private message
+ communication. In case of static or pre-shared keys the IV used in
+ the encryption SHOULD be chosen randomly. Sending the
+ SILC_PACKET_PRIVATE_MESSAGE_KEY is not mandatory, and clients may
+ naturally agree to use a key without sending the packet.
+
+ Another choice to use private message keys is to negotiate fresh key
+ material by performing the Key Agreement. The SILC_PACKET_KEY_AGREEMENT
+ packet MAY be used to negotiate the fresh key material. In this case
+ the resulting key material is used to secure the private messages.
+ Also, the IV used in encryption is used as defined in [SILC3], unless
+ otherwise stated by the encryption mode used. By performing Key
+ Agreement the clients can also negotiate the cipher and HMAC to be used
+ in the private message encryption and to negotiate additional security
+ parameters. The actual Key Agreement [SILC2] is performed by executing
+ the SILC Key Exchange protocol [SILC3], peer to peer. Because of NAT
+ devices in the network, it might be impossible to perform the Key
+ Agreement. In this case using static or pre-shared key and sending the
+ SILC_PACKET_PRIVATE_MESSAGE_KEY to indicate the use of a private message
+ key is a working alternative.
+
+ If the key is pre-shared key or other key material not generated by
+ Key Agreement, then the key material SHOULD be processed as defined
+ in [SILC3]. In the processing, however, the HASH, as defined in [SILC3]
+ MUST be ignored. After processing the key material it is employed as
+ defined in [SILC3]. If the SILC_PACKET_PRIVATE_MESSAGE_KEY was sent,
+ then it defines the cipher and HMAC to be used. The hash algorithm to be
+ used in the key material processing is the one that HMAC algorithm is
+ defined to use. If the SILC_PACKET_PRIVATE_MESSAGE_KEY was not sent at
+ all, then the hash algorithm to be used SHOULD be SHA1. In this case
+ also, implementations SHOULD use the SILC protocol's mandatory cipher
+
+
+
+Riikonen [Page 46]
+\f
+Internet Draft 15 January 2007
+
+
+ and HMAC in private message encryption.
+
+
+4.7 Channel Message Sending and Reception
+
+ Channel messages are delivered to a group of users. The group forms a
+ channel and all clients on the channel receives messages sent to the
+ channel. The Source ID in the SILC Packet Header MUST be the ID
+ of the sender of the message.
+
+ Channel messages are destined to a channel by specifying the Channel ID
+ as Destination ID in the SILC Packet Header. The server MUST then
+ distribute the message to all clients, except to the original sender,
+ on the channel by sending the channel message destined explicitly to a
+ client on the channel. However, the Destination ID MUST still remain
+ as the Channel ID.
+
+ If server receives a channel message packet which includes invalid
+ destination Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
+ notify to the sender with error status indicating that such Channel ID
+ does not exist.
+
+ See the [SILC2] for description of channel message routing for router
+ servers, and channel message encryption and decryption process.
+
+
+4.8 Session Key Regeneration
+
+ Session keys MUST be regenerated periodically, say, once in an hour.
+ The re-key process is started by sending SILC_PACKET_REKEY packet to
+ other end, to indicate that re-key must be performed. The initiator
+ of the connection SHOULD initiate the re-key.
+
+ If perfect forward secrecy (PFS) flag was selected in the SILC Key
+ Exchange protocol [SILC3] the re-key MUST cause new key exchange with
+ SKE protocol. In this case the protocol is secured with the old key
+ and the protocol results to new key material. See [SILC3] for more
+ information. After the SILC_PACKET_REKEY packet is sent the sender
+ will perform the SKE protocol.
+
+ If PFS flag was set the resulted key material is processed as described
+ in the section Processing the Key Material in [SILC3]. The difference
+ with re-key in the processing is that the initial data for the hash
+ function is just the resulted key material and not the HASH as it
+ is not computed at all with re-key. Other than that, the key processing
+ it equivalent to normal SKE negotiation.
+
+ If PFS flag was not set, which is the default case, then re-key is done
+
+
+
+Riikonen [Page 47]
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+Internet Draft 15 January 2007
+
+
+ without executing SKE protocol. In this case, the new key is created by
+ providing the current sending encryption key to the SKE protocol's key
+ processing function. The process is described in the section Processing
+ the Key Material in [SILC3]. The difference in the processing is that
+ the initial data for the hash function is the current sending encryption
+ key and not the SKE's KEY and HASH values. Other than that, the key
+ processing is equivalent to normal SKE negotiation.
+
+ After both parties have regenerated the session key, both MUST send
+ SILC_PACKET_REKEY_DONE packet to each other. These packets are still
+ secured with the old key. After these packets, the subsequent packets
+ MUST be protected with the new key. Note that, in case SKE was performed
+ again the SILC_PACKET_SUCCESS is not sent. The SILC_PACKET_REKEY_DONE
+ is sent in its stead.
+
+
+4.9 Command Sending and Reception
+
+ Client usually sends the commands in the SILC network. In this case
+ the client simply sends the command packet to server and the server
+ processes it and replies with command reply packet. See the [SILC4]
+ for detailed description of all commands.
+
+ However, if the server is not able to process the command, it is sent to
+ the server's router. This is case for example with commands such as
+ SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there are
+ other commands as well [SILC4]. For example, if client sends the WHOIS
+ command requesting specific information about some client the server must
+ send the WHOIS command to router so that all clients in SILC network are
+ searched. The router, on the other hand, sends the WHOIS command further
+ to receive the exact information about the requested client. The WHOIS
+ command travels all the way to the server which owns the client and it
+ replies with command reply packet. Finally, the server which sent the
+ command receives the command reply and it must be able to determine which
+ client sent the original command. The server then sends command reply to
+ the client. Implementations should have some kind of cache to handle, for
+ example, WHOIS information. Servers and routers along the route could all
+ cache the information for faster referencing in the future.
+
+ The commands sent by server may be sent hop by hop until someone is able
+ to process the command. However, it is preferred to destine the command
+ as precisely as it is possible. In this case, other routers en route
+ MUST route the command packet by checking the true sender and true
+ destination of the packet. However, servers and routers MUST NOT route
+ command reply packets to clients coming from other servers. Client
+ MUST NOT accept command reply packet originated from anyone else but
+ from its own server.
+
+
+
+
+Riikonen [Page 48]
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+Internet Draft 15 January 2007
+
+
+4.10 Closing Connection
+
+ When remote client connection is closed the server MUST send the notify
+ type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
+ the client was joined. The server MUST also save the client's information
+ for a period of time for history purposes.
+
+ When remote server or router connection is closed the server or router
+ MUST also remove all the clients that was behind the server or router
+ from the SILC Network. The server or router MUST also send the notify
+ type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
+ local clients that are joined on the same channels with the remote
+ server's or router's clients.
+
+ Router server MUST also check whether some client in the local cell
+ is watching for the nickname this client has, and send the
+ SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
+ the network has the SILC_UMODE_REJECT_WATCHING user mode set.
+
+
+4.11 Detaching and Resuming a Session
+
+ SILC protocol provides a possibility for a client to detach itself from
+ the network without actually signing off from the network. The client
+ connection to the server is closed but the client remains as valid client
+ in the network. The client may then later resume its session back from
+ any server in the network.
+
+ When client wishes to detach from the network it MUST send the
+ SILC_COMMAND_DETACH command to its server. The server then MUST set
+ SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
+ notify to its primary router, which then MUST broadcast it further
+ to other routers in the network. This user mode indicates that the
+ client is detached from the network. Implementations MUST NOT use
+ the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
+ to the client. All packets MUST still be sent to the client even if
+ client is detached from the network. Only the server that originally
+ had the active client connection is able to make the decision after it
+ notices that the network connection is not active. In this case the
+ default case is to discard the packet.
+
+ The SILC_UMODE_DETACHED flag cannot be set by client itself directly
+ with SILC_COMMAND_UMODE command, but only implicitly by sending the
+ SILC_COMMAND_DETACH command. The flag also cannot be unset by the
+ client, server or router with SILC_COMMAND_UMODE command, but only
+ implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
+ packet.
+
+
+
+
+Riikonen [Page 49]
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+
+
+ When the client wishes to resume its session in the SILC Network it
+ connects to a server in the network, which MAY also be a different
+ from the original server, and performs normal procedures regarding
+ creating a connection as described in section 4.1. After the SKE
+ and the Connection Authentication protocols has been successfully
+ completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
+ MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
+ perform the resuming procedure. The packet MUST include the detached
+ client's Client ID, which the client must know. It also includes
+ Authentication Payload which includes signature computed with the
+ client's private key. The signature is computed as defined in the
+ section 3.9.1. Thus, the authentication method MUST be based in
+ public key authentication.
+
+ When server receive the SILC_PACKET_RESUME_CLIENT packet it MUST
+ do the following: Server checks that the Client ID is valid client
+ and that it has the SILC_UMODE_DETACHED mode set. Then it verifies
+ the Authentication Payload with the detached client's public key.
+ If it does not have the public key it retrieves it by sending
+ SILC_COMMAND_GETKEY command to the server that has the public key from
+ the original client connection. The server MUST NOT use the public
+ key received in the SKE protocol for this connection. If the
+ signature is valid the server unsets the SILC_UMODE_DETACHED flag,
+ and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
+ The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
+ flag when the packet is received. If the server is router server it
+ also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
+ server whom owned the detached client.
+
+ The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
+ packet MUST know whether the packet already has been received for
+ the client. It is a protocol error to attempt to resume the client
+ session from more than one server. The implementations could set
+ internal flag that indicates that the client is resumed. If router
+ receive SILC_PACKET_RESUME_CLIENT packet for client that is already
+ resumed the client MUST be killed from the network. This would
+ indicate that the client is attempting to resume the session more
+ than once which is a protocol error. In this case the router sends
+ SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
+ the same situation MUST also send the notify for the client.
+
+ The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
+ must also understand that the client may not be found behind the
+ same server that it originally came from. They must update their
+ caches according to this. The server that now owns the client session
+ MUST check whether the Client ID of the resumed client is based
+ on the server's Server ID. If it is not it creates a new Client
+ ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
+
+
+
+Riikonen [Page 50]
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+Internet Draft 15 January 2007
+
+
+ also send the channel keys of all channels that the client has
+ joined to the client since it does not have them. Whether the
+ Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
+ packet to the client. Only after this is the client resumed back
+ to the network and may start sending packets and messages.
+
+ It is also possible that the server did not know about the global
+ channels before the client resumed. In this case it joins the client
+ to the channels, generates new channel keys and distributes the keys
+ to the channels as described in section 4.4.
+
+ It is an implementation issue for how long servers keep detached client
+ sessions. It is RECOMMENDED that the detached sessions would be
+ persistent as long as the server is running.
+
+
+
+4.12 UDP/IP Connections
+
+ SILC protocol allows the use of UDP/IP instead of TCP/IP. There may be
+ many reasons to use UDP, such as video and audio conferencing might
+ be more efficient with UDP.
+
+ When UDP/IP is used, in the SILC Key Exchange protocol the IV Included
+ flag MUST be set and the first 16-bits of the Cookie field in the Key
+ Exchange Start Payload MUST include the port that the other end will use
+ as the SILC session port. The port is in MSB first order. Both initiator
+ and responder will set the port they are going to use and all packets
+ after the SKE has been completed with the SILC_PACKET_SUCCESS packet MUST
+ be sent to the specified port. Initiator will send them to the port
+ responder specified and vice versa. When verifying the cookie for
+ modifications the first two bytes are to be ignored in case IV Included
+ flag has been set.
+
+ The default SILC port or port where the SILC server is listenning for
+ incoming packets is used only during initial key exchange protocol. After
+ SKE has been completed all packets are sent to the specified ports,
+ including connection authentication packets and rekey packets even when
+ PFS is used in rekey.
+
+ Changing the ports during SILC session is possible only by first detaching
+ from the server (with client-server connections) and then performing the
+ SILC Key Exchange protocol from the beginning and resuming the detached
+ session.
+
+ Since the UDP is unreliable transport the SKE packets may not arrive to
+ the recipient. Implementation should support retransmission of SKE
+ packets by using exponential backoff algorithm. Also other SILC packets
+
+
+
+Riikonen [Page 51]
+\f
+Internet Draft 15 January 2007
+
+
+ such as messages may drop en route. With message packets only way to
+ assure reliable delivery is to use message acking and retransmit the
+ message by using for example exponential backoff algorithm. With SKE
+ packets the initial timeout value should be no more than 1000
+ milliseconds. With message packets the initial timeout value should be
+ around 5000 milliseconds.
+
+
+5 Security Considerations
+
+ Security is central to the design of this protocol, and these security
+ considerations permeate the specification. Common security considerations
+ such as keeping private keys truly private and using adequate lengths for
+ symmetric and asymmetric keys must be followed in order to maintain the
+ security of this protocol.
+
+ Special attention must also be paid to the servers and routers that are
+ running the SILC service. The SILC protocol's security depends greatly
+ on the security and the integrity of the servers and administrators that
+ are running the service. It is recommended that some form of registration
+ is required by the server and router administrator prior to acceptance to
+ the SILC Network. Even though the SILC protocol is secure in a network
+ of mutual distrust between clients, servers, routers and administrators
+ of the servers, the client should be able to trust the servers they are
+ using if they wish to do so.
+
+ It however must be noted that if the client requires absolute security
+ by not trusting any of the servers or routers in the SILC Network, it can
+ be accomplished by negotiating private secret keys outside the SILC
+ Network, either using SKE or some other key exchange protocol, or to use
+ some other external means for distributing the keys. This applies for
+ all messages, private messages and channel messages.
+
+ It is important to note that SILC, like any other security protocol, is
+ not a foolproof system; the SILC servers and routers could very well be
+ compromised. However, to provide an acceptable level of security and
+ usability for end users, the protocol uses many times session keys or
+ other keys generated by the servers to secure the messages. This is an
+ intentional design feature to allow ease of use for end users. This way
+ the network is still usable, and remains encrypted even if the external
+ means of distributing the keys is not working. The implementation,
+ however, may like to not follow this design feature, and may always
+ negotiate the keys outside SILC network. This is an acceptable solution
+ and many times recommended. The implementation still must be able to
+ work with the server generated keys.
+
+ If this is unacceptable for the client or end user, the private keys
+ negotiated outside the SILC Network should always be used. In the end
+
+
+
+Riikonen [Page 52]
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+Internet Draft 15 January 2007
+
+
+ it is the implementor's choice whether to negotiate private keys by
+ default or whether to use the keys generated by the servers.
+
+ It is also recommended that router operators in the SILC Network would
+ form a joint forum to discuss the router and SILC Network management
+ issues. Also, router operators along with the cell's server operators
+ should have a forum to discuss the cell management issues.
+
+
+6 References
+
+ [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
+ January 2007.
+
+ [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
+ Protocols", Internet Draft, January 2007.
+
+ [SILC4] Riikonen, P., "SILC Commands", Internet Draft, January 2007.
+
+ [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
+ RFC 1459, May 1993.
+
+ [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
+ April 2000.
+
+ [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
+ 2811, April 2000.
+
+ [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
+ 2812, April 2000.
+
+ [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
+ 2813, April 2000.
+
+ [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
+ Internet Draft.
+
+ [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
+ November 1998.
+
+ [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
+ September 1999.
+
+ [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
+ Infrastructure, Certificate and CRL Profile", RFC 2459,
+ January 1999.
+
+ [Schneier] Schneier, B., "Applied Cryptography Second Edition",
+
+
+
+Riikonen [Page 53]
+\f
+Internet Draft 15 January 2007
+
+
+ John Wiley & Sons, New York, NY, 1996.
+
+ [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
+ CRC Press 1997.
+
+ [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
+ RFC 2412, November 1998.
+
+ [ISAKMP] Maughan D., et al, "Internet Security Association and
+ Key Management Protocol (ISAKMP)", RFC 2408, November
+ 1998.
+
+ [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
+ (IKE)", RFC 2409, November 1998.
+
+ [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
+ Authentication", RFC 2104, February 1997.
+
+ [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
+ Specifications, Version 2.0", RFC 2437, October 1998.
+
+ [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
+ 10646", RFC 3629, November 2003.
+
+ [RFC1321] Rivest R., "The MD5 Message-Digest Algorithm", RFC 1321,
+ April 1992.
+
+ [RFC3174] Eastlake, F., et al., "US Secure Hash Algorithm 1 (SHA1)",
+ RFC 3174, September 2001.
+
+ [PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
+ Version 1.5", RFC 2315, March 1998.
+
+ [RFC2253] Wahl, M., et al., "Lightweight Directory Access Protocol
+ (v3): UTF-8 String Representation of Distinguished Names",
+ RFC 2253, December 1997.
+
+ [RFC3454] Hoffman, P., et al., "Preparation of Internationalized
+ Strings ("stringprep")", RFC 3454, December 2002.
+
+ [SHA256] Eastlake 3rd, D., et al., "US Secure Hash Algorithms (SHA
+ and HMAC-SHA)", RFC 4634, July 2006.
+
+
+
+
+
+
+Riikonen [Page 54]
+\f
+Internet Draft 15 January 2007
+
+
+7 Author's Address
+
+ Pekka Riikonen
+ Helsinki
+ Finland
+
+ EMail: priikone@iki.fi
+
+
+Appendix A
+
+ This appendix defines the stringprep [RFC3454] profile for string
+ identifiers in SILC protocol. Compliant implementation MUST use this
+ profile to prepare the identifier strings in the SILC protocol. The
+ profile defines the following as required by [RFC3454].
+
+ - Intended applicability of the profile: the following identifiers in
+ the SILC Protocol; nicknames, usernames, server names, hostnames,
+ service names, algorithm names and other security property names [SILC3],
+ and SILC Public Key name.
+
+ - The character repertoire that is the input and output to
+ stringprep: Unicode 3.2 with the list of unassigned code points
+ being the Table A.1, as defined in [RFC3454].
+
+ - The mapping tables used: the following tables are used, in order,
+ as defined in [RFC3454].
+
+ Table B.1
+ Table B.2
+
+ The mandatory case folding is done using the Table B.2 which includes
+ the characters for the normalization form KC.
+
+ - The Unicode normalization used: the Unicode normalization form
+ KC is used, as defined in [RFC3454].
+
+ - The prohibited characters as output: the following tables are used
+ to prohibit characters, as defined in [RFC3454];
+
+ Table C.1.1
+ Table C.1.2
+ Table C.2.1
+ Table C.2.2
+ Table C.3
+ Table C.4
+ Table C.5
+ Table C.6
+
+
+
+Riikonen [Page 55]
+\f
+Internet Draft 15 January 2007
+
+
+ Table C.7
+ Table C.8
+ Table C.9
+
+ - Additional prohibited characters as output: in addition, the following
+ tables are used to prohibit characters, as defined in this document;
+
+ Appendix C
+ Appendix D
+
+ - The bidirectional string testing used: bidirectional string testing
+ is ignored in this profile.
+
+ This profile is to be maintained in the IANA registry for stringprep
+ profiles. The name of this profile is "silc-identifier-prep" and this
+ document defines the profile. This document defines the first version of
+ this profile.
+
+
+Appendix B
+
+ This appendix defines the stringprep [RFC3454] profile for channel name
+ strings in SILC protocol. Compliant implementation MUST use this profile
+ to prepare the channel name strings in the SILC protocol. The profile
+ defines the following as required by [RFC3454].
+
+ - Intended applicability of the profile: channel names.
+
+ - The character repertoire that is the input and output to
+ stringprep: Unicode 3.2 with the list of unassigned code points
+ being the Table A.1, as defined in [RFC3454].
+
+ - The mapping tables used: the following tables are used, in order,
+ as defined in [RFC3454].
+
+ Table B.1
+ Table B.2
+
+ The mandatory case folding is done using the Table B.2 which includes
+ the characters for the normalization form KC.
+
+ - The Unicode normalization used: the Unicode normalization form
+ KC is used, as defined in [RFC3454].
+
+ - The prohibited characters as output: the following tables are used
+ to prohibit characters, as defined in [RFC3454];
+
+ Table C.1.1
+
+
+
+Riikonen [Page 56]
+\f
+Internet Draft 15 January 2007
+
+
+ Table C.1.2
+ Table C.2.1
+ Table C.2.2
+ Table C.3
+ Table C.4
+ Table C.5
+ Table C.6
+ Table C.7
+ Table C.8
+ Table C.9
+
+ - Additional prohibited characters as output: in addition, the following
+ tables are used to prohibit characters, as defined in this document;
+
+ Appendix D
+
+ - The bidirectional string testing used: bidirectional string testing
+ is ignored in this profile.
+
+ This profile is to be maintained in the IANA registry for stringprep
+ profiles. The name of this profile is "silc-identifier-ch-prep" and this
+ document defines the profile. This document defines the first version of
+ this profile.
+
+
+Appendix C
+
+ This appendix defines additional prohibited characters in the identifier
+ strings as defined in the stringprep profile in Appendix A.
+
+ Reserved US-ASCII characters
+ 0021 002A 002C 003F 0040
+
+
+Appendix D
+
+ This appendix defines additional prohibited characters in the identifier
+ strings as defined in the stringprep profile in Appendix A and Appendix B.
+ Note that the prohibited character tables listed in the Appendix A and
+ Appendix B may include some of the same characters listed in this
+ appendix as well.
+
+ Symbol characters and other symbol like characters
+ 00A2-00A9 00AC 00AE 00AF 00B0 00B1 00B4 00B6 00B8 00D7 00F7
+ 02C2-02C5 02D2-02FF 0374 0375 0384 0385 03F6 0482 060E 060F
+ 06E9 06FD 06FE 09F2 09F3 09FA 0AF1 0B70 0BF3-0BFA 0E3F
+ 0F01-0F03 0F13-0F17 0F1A-0F1F 0F34 0F36 0F38 0FBE 0FBF
+ 0FC0-0FC5 0FC7-0FCF 17DB 1940 19E0-19FF 1FBD 1FBF-1FC1
+
+
+
+Riikonen [Page 57]
+\f
+Internet Draft 15 January 2007
+
+
+ 1FCD-1FCF 1FDD-1FDF 1FED-1FEF 1FFD 1FFE 2044 2052 207A-207C
+ 208A-208C 20A0-20B1 2100-214F 2150-218F 2190-21FF 2200-22FF
+ 2300-23FF 2400-243F 2440-245F 2460-24FF 2500-257F 2580-259F
+ 25A0-25FF 2600-26FF 2700-27BF 27C0-27EF 27F0-27FF 2800-28FF
+ 2900-297F 2980-29FF 2A00-2AFF 2B00-2BFF 2E9A 2EF4-2EFF
+ 2FF0-2FFF 303B-303D 3040 3095-3098 309F-30A0 30FF-3104
+ 312D-3130 318F 31B8-31FF 321D-321F 3244-325F 327C-327E
+ 32B1-32BF 32CC-32CF 32FF 3377-337A 33DE-33DF 33FF 4DB6-4DFF
+ 9FA6-9FFF A48D-A48F A4A2-A4A3 A4B4 A4C1 A4C5 A4C7-ABFF
+ D7A4-D7FF FA2E-FAFF FFE0-FFEE FFFC 10000-1007F 10080-100FF
+ 10100-1013F 1D000-1D0FF 1D100-1D1FF 1D300-1D35F 1D400-1D7FF
+
+ Other characters
+ E0100-E01EF
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (2007).
+
+ This document is subject to the rights, licenses and restrictions
+ contained in BCP 78, and except as set forth therein, the authors
+ retain all their rights.
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Riikonen [Page 58]
+\f
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