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Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-spec-07.txt                             29 July 2003
Expires: 29 January 2004


                 Secure Internet Live Conferencing (SILC),
                          Protocol Specification
                     <draft-riikonen-silc-spec-07.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   The distribution of this memo is unlimited.


Abstract

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









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Table of Contents

   1 Introduction ..................................................  3
     1.1 Requirements Terminology ..................................  4
   2 SILC Concepts .................................................  4
     2.1 SILC Network Topology .....................................  4
     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 ...........................................  9
     3.2 Server .................................................... 10
         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 .................................. 14
     3.4 Channels .................................................. 14
         3.4.1 Channel ID .......................................... 16
     3.5 Operators ................................................. 16
     3.6 SILC Commands ............................................. 17
     3.7 SILC Packets .............................................. 17
     3.8 Packet Encryption ......................................... 17
         3.8.1 Determination of the Source and the Destination ..... 18
         3.8.2 Client To Client .................................... 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 .............................. 21
     3.10 Algorithms ............................................... 23
         3.10.1 Ciphers ............................................ 23
                3.10.1.1 CBC Mode .................................. 24
                3.10.1.2 CTR Mode .................................. 24
                3.10.1.3 Randomized CBC Mode ....................... 26
         3.10.2 Public Key Algorithms .............................. 26
                3.10.2.1 Multi-Precision Integers .................. 27
         3.10.3 Hash Functions ..................................... 27
         3.10.4 MAC Algorithms ..................................... 27
         3.10.5 Compression Algorithms ............................. 28
     3.11 SILC Public Key .......................................... 29
     3.12 SILC Version Detection ................................... 31
     3.13 Backup Routers ........................................... 31
         3.13.1 Switching to Backup Router ......................... 33
         3.13.2 Resuming Primary Router ............................ 34



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   4 SILC Procedures ............................................... 36
     4.1 Creating Client Connection ................................ 37
     4.2 Creating Server Connection ................................ 38
         4.2.1 Announcing Clients, Channels and Servers ............ 39
     4.3 Joining to a Channel ...................................... 40
     4.4 Channel Key Generation .................................... 41
     4.5 Private Message Sending and Reception ..................... 42
     4.6 Private Message Key Generation ............................ 42
     4.7 Channel Message Sending and Reception ..................... 43
     4.8 Session Key Regeneration .................................. 44
     4.9 Command Sending and Reception ............................. 44
     4.10 Closing Connection ....................................... 45
     4.11 Detaching and Resuming a Session ......................... 46
   5 Security Considerations ....................................... 47
   6 References .................................................... 48
   7 Author's Address .............................................. 50
   8 Full Copyright Statement ...................................... 50

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


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
   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].




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   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 TCP/IP network
   protocol, although it could be made to work on other network protocols
   with only minor changes.  However, it is recommended that TCP/IP
   protocol is used under SILC protocol.  Typical implementation would
   be made in client-server model.


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 routers and servers in secure manner.  The messages may also
   be delivered from one client to many clients forming a group, also
   known as a channel.

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


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



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   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
   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
   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



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   as normal SILC server; clients may connect to it the same way as to
   normal SILC server.  Normal server also cannot have active connections
   to more than one router.  Normal server cannot be connected to two
   different cells.  Router servers, on the other hand, may have as many
   router to router connections as needed.  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
             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



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   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.


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



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   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.  There may
             be other direct connections between the routers but they must
             not be used as primary routes.

   The above example is applicable to any amount of routers in the network
   except for two routers.  If there are only two routers in the network both
   routers must be able to handle situation where they use each other as their
   primary routes.

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



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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.  The maximum length of
   nickname is 128 bytes.


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



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      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 lowercase nickname is
        truncated taking 88 bits from the start of the hash value.
        This hash value is used to search the user's Client ID
        from the ID lists.  Note that the nickname MUST be in
        lowercase format.

   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
   produce same truncated hash value.


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



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   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
         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






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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:

          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.






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3.3 Router

   Router server in SILC network is responsible for keeping the cell together
   and routing messages to other servers and to other routers.  Router server
   is also a normal server thus clients may connect to it as it would be
   just normal SILC server.

   However, router servers has a lot of important tasks that normal servers
   do not have.  Router server knows everything 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

      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



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   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



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



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   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.  The channel name is a string of
   maximum length of 256 bytes.  Channel names MUST NOT contain any
   whitespaces (`  '), any non-printable ASCII characters, commas (`,')
   and wildcard characters.

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

      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
        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.







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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 router knows
        where this channel resides in the SILC network.

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

      o Random number 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.


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.




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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
   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



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   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 whom is the next
   receiver of the packet along the route.  The receiver of the packet, for
   example a router along the route, is able to determine the sender and the
   destination of the packet by decrypting the SILC Packet Header and
   checking the 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



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   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.  However, the header and the packet may be encrypted
   with same key.  This is the case, for example, with 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
     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 is encrypted with the private



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     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.
             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.



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   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
   key based on digital signatures.  All passphrases sent in SILC protocol
   MUST be UTF-8 [RFC2279] encoded. The connection authentication protocol
   is described in detail in [SILC3].


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:




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                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        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
        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.



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   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 encrypted.


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)



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   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 CBC mode with inter-packet chaining.  This
   means that the Initialization Vector (IV) for the next encryption block
   is the previous ciphertext block.  The very first IV MUST be random and
   is generated as described in [SILC3].


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



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   decryption operation is not needed since both encryption and decryption
   process is simple XOR with the plaintext block and the key stream block.

   The counter block is used to create the key for the CTR mode.  When
   SILC specifications refer to Initialization Vector (IV) in general cases,
   in case of CTR mode it refers to the counter block.  The format of the
   128 bit counter block is as follows:

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

                         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 (8 bytes) - This value is the
        first 8 bytes from the Sending IV or Receiving IV generated in
        the SKE protocol.  When this mode is used to encrypt sending
        traffic the Sending IV is used, when used to decrypt receiving
        traffic the Receiving IV is used.  This assures that two parties
        of the protocol use different IV for sending traffic.  Each rekey
        MUST produce a new value.

      o Block Counter (4 bytes) - This is the counter value for the
        counter block and is MSB ordered number starting from one (1)
        value for first block and incrementing for subsequent blocks.
        The same value MUST NOT be used twice.  The rekey MUST be
        performed before this counter value wraps.

   CTR mode MUST NOT be used with "none" MAC.  Implementations also MUST
   assure that the same counter block is not used to encrypt more than
   one block.  Also, the key material used with CTR mode MUST be fresh
   key material.  Static keys (pre-shared keys) MUST NOT be used with
   CTR mode.  For this reason using CTR mode to encrypt for example
   channel messages or private messages with a pre-shared key is
   inappropriate.  For private messages, the Key Agreement could be
   performed to produce fresh key material.



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   If the IV Included flag was negotiated in SKE, implementations SHOULD
   still use the same counter block format as defined above.  However,
   implementations are RECOMMENDED to replace the Truncated HASH field
   with a 32 bit random value for each IV (counter block) per encrypted
   SILC packet.  Also note, that in this case the decryption process is
   not stateful and receiver cannot precompute the key stream.


3.10.1.3 Randomized CBC Mode

   The "rcbc" encryption mode is CBC mode with randomized IV.  This means
   that each IV for each packet MUST be chosen randomly.  When encrypting
   more than one block the normal inter-packet chaining is used, but for
   the first block new random IV is selected in each packet.  In this mode
   the IV is appended at the end of the last ciphertext block and thus
   delivered to the recipient.  This mode increases the ciphertext size by
   one ciphertext block.  Note also that some data payloads in SILC are
   capable of delivering the IV to the recipient.  When explicitly
   encrypting these payloads with randomized CBC the IV MUST NOT be appended
   at the end of the ciphertext, but is placed at the specified location
   in the payload.  However, Message Payload for example has the IV at
   the location which is equivalent to placing it after the last ciphertext
   block.  When using CBC mode with such payloads it is actually equivalent
   to using randomized CBC since the IV is selected in random and included
   in the ciphertext.


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].  The mandatory PKCS #1 implementation in SILC MUST be
   compliant to either PKCS #1 version 1.5 or newer with the following
   notes: The signature encoding is always in same format as the encryption
   encoding regardless of the PKCS #1 version.  The signature with appendix
   (with hash algorithm OID in the data) MUST NOT be used in the SILC.  The
   rationale for this is that there is no binding between the PKCS #1 OIDs
   and the hash algorithms used in the SILC protocol.  Hence, the encoding
   is always in PKCS #1 version 1.5 format.




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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
   represented as sign().  The signature computing procedure is dependent
   of the public key algorithm, and the public key or certificate encoding.
   When using SILC public key the signature is computed as described in
   previous paragraph for RSA and DSS keys.  If the hash function is not
   specified separately for signing process SHA-1 MUST be used.  When using
   SSH2 public keys the signature is computed as described in [SSH-TRANS].
   When using X.509 version 3 certificates the signature is computed as
   described in [PKCS7].  When using OpenPGP certificates the signature is
   computed as described in [PGP].


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 desired octet
   length.  This means that if the octet length is more than the actual
   length of the integer one or more leading zero octets will appear at the
   start of the encoding.  The actual length of the integer is the bit size
   of the integer not counting any leading zero bits.


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      (REQUIRED)
      md5              MD5, length = 16        (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:

      hmac-sha1-96     HMAC-SHA1, length = 12 bytes  (REQUIRED)
      hmac-md5-96      HMAC-MD5, length = 12 bytes   (OPTIONAL)
      hmac-sha1        HMAC-SHA1, length = 20 bytes  (OPTIONAL)



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      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].

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


3.10.5 Compression Algorithms

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

   The following compression algorithms are defined:

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

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


3.11 SILC Public Key

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

   The format of the SILC Public Key is as follows:











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

                        Figure 5:  SILC Public Key


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

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

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

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

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


        Examples of an identifier:

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

          `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'

        At least user name (UN) and host name (HN) MUST be provided as
        identifier.  The fields are separated by commas (`,').  If
        comma is in the identifier string it must be 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.

      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.

   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.



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   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).


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
      SILC-1.2-2.4.5 Vendor Limited


3.13 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



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   to maintain working connections inside the cell and outside the cell and
   to avoid netsplits.

   Backup routers are normal servers in the cell that are prepared to take
   over the tasks of the primary router if needed.  They need to have at
   least one direct and active connection to the primary router of the cell.
   This communication channel is used to send the router information to
   the backup router.  When the backup router connects to the primary router
   of the cell it MUST present itself as router server in the Connection
   Authentication protocol, even though it is normal server as long as the
   primary router is available.  Reason for this is that the configuration
   needed in the responder end requires usually router connection level
   configuration.  The responder, however must understand and treat the
   connection as normal server (except when feeding router level data to
   the backup router).

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



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   be configured accordingly and care must be taken when configuring the
   backup routers, servers and other routers in the network.

   It must be noted that some of the channel messages and private messages
   may be lost during the switch to the backup router.  The announcements
   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.


3.13.1 Switching to Backup Router

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

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

   Servers connected to the backup router MUST send SILC_PACKET_RESUME_ROUTER
   packet with type 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



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   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
   SILC_COMMAND_PING command to detect whether primary router is responsive.

   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.


3.13.2 Resuming Primary Router

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

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

     1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
        value 1 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



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        order) back to the backup router.  If backup receives this it
        MUST NOT continue with the backup resuming protocol.

     2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
        value 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 this packet
        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



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        not originated from the backup router but from the locally connected
        servers.  After that they MUST announce their channels, channel
        users, modes etc. to the primary router.  They MUST NOT use the
        backup router connection after this and the connection is considered
        to be 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.







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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
   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.




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   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.

   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



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   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.


4.2.1 Announcing Clients, Channels and Servers

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

   All clients are announced by compiling a list of ID Payloads into the
   SILC_PACKET_NEW_ID packet.  All channels are announced by compiling a
   list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet.
   Channels' mode and founder public key and other channel mode specific
   data is announced by sending SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.
   Also, the channel users on the channels must be announced by compiling a
   list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
   the SILC_PACKET_NOTIFY packet.  The users' modes on the channel must
   also be announced by compiling list of Notify Payloads with the
   SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
   packet.

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

   Also, clients' modes (user modes in SILC) MUST be announced.  This is
   done by compiling a list of Notify Payloads with SILC_NOTIFY_UMODE_CHANGE
   notify type into the SILC_PACKET_NOTIFY packet.  Also, 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.

   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.

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

   If the channel has channel founder already on the router, the router



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   MUST send the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server
   to force the mode change for the channel founder on the server.  The
   channel founder privileges MUST be removed.

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


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
   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



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   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
   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



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   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 HMAC key that is used to compute
   the MACs of the channel messages.  The processing is as follows:

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

   The raw key data is the key data received in the Channel Key Payload.
   The hash() function is the hash function used in the HMAC of the channel.
   Note that the server also MUST save the channel key.


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
   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.
   The key may be generated and sent to the other client by sending packet



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   SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
   and is secured by session keys.  After that the private message key
   is used in the private message communication between those clients.
   The key sent inside the payload SHOULD be randomly generated.  This
   packet MUST NOT be used to send pre-shared keys.

   Another choice is to entirely use keys that are not sent through
   the SILC network at all.  This significantly adds security.  This key
   could be a pre-shared key that is known by both of the clients.  Both
   agree about using the key and start sending packets that indicate
   that the private message is secured using private message key.  In
   case of pre-shared keys (static keys) the IV used in encryption SHOULD
   be chosen randomly.

   It is also possible to negotiate fresh key material by performing
   Key Agreement.  The SILC_PACKET_KEY_AGREEMENT packet MAY be used to
   negotiate the fresh key material.  In this case the 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
   may negotiate the cipher and HMAC to be used in the private message
   encryption and to negotiate additional security parameters.

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


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 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



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   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
   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.


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.



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   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.


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.







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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.

   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



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   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
   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.


5 Security Considerations

   Security is central to the design of this protocol, and these security
   considerations permeate the specification.  Common security considerations



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   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 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
   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,



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                May 2002.

   [SILC3]      Riikonen, P., "SILC Key Exchange and Authentication
                Protocols", Internet Draft, May 2002.

   [SILC4]      Riikonen, P., "SILC Commands", Internet Draft, May 2002.

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

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

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

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

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

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

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

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

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

   [Schneier]   Schneier, B., "Applied Cryptography Second Edition",
                John Wiley & Sons, New York, NY, 1996.

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

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

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




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

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

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

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

   [RFC2279]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 2279, January 1998.

   [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.


7 Author's Address

   Pekka Riikonen
   Snellmaninkatu 34 A 15
   70100 Kuopio
   Finland

   EMail: priikone@iki.fi


8 Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are



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   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Riikonen                                                       [Page 51]


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