X-Git-Url: http://git.silcnet.org/gitweb/?a=blobdiff_plain;f=doc%2Fdraft-riikonen-silc-spec-09.nroff;fp=doc%2Fdraft-riikonen-silc-spec-09.nroff;h=d1c3dd5f7212c4bc0563a073fd7b50ed3e6ce733;hb=c257b555225193e54d85daf541d29578b3c93882;hp=0000000000000000000000000000000000000000;hpb=f658940d02cf2fd893296b6a7825b42502573668;p=crypto.git diff --git a/doc/draft-riikonen-silc-spec-09.nroff b/doc/draft-riikonen-silc-spec-09.nroff new file mode 100644 index 00000000..d1c3dd5f --- /dev/null +++ b/doc/draft-riikonen-silc-spec-09.nroff @@ -0,0 +1,2848 @@ +.pl 10.0i +.po 0 +.ll 7.2i +.lt 7.2i +.nr LL 7.2i +.nr LT 7.2i +.ds LF Riikonen +.ds RF FORMFEED[Page %] +.ds CF +.ds LH Internet Draft +.ds RH 11 February 2004 +.ds CH +.na +.hy 0 +.in 0 +.nf +Network Working Group P. Riikonen +Internet-Draft +draft-riikonen-silc-spec-09.txt XX +Expires: XXX + +.in 3 + +.ce 3 +Secure Internet Live Conferencing (SILC), +Protocol Specification + + +.ti 0 +Status of this Memo + +This document is an Internet-Draft and is in full conformance with +all provisions of Section 10 of RFC 2026. Internet-Drafts are +working documents of the Internet Engineering Task Force (IETF), its +areas, and its working groups. Note that other groups may also +distribute working documents as Internet-Drafts. + +Internet-Drafts are draft documents valid for a maximum of six months +and may be updated, replaced, or obsoleted by other documents at any +time. It is inappropriate to use Internet-Drafts as reference +material or to cite them other than as "work in progress." + +The list of current Internet-Drafts can be accessed at +http://www.ietf.org/ietf/1id-abstracts.txt + +The list of Internet-Draft Shadow Directories can be accessed at +http://www.ietf.org/shadow.html + +The distribution of this memo is unlimited. + + +.ti 0 +Abstract + +This memo describes a Secure Internet Live Conferencing (SILC) +protocol which provides secure conferencing services over insecure +network channel. SILC 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]. + + + + + + +.ti 0 +Table of Contents + +.nf +1 Introduction .................................................. 3 + 1.1 Requirements Terminology .................................. 4 +2 SILC Concepts ................................................. 4 + 2.1 SILC Network Topology ..................................... 5 + 2.2 Communication Inside a Cell ............................... 6 + 2.3 Communication in the Network .............................. 7 + 2.4 Channel Communication ..................................... 7 + 2.5 Router Connections ........................................ 8 +3 SILC Specification ............................................ 9 + 3.1 Client .................................................... 9 + 3.1.1 Client ID ........................................... 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 .................................................. 15 + 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 .......................................... 28 + 3.12 SILC Version Detection ................................... 31 + 3.13 UTF-8 Strings in SILC .................................... 31 + 3.13.1 UTF-8 Identifier Strings ........................... 32 + 3.14 Backup Routers ........................................... 33 + 3.14.1 Switching to Backup Router ......................... 35 + 3.14.2 Resuming Primary Router ............................ 36 +4 SILC Procedures ............................................... 38 + 4.1 Creating Client Connection ................................ 38 + 4.2 Creating Server Connection ................................ 40 + 4.2.1 Announcing Clients, Channels and Servers ............ 40 + 4.3 Joining to a Channel ...................................... 42 + 4.4 Channel Key Generation .................................... 43 + 4.5 Private Message Sending and Reception ..................... 44 + 4.6 Private Message Key Generation ............................ 44 + 4.7 Channel Message Sending and Reception ..................... 45 + 4.8 Session Key Regeneration .................................. 46 + 4.9 Command Sending and Reception ............................. 46 + 4.10 Closing Connection ....................................... 47 + 4.11 Detaching and Resuming a Session ......................... 48 +5 Security Considerations ....................................... 49 +6 References .................................................... 50 +7 Author's Address .............................................. 52 +Appendix A ...................................................... 52 +Appendix B ...................................................... 54 +Appendix C ...................................................... XXX +Appendix D ...................................................... XXX +Full Copyright Statement ........................................ XXX + +.ti 0 +List of Figures + +.nf +Figure 1: SILC Network Topology +Figure 2: Communication Inside cell +Figure 3: Communication Between Cells +Figure 4: Router Connections +Figure 5: SILC Public Key +Figure 6: Counter Block + + +.ti 0 +1. Introduction + +This document describes a Secure Internet Live Conferencing (SILC) +protocol which provides secure conferencing services over insecure +network channel. SILC 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]. + +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. + + +.ti 0 +1.1 Requirements Terminology + +The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, +MAY, and OPTIONAL, when they appear in this document, are to be +interpreted as described in [RFC2119]. + + +.ti 0 +2. SILC Concepts + +This section describes various SILC protocol concepts that forms the +actual protocol, and in the end, the actual SILC network. The mission +of the protocol is to deliver messages from clients to other clients +through routers and servers in secure manner. The messages may also +be delivered from one client to many clients forming a group, also +known as a channel. + +This section does not focus to security issues. Instead, basic network +concepts are introduced to make the topology of the SILC network +clear. + + + +.ti 0 +2.1 SILC Network Topology + +SILC network forms a ring as opposed to tree style network topology that +conferencing protocols usually have. The network has a cells which are +constructed from a router and zero or more servers. The servers are +connected to the router in a star like network topology. Routers in the +network are connected to each other forming a ring. The rationale for +this is to have servers that can perform specific kind of tasks what +other servers cannot perform. This leads to two kinds of servers; normal +SILC servers and SILC router servers. + +A difference between normal server and router server is that routers +knows all global information and keep the global network state up to date. +They also do the actual routing of the messages to the correct receiver +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. + +.in 8 +.nf + ---- ---- ---- ---- ---- ---- + | S8 | S5 | S4 | | S7 | S5 | S6 | + ----- ---- ----- ----- ---- ----- +| S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 | + ---- ------ ---- ---- ------ ---- + | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ---- + ---- ---- ---- ---- ---- ---- | S3 | S1 | + Cell 1. \\ Cell 2. | \\____ ----- ----- + | | | S4 | S/R4 | + ---- ---- ---- ---- ---- ---- ---- ------ + | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 | + ----- ---- ----- ----- ---- ----- ---- ---- + | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4. + ---- ------ ---- ---- ------ ---- + | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ... + ---- ---- ---- ---- ---- ---- + Cell 3. Cell 5. +.in 3 + +.ce +Figure 1: SILC Network Topology + + +A cell is formed when a server or servers connect to one router. In +SILC network normal server cannot directly connect to other normal +server. Normal server may only connect to SILC router which then +routes the messages to the other servers in the cell. Router servers +on the other hand may connect to other routers to form the actual SILC +network, as seen in above figure. However, router is also able to act +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. + + +.ti 0 +2.2 Communication Inside a Cell + +It is always guaranteed that inside a cell message is delivered to the +recipient with at most two server hops. A client which is connected to +server in the cell and is talking on channel to other client connected +to other server in the same cell, will have its messages delivered from +its local server first to the router of the cell, and from the router +to the other server in the cell. + +The following diagram represents this scenario: + + +.in 25 +.nf +1 --- S1 S4 --- 5 + S/R + 2 -- S2 S3 + / | + 4 3 +.in 3 + + +.ce +Figure 2: Communication Inside cell + + +Example: Client 1. connected to Server 1. send message to + Client 4. connected to Server 2. travels from Server 1. + first to Router which routes the message to Server 2. + which then sends it to the Client 4. All the other + servers in the cell will not see the routed message. + + +If the client is connected directly to the router, as router is also normal +SILC server, the messages inside the cell are always delivered only with +one server hop. If clients communicating with each other are connected +to the same server, no router interaction is needed. This is the optimal +situation of message delivery in the SILC network. + + +.ti 0 +2.3 Communication in the Network + +If the message is destined to 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. + + + +.in 16 +.nf +1 --- S1 S4 --- 5 S2 --- 1 + S/R - - - - - - - - S/R + 2 -- S2 S3 S1 + / | \\ + 4 3 2 + + Cell 1. Cell 2. +.in 3 + + +.ce +Figure 3: Communication Between Cells + + +Example: Client 5. connected to Server 4. in Cell 1. sends message + to Client 2. connected to Server 1. in Cell 2. travels + from Server 4. to Router which routes the message to + Router in Cell 2, which then routes the message to + Server 1. All the other servers and routers in the + network will not see the routed message. + + +The optimal case of message delivery from the client point of view is +when clients are connected directly to the routers and the messages +are delivered from one router to the other. + + +.ti 0 +2.4 Channel Communication + +Messages may be sent to group of clients as well. Sending messages to +many clients works the same way as sending messages point to point, from +message delivery point of view. Security issues are another matter +which are not discussed in this section. + +Router server handles the message routing to multiple recipients. If +any recipient is not in the same cell as the sender the messages are +routed further. + +Server distributes the channel message to its local clients which are +joined to the channel. Router also distributes the message to its +local clients on the channel. + + +.ti 0 +2.5 Router Connections + +Router connections play very important role in making the SILC like +network topology to work. For example, sending broadcast packets in +SILC network require special connections between routers; routers must +be connected in a specific way. + +Every router has their primary route which is a connection to another +router in the network. Unless there is only two routers in the network +must not routers use each other as their primary routes. The router +connections in the network must form a ring. + +Example with three routers in the network: + + +.in 16 +.nf + S/R1 - < - < - < - < - < - < - S/R2 + \\ / + v ^ + \\ - > - > - S/R3 - > - > - / +.in 3 + + +.ce +Figure 4: Router Connections + + +Example: Network with three routers. Router 1. uses Router 2. as its + primary router. Router 2. uses Router 3. as its primary router, + and Router 3. uses Router 1. as its primary router. There may + be other direct connections between the routers but they must + not be used as primary routes. + +The above example is applicable to any amount of routers in the network +except for two routers. If there are only two routers in the network both +routers must be able to handle situation where they use each other as their +primary routes. + +The issue of router connections are very important especially with SILC +broadcast packets. Usually all router wide information in the network is +distributed by SILC broadcast packets. This sort of ring network, with +ability to have other direct routes in the network can cause interesting +routing problems. The [SILC2] discusses the routing of packets in this +sort of network in more detail. + + +.ti 0 +3. SILC Specification + +This section describes the SILC protocol. However, [SILC2] and +[SILC3] describes other important protocols that are part of this SILC +specification and must be read. + + +.ti 0 +3.1 Client + +A client is a piece of software connecting to SILC server. SILC client +cannot be SILC server. Purpose of clients is to provide the user +interface of the SILC services for end user. Clients are distinguished +from other clients by unique Client ID. Client ID is a 128 bit ID that +is used in the communication in the SILC network. The client ID is +based on the user's IP address and nickname. User use logical nicknames +in communication which are then mapped to the corresponding Client ID. +Client IDs are low level identifications and should not be seen by the +end user. + +Clients provide other information about the end user as well. Information +such as the nickname of the user, username and the host name of the end +user and user's real name. See section 3.2 Server for information of +the requirements of keeping this information. + +The nickname selected by the user is not unique in the SILC network. +There can be 2^8 same nicknames for one IP address. As for comparison to +IRC [IRC] where nicknames are unique this is a fundamental difference +between SILC and IRC. This typically causes the server names or client's +host names to be used along with the nicknames on user interface to +identify specific users when sending messages. This feature of SILC +makes IRC style nickname-wars obsolete as no one owns their nickname; +there can always be someone else with the same nickname. Also, any kind +of nickname registering service becomes obsolete. See the section 3.13.1 +for more information about nicknames. + + +.ti 0 +3.1.1 Client ID + +Client ID is used to identify users in the SILC network. The Client ID +is unique to the extent that there can be 2^128 different Client 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. + +.in 6 +128 bit Client ID based on IPv4 addresses: + +32 bit Server ID IP address (bits 1-32) + 8 bit Random number or counter +88 bit Truncated MD5 hash value of the nickname + +224 bit Client ID based on IPv6 addresses: + +128 bit Server ID IP address (bits 1-128) + 8 bit Random number or counter + 88 bit Truncated MD5 hash value of the nickname + +o Server ID IP address - Indicates the server where this + client is coming from. The IP address hence equals the + server IP address where the client is connected. + +o Random number or counter - Random number to further + randomize the Client ID. Another choice is to use + a counter starting from the zero (0). This makes it + possible to have 2^8 same nicknames from the same + server IP address. + +o MD5 hash - MD5 hash value of the case folded nickname is + truncated taking 88 bits from the start of the hash value. + This hash value is used to search the user's Client ID + from the ID lists. Note that the nickname MUST be prepared + using the stringprep [RFC3454] profile described in the + Appendix A before computing the MD5 hash. See also the + section 3.13.1 for more information. + +.in 3 +Collisions could occur when more than 2^8 clients using same nickname +from the same server IP address is connected to the SILC network. +Server MUST be able to handle this situation by refusing to accept +anymore of that nickname. + +Another possible collision may happen with the truncated hash value of +the nickname. It could be possible to have same truncated hash value +for two different nicknames. However, this is not expected to happen +nor cause any 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. + + +.ti 0 +3.2 Server + +Servers are the most important parts of the SILC network. They form the +basis of the SILC, providing a point to which clients may connect to. +There are two kinds of servers in SILC; normal servers and router servers. +This section focus on the normal server and router server is described +in the section 3.3 Router. + +Normal servers MUST NOT directly connect to other normal server. Normal +servers may only directly connect to router server. If the message sent +by the client is destined outside the local server it is always sent to +the router server for further routing. Server may only have one active +connection to router on same port. Normal server MUST NOT connect to other +cell's router except in situations where its cell's router is unavailable. + + +.ti 0 +3.2.1 Server's Local ID List + +Normal server keeps various information about the clients and their end +users connected to it. Every normal server MUST keep list of all locally +connected clients, Client 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: + +.in 6 +server list - Router connection + o Server name + o Server IP address + o Server ID + o Sending key + o Receiving key + o Public key + +client list - All clients in server + o Nickname + o Username@host + o Real name + o Client ID + o Sending key + o Receiving key + o Public key + +channel list - All channels in server + o Channel name + o Channel ID + o Client IDs on channel + o Client ID modes on channel + o Channel key +.in 3 + + +.ti 0 +3.2.2 Server ID + +Servers are distinguished from other servers by unique 64 bit Server ID +(for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in +the SILC to route messages to correct servers. Server IDs also provide +information for Client IDs, see section 3.1.1 Client ID. Server ID is +defined as follows. + +.in 6 +64 bit Server ID based on IPv4 addresses: + +32 bit IP address of the server +16 bit Port +16 bit Random number + +160 bit Server ID based on IPv6 addresses: + +128 bit IP address of the server + 16 bit Port + 16 bit Random number + +o IP address of the server - This is the real IP address of + the server. + +o Port - This is the port the server is bound to. + +o Random number - This is used to further randomize the Server ID. + +.in 3 +Collisions are not expected to happen in any conditions. The Server ID +is always created by the server itself and server is responsible of +distributing it to the router. + + +.ti 0 +3.2.3 SILC Server Ports + +The following ports has been assigned by IANA for the SILC protocol: + +.in 10 +silc 706/tcp SILC +silc 706/udp SILC +.in 3 + + +If there are needs to create new SILC networks in the future the port +numbers must be officially assigned by the IANA. + +Server on network above privileged ports (>1023) SHOULD NOT be trusted +as they could have been set up by untrusted party. + + +.ti 0 +3.3 Router + +Router server in SILC network is responsible for keeping the cell together +and routing messages to other servers and to other routers. Router server +is also a normal server thus clients may connect to it as it would be +just normal SILC server. + +However, router servers has a lot of important tasks that normal servers +do not have. Router server knows everything 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. + + +.ti 0 +3.3.1 Router's Local ID List + +Router server as well MUST keep local list of connected clients and +locally created channels. However, this list is extended to include all +the informations of the entire cell, not just the server itself as for +normal servers. + +However, on router this list is a lot smaller since routers do not need +to keep information about user's nickname, username and host name and real +name since these are not needed by the router. The router keeps only +information that it needs. + +Hence, local list for router includes: + +.in 6 +server list - All servers in the cell + o Server name + o Server ID + o Router's Server ID + o Sending key + o Receiving key + +client list - All clients in the cell + o Client ID + +channel list - All channels in the cell + o Channel ID + o Client IDs on channel + o Client ID modes on channel + o Channel key +.in 3 + + +Note that locally connected clients and other information include all the +same information as defined in section section 3.2.1 Server's Local ID +List. Router MAY also cache same detailed information for other clients +if needed. + + +.ti 0 +3.3.2 Router's Global ID List + +Router server MUST also keep global list. Normal servers do not have +global list as they know only about local information. Global list +includes all the clients on SILC, their Client 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: + +.in 6 +server list - All servers in SILC + o Server name + o Server ID + o Router's Server ID + +client list - All clients in SILC + o Client ID + +channel list - All channels in SILC + o Channel ID + o Client IDs on channel + o Client ID modes on channel +.in 3 + + + +.ti 0 +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. + + + + +.ti 0 +3.4 Channels + +A channel is a named group of one or more clients which will all receive +messages addressed to that channel. The channel is created when first +client requests JOIN command to the channel, and the channel ceases to +exist when the last client has left it. When channel exists, any client +can reference it using the Channel ID of the channel. If the channel has +a founder mode set and last client leaves the channel the channel does +not cease to exist. The founder mode can be used to make permanent +channels in the network. The founder of the channel can regain the +channel founder privileges on the channel later when he joins the +channel. + +Channel names are unique although the real uniqueness comes from 64 bit +Channel ID. However, channel names are still unique and no two global +channels with same name may exist. See the section 3.13.1 for more +information about channel names. + +Channels can have operators that can administrate the channel and operate +all of its modes. The following operators on channel exist on the +SILC network. + +.in 6 +o Channel founder - When channel is created the joining client becomes + channel founder. Channel founder is channel operator with some more + privileges. Basically, channel founder can fully operate the channel + and all of its modes. The privileges are limited only to the + particular channel. There can be only one channel founder per + channel. Channel founder supersedes channel operator's privileges. + + Channel founder privileges cannot be removed by any other operator on + channel. When channel founder leaves the channel there is no channel + founder on the channel. However, it is possible to set a mode for + the channel which allows the original channel founder to regain the + founder privileges even after leaving the channel. Channel founder + also cannot be removed by force from the channel. + +o Channel operator - When client joins to channel that has not existed + previously it will become automatically channel operator (and channel + founder discussed above). Channel operator is able 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. +.in 3 + + + + +.ti 0 +3.4.1 Channel ID + +Channels are distinguished from other channels by unique Channel ID. +The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and +collisions are not expected to happen in any conditions. Channel names +are just for logical use of channels. The Channel ID is created by the +server where the channel is created. The Channel ID is defined as +follows. + +.in 6 +64 bit Channel ID based on IPv4 addresses: + +32 bit Router's Server ID IP address (bits 1-32) +16 bit Router's Server ID port (bits 33-48) +16 bit Random number 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. +.in 3 + + +.ti 0 +3.5 Operators + +Operators are normal users with extra privileges to their server or +router. Usually these people are SILC server and router administrators +that take care of their own server and clients on them. The purpose of +operators is to administrate the SILC server or router. However, even +an operator with highest privileges is not able to enter invite-only +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. + + +.ti 0 +3.6 SILC Commands + +Commands are very important part on SILC network especially for client +which uses commands to operate on the SILC network. Commands are used +to set nickname, join to channel, change modes and many other things. + +Client usually sends the commands and server replies by sending a reply +packet to the command. Server MAY also send commands usually to serve +the original client's request. Usually server cannot send commands to +clients, however there MAY be commands that allow the server to send +commands to client. By default servers MAY send commands only to other +servers and routers. + +Note that the command reply is usually sent only after client has sent +the command request but server is allowed to send command reply packet +to client even if client has not requested the command. Client MAY +choose to ignore the command reply. + +It is expected that some of the commands may be 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]. + + +.ti 0 +3.7 SILC Packets + +Packets are naturally the most important part of the protocol and the +packets are what actually makes the protocol. Packets in SILC network +are always encrypted using, usually the shared secret session key +or some other key, for example, channel key, when encrypting channel +messages. It is not possible to send 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. + + +.ti 0 +3.8 Packet Encryption + +All packets passed in SILC network MUST be encrypted. This section +gives generic description of how packets must be encrypted in the SILC +network. The detailed description of the actual encryption process +of the packets are described in [SILC2]. + +Client and its server shares secret symmetric session key which is +established by the SILC Key Exchange Protocol, described in [SILC3]. +Every packet sent from client to server, with exception of packets for +channels, are encrypted with this session key. + +Channels have a channel key that are shared by every client on the channel. +However, the channel keys are cell specific thus one cell does not know +the channel key of the other cell, even if that key is for same channel. +Channel key is also known by the routers and all servers that have clients +on the channel. However, channels MAY have channel private keys that are +entirely local setting for the client. All clients on the channel MUST +know the channel private key beforehand to be able to talk on the +channel. In this case, no server or router knows the key for the channel. + +Server shares secret symmetric session key with router which is +established by the SILC Key Exchange Protocol. Every packet passed from +server to router, with exception of packets for channels, are encrypted +with the shared session key. Same way, router server shares secret +symmetric key with its primary router. However, every packet passed +from router to other router, including packets for channels, are +encrypted with the shared session key. Every router connection MUST +have their own session keys. + + +.ti 0 +3.8.1 Determination of the Source and the Destination + +The source and the destination of the packet needs to be determined +to be able to route the packets to correct receiver. This information +is available in the SILC Packet Header which is included in all packets +sent in SILC network. The SILC Packet Header is described in [SILC2]. + +The header MUST be encrypted with the session key 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 +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. + + +.ti 0 +3.8.2 Client To Client + +The process of message delivery and encryption from client to another +client is as follows. + +Example: Private message from client to another client on different + servers. Clients do not share private message delivery + keys; normal session keys are used. + +o Client 1 sends encrypted packet to its server. The packet is + encrypted with the session key shared between client and its + server. + +o Server determines the destination of the packet and decrypts + the packet. Server encrypts the packet with session key shared + between the server and its router, and sends the packet to the + router. + +o Router determines the destination of the packet and decrypts + the packet. Router encrypts the packet with session key + shared between the router and the destination server, and sends + the packet to the server. + +o Server determines the client to which the packet is destined + to and decrypts the packet. Server encrypts the packet with + session key shared between the server and the destination client, + and sends the packet to the client. + +o Client 2 decrypts the packet. + + +Example: Private message from client to another client on different + servers. Clients 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 + 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. + + +.ti 0 +3.8.3 Client To Channel + +Process of message delivery from client on channel to all the clients +on the channel. + +Example: Channel of four users; two on same server, other two on + different cells. Client sends message to the channel. + Packet header is encrypted with the session key, message + data is encrypted with channel key. + +o Client 1 encrypts the packet with channel key and sends the + packet to its server. + +o Server determines local clients on the channel and sends the + packet to the Client on the same server. Server then sends + the packet to its router for further routing. + +o Router determines local clients on the channel, if found + sends packet to the local clients. Router determines global + clients on the channel and sends the packet to its primary + router or fastest route. + +o (Other router(s) do the same thing and sends the packet to + the server(s).) + +o Server determines local clients on the channel and sends the + packet to the client. + +o All clients receiving the packet decrypts it. + + +.ti 0 +3.8.4 Server To Server + +Server to server packet delivery and encryption is described in above +examples. Router to router packet delivery is analogous to server to +server. However, some packets, such as channel packets, are processed +differently. These cases are described later in this document and +more in detail in [SILC2]. + + +.ti 0 +3.9 Key Exchange And Authentication + +Key exchange is done always when for example client connects to server +but also when server and router, and router and 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 [RFC3629] encoded. The connection authentication protocol +is described in detail in [SILC3]. + + +.ti 0 +3.9.1 Authentication Payload + +Authentication Payload is used separately from the SKE and the Connection +Authentication 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: + +.in 5 +.nf + 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Payload Length | Authentication Method | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Public Data Length | | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +| | +~ Public Data ~ +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Authentication Data Length | | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +| | +~ Authentication Data ~ +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +.in 3 + +.ce +Figure 5: Authentication Payload + + +.in 6 +o Payload Length (2 bytes) - Length of the entire payload. + +o Authentication Method (2 bytes) - The method of the + authentication. The authentication methods are defined + in [SILC2] in the Connection Auth Request Payload. The NONE + authentication method SHOULD NOT be used. + +o Public Data Length (2 bytes) - Indicates the length of + the Public Data field. + +o Public Data (variable length) - This is defined only if + the authentication method is public key. If it is any other + this field MAY include random data for padding purposes. + However, in this case the field MUST be ignored by the + receiver. + + When the authentication method is public key this includes + 128 to 4096 bytes of non-zero random data that is used in + the signature process, described subsequently. + +o Authentication Data Length (2 bytes) - Indicates the + length of the Authentication Data field. If zero (0) + value is found in this field the payload MUST be + discarded. + +o Authentication Data (variable length) - Authentication + method dependent authentication data. +.in 3 + + +If the authentication method is 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. + + +.ti 0 +3.10 Algorithms + +This section defines all the allowed algorithms that can be used in +the SILC protocol. This includes mandatory cipher, mandatory public +key algorithm and MAC algorithms. + + +.ti 0 +3.10.1 Ciphers + +Cipher is the encryption algorithm that is used to protect the data +in the SILC packets. See [SILC2] for the actual encryption process and +definition of how it must be done. SILC has a mandatory algorithm that +must be supported in order to be compliant with this protocol. + +The following ciphers are defined in SILC protocol: + +aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED) +aes-256-ctr AES in CTR mode, 256 bit key (RECOMMENDED) +aes-256-rcbc AES in randomized CBC mode, 256 bit key (OPTIONAL) +aes-192- AES in mode, 192 bit key (OPTIONAL) +aes-128- AES in mode, 128 bit key (RECOMMENDED) +twofish-256- Twofish in mode, 256 bit key (OPTIONAL) +twofish-192- Twofish in mode, 192 bit key (OPTIONAL) +twofish-128- Twofish in mode, 128 bit key (OPTIONAL) +cast-256- CAST-256 in mode, 256 bit key (OPTIONAL) +cast-192- CAST-256 in mode, 192 bit key (OPTIONAL) +cast-128- CAST-256 in mode, 128 bit key (OPTIONAL) +serpent-- Serpent in mode, bit key (OPTIONAL) +rc6-- RC6 in mode, bit key (OPTIONAL) +mars-- MARS in mode, bit key (OPTIONAL) +none No encryption (OPTIONAL) + +The is either "cbc", "ctr" or "rcbc". Other encryption modes MAY +be defined to be used in SILC using the same name format. The is +either 256, 192 or 128 bit key length. Also, additional ciphers MAY be +defined to be used in SILC by using the same name format as above. + +Algorithm "none" does not perform any encryption process at all and +thus is not recommended to be used. It is recommended that no client +or server implementation would accept none algorithm except in special +debugging mode. + + +.ti 0 +3.10.1.1 CBC Mode + +The "cbc" encryption mode is CBC mode with inter-packet chaining. This +means that the 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]. + + +.ti 0 +3.10.1.2 CTR Mode + +The "ctr" encryption mode is Counter Mode (CTR). The CTR mode in SILC is +stateful in encryption and decryption. Both sender and receiver maintain +the counter for the CTR mode and thus can precompute the key stream for +encryption and decryption. By default, CTR mode does not require +plaintext padding, however implementations MAY apply padding to the +packets. If the last key block is larger than the last plaintext block +the resulted value is truncated to the size of the plaintext block and +the most significant bits are used. When sending authentication data +inside packets the maximum amount of padding SHOULD be applied with +CTR mode as well. + +In CTR mode only the encryption operation of the cipher is used. The +decryption operation is not needed since both encryption and decryption +process is simple XOR with the plaintext block and the key stream block. + +The counter block is used to create the key for the CTR mode. 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: + +.in 5 +.nf + 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Truncated HASH from SKE | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Sending/Receiving IV from SKE | +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Block Counter | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +.in 3 + +.ce +Figure 6: Counter Block + +.in 6 +o Truncated HASH from SKE (4 bytes) - This value is the first 4 + bytes from the HASH value that was computed as a result of SKE + protocol. This acts as session identifier and each rekey MUST + produce a new HASH value. + +o Sending/Receiving IV from SKE (8 bytes) - This value is the + first 8 bytes from the Sending IV or Receiving IV generated in + the SKE protocol. When this mode is used to encrypt sending + traffic the Sending IV is used, when used to decrypt receiving + traffic the Receiving IV is used. This assures that two parties + of the protocol use different IV for sending traffic. Each rekey + MUST produce a new value. + +o Block Counter (4 bytes) - This is the counter value for the + counter block and is MSB ordered number starting from one (1) + value for first block and incrementing for subsequent blocks. + The same value MUST NOT be used twice. The rekey MUST be + performed before this counter value wraps. +.in 3 + +CTR mode MUST NOT be used with "none" MAC. Implementations also MUST +assure that the same counter block is not used to encrypt more than +one block. Also, the key material used with CTR mode MUST be fresh +key material. Static keys (pre-shared keys) MUST NOT be used with +CTR mode. For this reason using CTR mode to encrypt for example +channel messages or private messages with a pre-shared key is +inappropriate. For private messages, the Key Agreement could be +performed to produce fresh key material. + +If the IV Included flag was negotiated in SKE, or CTR mode is used to +protect channel messages where the counter block will be included in the +Message Payload, implementations SHOULD still use the same counter block +format as defined above. However, implementations are RECOMMENDED to +replace the Truncated HASH field with a 32 bit random value for each IV +(counter block) per encrypted SILC packet. Also note, that in this case +the decryption process is not stateful and receiver cannot precompute the +key stream. + + +.ti 0 +3.10.1.3 Randomized CBC Mode + +The "rcbc" encryption mode is CBC mode with randomized IV. This means +that each IV for each packet MUST be chosen randomly. When encrypting +more than one block the normal inter-packet chaining is used, but for +the first block new random IV is selected in each packet. In this mode +the IV is appended at the end of the last ciphertext block and thus +delivered to the recipient. This mode increases the ciphertext size by +one ciphertext block. Note also that some data payloads in SILC are +capable of delivering the IV to the recipient. When explicitly +encrypting these payloads with randomized CBC the IV MUST NOT be appended +at the end of the ciphertext, 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. + + +.ti 0 +3.10.2 Public Key Algorithms + +Public keys are used in SILC to authenticate entities in SILC network +and to perform other tasks related to public key cryptography. The +public keys are also used in the SILC Key Exchange protocol [SILC3]. + +The following public key algorithms are defined in SILC protocol: + +.in 6 +rsa RSA (REQUIRED) +dss DSS (OPTIONAL) +.in 3 + +DSS is described in [Menezes]. The RSA MUST be implemented according +PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be +compliant to either PKCS #1 version 1.5 or newer with the following +notes: The signature encoding is always in same format as the encryption +encoding regardless of the PKCS #1 version. The signature with appendix +(with hash algorithm OID in the data) MUST NOT be used in the SILC. The +rationale for this is that there is no binding between the PKCS #1 OIDs +and the hash algorithms used in the SILC protocol. Hence, the encoding +is always in PKCS #1 version 1.5 format. + +Additional public key algorithms MAY be defined to be used in SILC. + +When signatures are computed in SILC the computing of the signature is +represented as sign(). The signature computing procedure is dependent +of the public key algorithm, and the public key or certificate encoding. +When using SILC public key the signature is computed as described in +previous paragraph for RSA and DSS keys. If the hash function is not +specified separately for signing process 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]. + + +.ti 0 +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. + + +.ti 0 +3.10.3 Hash Functions + +Hash functions are used as part of MAC algorithms defined in the next +section. They are also used in the SILC Key Exchange protocol defined +in the [SILC3]. + +The following Hash algorithm are defined in SILC protocol: + +.in 6 +sha1 SHA-1, length = 20 (REQUIRED) +md5 MD5, length = 16 (RECOMMENDED) +.in 3 + + +.ti 0 +3.10.4 MAC Algorithms + +Data integrity is protected by computing a message authentication code +(MAC) of the packet data. See [SILC2] for details how to compute the +MAC for a packet. + +The following MAC algorithms are defined in SILC protocol: + +.in 6 +hmac-sha1-96 HMAC-SHA1, length = 12 bytes (REQUIRED) +hmac-md5-96 HMAC-MD5, length = 12 bytes (OPTIONAL) +hmac-sha1 HMAC-SHA1, length = 20 bytes (OPTIONAL) +hmac-md5 HMAC-MD5, length = 16 bytes (OPTIONAL) +none No MAC (OPTIONAL) +.in 3 + +The "none" MAC is not recommended to be used as the packet is not +authenticated when MAC is not computed. It is recommended that no +client or server would accept none MAC except in special debugging +mode. + +The HMAC algorithm is described in [HMAC]. 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. + + +.ti 0 +3.10.5 Compression Algorithms + +SILC protocol supports compression that may be applied to unencrypted +data. It is recommended to use compression on slow links as it may +significantly speed up the data transmission. By default, SILC does not +use compression which is the mode that must be supported by all SILC +implementations. + +The following compression algorithms are defined: + +.in 6 +none No compression (REQUIRED) +zlib GNU ZLIB (LZ77) compression (OPTIONAL) +.in 3 + +Additional compression algorithms MAY be defined to be used in SILC. + + +.ti 0 +3.11 SILC Public Key + +This section defines the type and format of the SILC public key. All +implementations MUST support this public key type. See [SILC3] for +other optional public key and certificate types allowed in the SILC +protocol. Public keys in SILC may be used to authenticate entities +and to perform other tasks related to public key cryptography. + +The format of the SILC Public Key is as follows: + + + + + + + + +.in 5 +.nf + 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Public Key Length | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Algorithm Name Length | | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +| | +~ Algorithm Name ~ +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| Identifier Length | | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +| | +~ Identifier ~ +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| | +~ Public Data ~ +| | ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +.in 3 + +.ce +Figure 5: SILC Public Key + + +.in 6 +o Public Key Length (4 bytes) - Indicates the full length + of the 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 + 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. +.in 3 + +All fields in the public key are in MSB (most significant byte first) +order. All strings in the public key MUST be UTF-8 encoded. + +If an external protocol needs to refer to SILC Public Key by name, the +names "silc-rsa" and "silc-dss" for SILC Public Key based on RSA algorithm +and SILC Public Key based on DSS algorithm, respectively, are to be used. +However, this SILC specification does not use these names directly, and +they are defined here for external protocols (protocols that may like +to use SILC Public Key). + + +.ti 0 +3.12 SILC Version Detection + +The version detection of both client and server is performed at the +connection phase while executing the SILC Key Exchange protocol. The +version identifier is exchanged between initiator and responder. The +version identifier is of the following format: + +.in 6 +SILC-- +.in 3 + +The version strings are of the following format: + +.in 6 +protocol version = . +software version = [.[.]] +.in 3 + +Protocol version MUST provide both major and minor version. Currently +implementations MUST set the protocol version and accept at least the +protocol version as SILC-1.2-. If new protocol version +causes incompatibilities with older version the version number +MUST be incremented. The is incremented if new protocol version +is fully incompatible. + +Software version MAY provide major, minor and build (vendor) version. +The software version MAY be freely set and accepted. The version string +MUST consist of printable US-ASCII characters. + +Thus, the version strings could be, for example: + +.in 6 +SILC-1.1-2.0.2 +SILC-1.0-1.2 +SILC-1.2-1.0.VendorXYZ +SILC-1.2-2.4.5 Vendor Limited +.in 3 + + +.ti 0 +3.13 UTF-8 Strings in SILC + +By default all strings that are sent in SILC protocol MUST be UTF-8 +[RFC3269] encoded, unless otherwise defined. This means that any string +sent inside for example, command, command reply, notify or any packet +payload is UTF-8 encoded. Also nicknames, channel names, server names, +and hostnames are UTF-8 encoded. This definition does not affect +messages sent in SILC, as the Message Payload provides its own mechanism +to indicate whether a message is UTF-8 text message, data message, which +may use its own character encoding, or pure binary message [SILC2]. + +Certain limitations are imposed on the UTF-8 encoded strings in SILC. +The UTF-8 encoded strings MUST NOT include any characters that are +marked in the Unicode standard as control codes, noncharacters, +reserved or private range characters, or any other illegal Unicode +characters. Also the BOM (Byte-Order Mark) MUST NOT be used as byte +order signature in UTF-8 encoded strings. A string containing these +characters MUST be treated as malformed UTF-8 encoding. + +The Unicode standard defines that malformed sequences shall be signalled +by replacing the sequence with a replacement character. Even though, +in case of SILC these strings may not be malformed UTF-8 encodings +they MUST be treated as malformed strings. Implementation MAY use +a replacement character, however, the character Unicode standard defines +MUST NOT be used, but another character must be chosen. It is, however, +RECOMMENDED that an error is returned instead of using replacement +character if it is possible. For example, when setting a nickname +with SILC_COMMAND_NICK command, implementation is able to send error +indication back to the command sender. It must be noted that on server +implementation if a character sequence is merely outside of current +character subset, but is otherwise valid character, it MUST NOT be +replaced by a replacement character. + +On user interface where UTF-8 strings are displayed the implementation +is RECOMMENDED to escape any character that it is unable to render +properly. The escaping may be done for example as described in +[RFC2253]. The escaping makes it possible to retrieve the original +UTF-8 encoding. Alternatively, a replacement character may be used +if it does not cause practical problems to the implementation. + + +.ti 0 +3.13.1 UTF-8 Identifier Strings + +Identifier strings are special strings in SILC protocol that require +more careful processing, than the general UTF-8 strings described in the +previous section. These strings include the nicknames, server names, +hostnames and some other identifier strings. These strings are prepared +using the stringprep [RFC3454] standard. The Appendix A defines the +stringprep profile for SILC identifier strings and conforming +implementation MUST use the profile to prepare any identifier string. + +The stringprep profile describes how identifier strings are prepared, +what characters they may include, and which characters are prohibited. +Identifier strings with prohibited characters MUST be treated as +malformed strings. + +The channel name is also special identifier strings with some slight +differences to other identifier strings. The Appendix B defines the +stringprep profile for the channel name strings and conforming +implementation MUST use the profile to prepare any channel name string. + +Because of the profile the identifier strings in SILC may generally +include only letters, numbers, most punctuation characters, and some +other characters. For practical reasons most symbol characters and +many other special characters are prohibited. All identifier strings +are case folded and comparing the identifier strings MUST be done as +caseless matching. + +In general, the identifier strings does not have a maximum length. +However, the length of a nickname string MUST NOT exceed 128 bytes, and +the length of a channel name string MUST NOT exceed 256 bytes. Since +these strings are UTF-8 encoded the length of one character may be +longer than one byte. This means that the character length of these +strings may be shorter than the maximum length of the string in bytes. +The minimum length of an identifier string MUST be at least one character, +which may be one byte or more in length. Implementation MAY limit the +maximum length of an identifier string, with exception of the nickname +and channel name strings which has the explicit length definition. + + +.ti 0 +3.14 Backup Routers + +Backup routers may exist in the cell in addition to the primary router. +However, they must not be active routers or act as routers in the cell. +Only one router may be acting as primary router in the cell. In the case +of failure of the primary router one of the backup routers becomes active. +The purpose of backup routers are in case of failure of the primary router +to maintain working connections inside the cell and outside the cell and +to avoid netsplits. + +Backup routers are normal servers in the cell that are prepared to take +over the tasks of the primary router if needed. They need to have at +least one direct and active connection to the primary router of the cell. +This communication channel is used to send the router information to +the backup router. When the backup router connects to the primary router +of the cell it MUST present itself as router server in the Connection +Authentication protocol, even though it is normal server as long as the +primary router is available. Reason for this is that the configuration +needed in the responder end requires usually router connection level +configuration. The responder, however must understand and treat the +connection as normal server (except when feeding router level data to +the backup router). + +Backup router must know everything that the primary router knows to be +able to take over the tasks of the primary router. It is the primary +router's responsibility to feed the data to the backup router. If the +backup router does not know all the data in the case of failure some +connections may be lost. The primary router of the cell must consider +the backup router being an actual router server when it feeds the data +to it. + +In addition to having direct connection to the primary router of the +cell, the backup router must also have connection to the same router +to which the primary router of the cell is connected. However, it must +not be the active router connection meaning that the backup router must +not use that channel as its primary route and it must not notify the +router about having connected servers, channels and clients behind it. +It merely connects to the router. This sort of connection is later +referred to as being a passive connection. Some keepalive actions may +be needed by the router to keep the connection alive. + +It is required that other normal servers have passive connections to +the backup router(s) in the cell. Some keepalive actions may be needed +by the server to keep the connection alive. After they notice the +failure of the primary router they must start using the connection to +the first backup router as their primary route. + +Also, if any other router in the network is using the cell's primary +router as its own primary router, it must also have passive connection +to the cell's backup router. It too is prepared to switch to use the +backup router as its new primary router as soon as the original primary +router becomes unresponsive. + +All of the parties of this protocol know which one is the backup router +of the cell from their local configuration. Each of the entities must +be configured accordingly and care must be taken when configuring the +backup routers, servers and other routers in the network. + +It must be noted that some of the channel messages and private messages +may be lost during the switch to the backup router. 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. + + +.ti 0 +3.14.1 Switching to Backup Router + +When the primary router of the cell becomes unresponsive, for example +by sending EOF to the connection, all the parties of this protocol MUST +replace the old connection to the primary router with first configured +backup router. The backup router usually needs to do local modifications +to its database in order to update all the information needed to maintain +working routes. The backup router must understand that clients that +were originated from the primary router are now originated from some of +the existing server connections and must update them accordingly. It +must also remove those clients that were owned by the primary router +since those connections were lost when the primary router became +unresponsive. + +All the other parties of the protocol must also update their local +database to understand that the route to the primary router will now go +to the backup router. + +Servers connected to the backup router MUST send SILC_PACKET_RESUME_ROUTER +packet with type value 21, to indicate that the server will start using +the backup router as primary router. The backup router MUST NOT allow +this action if it detects that primary is still up and running. If +backup router knows that primary is up and running it MUST send +SILC_PACKET_FAILURE with type value 21 (4 bytes, MSB first order) back +to the server. The server then MUST NOT use the backup as primary +router, but must try to establish connection back to the primary router. +If the action is allowed type value 21 is sent back to the server from +the backup router. It is RECOMMENDED that implementations use the +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. + + +.ti 0 +3.14.2 Resuming Primary Router + +Usually the primary router is unresponsive only a short period of time +and it is intended that the original router of the cell will resume +its position as primary router when it comes back online. The backup +router that is now acting as primary router of the cell must constantly +try to connect to the original primary router of the cell. It is +RECOMMENDED that it would try to reconnect in 30 second intervals to +the primary router. + +When the connection is established to the primary router the backup +resuming protocol is executed. The protocol is advanced as follows: + + 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type + value 1 to the primary router that came back online. The packet + will indicate the primary router has been replaced by the backup + router. After sending the packet the backup router will announce + all of its channels, channel users, modes etc. to the primary + router. + + If the primary knows that it has not been replaced (for example + the backup itself disconnected from the primary router and thinks + that it is now primary in the cell) the primary router send + SILC_PACKET_FAILURE with the type value 1 (4 bytes, MSB first + order) back to the backup router. If backup receives this it + MUST NOT continue with the backup resuming protocol. + + 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type + value 1 to its current primary router to indicate that it will + resign as being primary router. Then, backup router sends the + SILC_PACKET_RESUME_ROUTER packet with type value 1 to all + connected servers to also indicate that it will resign as being + primary router. + + 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with + type value 1 to the router that is using the backup router + currently as its primary router. + + 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER + with type value 1 must reconnect immediately to the primary + router of the cell that came back online. After they have created + the connection they MUST NOT use that connection as active primary + route but still route all packets to the backup router. After + the connection is created they MUST send SILC_PACKET_RESUME_ROUTER + with type value 2 back to the backup router. The session ID value + found in the first packet MUST be set in this packet. + + 5. Backup router MUST wait for all packets with type value 2 before + it continues with the protocol. It knows from the session ID values + set in the packet when it has received all packets. The session + value should be different in all packets it has sent earlier. + After the packets are received the backup router sends the + SILC_PACKET_RESUME_ROUTER packet with type value 3 to the + primary router that came back online. This packet will indicate + that the backup router is now ready to resign as being primary + router. The session ID value in this packet MUST be the same as + in the first packet sent to the primary router. During this time + the backup router must still route all packets it is receiving + from server connections. + + 6. The primary router receives the packet and send the packet + SILC_PACKET_RESUME_ROUTER with type value 4 to all connected servers + including the backup router. It also sends the packet with type + value 4 to its primary router, and to the router that is using + it as its primary router. The Session ID value in these packets + SHOULD be zero (0). + + 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER + packet with type value 4 must switch their primary route to the new + primary router and remove the route for the backup router, since + it is no longer the primary router of the cell. They must also + update their local database to understand that the clients are + not originated from the backup router but from the locally connected + servers. After that they MUST announce their channels, channel + users, modes etc. to the primary router. They MUST NOT use the + 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]. + + +.ti 0 +4 SILC Procedures + +This section describes various SILC procedures such as how the +connections are created and registered, how channels are created and +so on. The references [SILC2], [SILC3] and [SILC4] permeate this +section's definitions. + + +.ti 0 +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. + +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. + + + + +.ti 0 +4.2 Creating Server Connection + +This section describes the procedure when server connects to its +router (or when router connects to other router, the cases are +equivalent). The procedure is very much alike to when a client +connects to the server thus it is not repeated here. + +One difference is that server MUST perform connection authentication +protocol with proper authentication. A proper authentication is based +on passphrase authentication or public key authentication based on +digital signatures. + +After server and router have successfully performed the key exchange +and connection authentication protocol, the server MUST register itself +to the router by sending SILC_PACKET_NEW_SERVER packet. This packet +includes the server's Server ID that it has created by itself and +other relevant information about the server. The router receiving the +ID MUST verify that the IP address in the Server ID is same as the +server's real IP address. + +After router has received the SILC_PACKET_NEW_SERVER packet it +distributes the information about newly registered server to all routers +in the SILC network. More information about this is in [SILC2]. + +As the client needed to resolve the destination ID this MUST be done by +the server that connected to the router, as well. The way to resolve it +is to get the ID from previously received packet. The server MAY also +use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start +using its own Server ID as Source ID in SILC Packet Header and the +router's Server ID as Destination when communicating with the router. + + +.ti 0 +4.2.1 Announcing Clients, Channels and Servers + +After server or router has connected to the remote router, and it already +has connected clients and channels it MUST announce them to the router. +If the server is router server, also all the local servers in the cell +MUST be announced. + +All clients are announced by compiling a list of ID Payloads into the +SILC_PACKET_NEW_ID packet. All channels are announced by compiling a +list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. +Channels' mode, founder public key, channel public keys, and other +channel mode specific data is announced by sending the +SILC_NOTIFY_TYPE_CMODE_CHANGE notify list. + +The channel public keys that are announced are compiled in Argument +List Payload where the argument type is 0x03, and each argument is +Public Key Payload containing one public key or certificate. + +Also, the channel users on the channels must be announced by compiling +a list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type +into the SILC_PACKET_NOTIFY packet. The users' modes on the channel +must also be announced by compiling list of Notify Payloads with the +SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY +packet. + +The router MUST also announce the local servers by compiling list of +ID Payloads into the SILC_PACKET_NEW_ID packet. + +Also, clients' modes (user modes in SILC) MUST be announced. This is +done by compiling a list of Notify Payloads with SILC_NOTIFY_UMODE_CHANGE +notify type into the SILC_PACKET_NOTIFY packet. Also, channels' topics +MUST be announced by compiling a list of Notify Payloads with the +SILC_NOTIFY_TOPIC_SET notify type into the SILC_PACKET_NOTIFY packet. +Also, channel's invite and ban lists MUST be announced by compiling list +of Notify Payloads with the SILC_NOTIFY_TYPE_INVITE and +SILC_NOTIFY_TYPE_BAN notify types, respectively, into the +SILC_PACKET_NOTIFY packet. + +The router which receives these lists MUST process them and broadcast +the packets to its primary router. When processing the announced channels +and channel users the router MUST check whether a channel exists already +with the same name. If channel exists with the same name it MUST check +whether the Channel ID is different. If the Channel ID is different the +router MUST send the notify type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the +server to force the channel ID change to the ID the router has. If the +mode of the channel is different the router MUST send the notify type +SILC_NOTIFY_TYPE_CMODE_CHANGE to the server to force the mode change +to the mode that the router has. + +The router MUST also generate new channel key and distribute it to the +channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode +is set. + +If the channel has a channel founder already on the router, the router +MUST send the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server +to force the mode change for the channel founder on the server. The +channel founder privileges MUST be removed on the server. + +If the channel public keys are already set on the on router, the router +MUST ignore the received channel public key list and send the notify +type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server which includes the +channel public key list that is on router. The server MUST change the +list to the one it receives from router. + +The router processing the channels MUST also compile a list of +Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the +SILC_PACKET_NOTIFY and send the packet to the server. This way the +server (or router) will receive the clients on the channel that +the router has. + + +.ti 0 +4.3 Joining to a Channel + +This section describes the procedure when client joins to a channel. +Client joins to channel by sending command SILC_COMMAND_JOIN to the +server. If the receiver receiving join command is normal server the +server MUST check its local list whether this channel already exists +locally. This would indicate that some client connected to the server +has already joined to the channel. If this is 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 +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. + + +.ti 0 +4.4 Channel Key Generation + +Channel keys are created by router which creates the channel by taking +enough randomness from cryptographically strong random number generator. +The key is generated always when channel is created, when new client +joins a channel and after the key has expired. Key could expire for +example in an hour. + +The key MUST also be re-generated whenever some client leaves a channel. +In this case the key is created from scratch by taking enough randomness +from the random number generator. After that the key is distributed to +all clients on the channel. However, channel keys are cell specific thus +the key is created only on the cell where the client, which left the +channel, exists. While the server or router is creating the new channel +key, no other client may join to the channel. Messages that are sent +while creating the new key are still processed with the old key. After +server has sent the SILC_PACKET_CHANNEL_KEY packet client MUST start +using the new key. If server creates the new key the server MUST also +send the new key to its router. See [SILC2] for more information about +how channel messages must be encrypted and decrypted when router is +processing them. + +If the key changes very often due to joining traffic on the channel it +is RECOMMENDED that client implementation would cache some of the old +channel keys for short period of time so that it is able to decrypt all +channel messages it receives. It is possible that on a heavy traffic +channel a message encrypted with channel key that was just changed +is received by client after the new key was set into use. This is +possible because not all clients may receive the new key at the same +time, and may still be sending messages encrypted with the old key. + +When client receives the SILC_PACKET_CHANNEL_KEY packet with the +Channel Key Payload it MUST process the key data to create encryption +and decryption key, and to create the HMAC key that is used to compute +the MACs of the channel messages. The processing is as follows: + + channel_key = raw key data + HMAC key = hash(raw key data) + +The raw key data is the key data received in the Channel Key Payload. +The hash() function is the hash function used in the HMAC of the channel. +Note that the server also MUST save the channel key. + + +.ti 0 +4.5 Private Message Sending and Reception + +Private messages are sent point to point. Client explicitly destine +a private message to specific client that is delivered to only to that +client. No other client may receive the private message. The receiver +of the private message is destined in the SILC Packet Header as 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. + + +.ti 0 +4.6 Private Message Key Generation + +Private message MAY be protected with a key generated by the client. +One way to generate private message key is to use static or pre-shared +keys in the client implementation. Client that wants to indicate other +client on the network that a private message key should be set, the +client MAY send SILC_PACKET_PRIVATE_MESSAGE_KEY packet to indicate this. +The actual key material has to be transferred outside the SILC network, +or it has to be pre-shared key. The client receiving this packet knows +that the sender wishes to use private message key in private message +communication. In case of static or pre-shared keys the IV used in +the encryption SHOULD be chosen randomly. Sending the +SILC_PACKET_PRIVATE_MESSAGE_KEY is not mandatory, and clients may +naturally agree to use a key without sending the packet. + +Another choice to use private message keys is to negotiate fresh key +material by performing the Key Agreement. The SILC_PACKET_KEY_AGREEMENT +packet MAY be used to negotiate the fresh key material. In this case +the resulting key material is used to secure the private messages. +Also, the IV used in encryption is used as defined in [SILC3], unless +otherwise stated by the encryption mode used. By performing Key +Agreement the clients can also negotiate the cipher and HMAC to be used +in the private message encryption and to negotiate additional security +parameters. The actual Key Agreement [SILC2] is performed by executing +the SILC Key Exchange protocol [SILC3], peer to peer. Because of NAT +devices in the network, it might be impossible to perform the Key +Agreement. In this case using static or pre-shared key and sending the +SILC_PACKET_PRIVATE_MESSAGE_KEY to indicate the use of a private message +key is a working alternative. + +If the key is pre-shared key or other key material not generated by +Key Agreement, then the key material SHOULD be processed as defined +in [SILC3]. In the processing, however, the HASH, as defined in [SILC3] +MUST be ignored. After processing the key material it is employed as +defined in [SILC3]. If the SILC_PACKET_PRIVATE_MESSAGE_KEY was sent, +then it defines the cipher and HMAC to be used. The hash algorithm to be +used in the key material processing is the one that HMAC algorithm is +defined to use. If the SILC_PACKET_PRIVATE_MESSAGE_KEY was not sent at +all, then the hash algorithm to be used SHOULD be SHA1. In this case +also, implementations SHOULD use the SILC protocol's mandatory cipher +and HMAC in private message encryption. + + +.ti 0 +4.7 Channel Message Sending and Reception + +Channel messages are delivered to a group of users. The group forms a +channel and all clients on the channel receives messages sent to the +channel. The Source ID in the SILC Packet Header MUST be the ID +of the sender of the message. + +Channel messages are destined to a channel by specifying the Channel ID +as Destination ID in the SILC Packet Header. The server MUST then +distribute the message to all clients, except to the original sender, +on the channel by sending the channel message destined explicitly to a +client on the channel. However, the Destination ID MUST still remain +as the Channel ID. + +If server receives a channel message packet which includes invalid +destination Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR +notify to the sender with error status indicating that such Channel ID +does not exist. + +See the [SILC2] for description of channel message routing for router +servers, and channel message encryption and decryption process. + + +.ti 0 +4.8 Session Key Regeneration + +Session keys MUST be regenerated periodically, say, once in an hour. +The re-key process is started by sending SILC_PACKET_REKEY packet to +other end, to indicate that re-key must be performed. The initiator +of the connection SHOULD initiate the re-key. + +If perfect forward secrecy (PFS) flag was selected in the SILC Key +Exchange protocol [SILC3] the re-key MUST cause new key exchange with +SKE protocol. In this case the protocol is secured with the old key +and the protocol results to new key material. See [SILC3] for more +information. After the SILC_PACKET_REKEY packet is sent the sender +will perform the SKE protocol. + +If PFS flag was set the resulted key material is processed as described +in the section Processing the Key Material in [SILC3]. The difference +with re-key in the processing is that the initial data for the hash +function is just the resulted key material and not the HASH as it +is not computed at all with re-key. Other than that, the key processing +it equivalent to normal SKE negotiation. + +If PFS flag was not set, which is the default case, then re-key is done +without executing SKE protocol. In this case, the new key is created by +providing the current sending encryption key to the SKE protocol's key +processing function. The process is described in the section Processing +the Key Material in [SILC3]. The difference in the processing is that +the initial data for the hash function is the current sending encryption +key and not the SKE's KEY and HASH values. Other than that, the key +processing is equivalent to normal SKE negotiation. + +After both parties 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. + + +.ti 0 +4.9 Command Sending and Reception + +Client usually sends the commands in the SILC network. In this case +the client simply sends the command packet to server and the server +processes it and replies with command reply packet. See the [SILC4] +for detailed description of all commands. + +However, if the server is not able to process the command, it is sent to +the server's router. This is case for example with commands such as +SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there are +other commands as well [SILC4]. For example, if client sends the WHOIS +command requesting specific information about some client the server must +send the WHOIS command to router so that all clients in SILC network are +searched. The router, on the other hand, sends the WHOIS command further +to receive the exact information about the requested client. The WHOIS +command travels all the way to the server which owns the client and it +replies with command reply packet. Finally, the server which sent the +command receives the command reply and it must be able to determine which +client sent the original command. The server then sends command reply to +the client. Implementations should have some kind of cache to handle, for +example, WHOIS information. Servers and routers along the route could all +cache the information for faster referencing in the future. + +The commands sent by server may be sent hop by hop until someone is able +to process the command. However, it is preferred to destine the command +as precisely as it is possible. In this case, other routers en route +MUST route the command packet by checking the true sender and true +destination of the packet. However, servers and routers MUST NOT route +command reply packets to clients coming from other servers. Client +MUST NOT accept command reply packet originated from anyone else but +from its own server. + + +.ti 0 +4.10 Closing Connection + +When remote client connection is closed the server MUST send the notify +type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels +the client was joined. The server MUST also save the client's information +for a period of time for history purposes. + +When remote server or router connection is closed the server or router +MUST also remove all the clients that was behind the server or router +from the SILC Network. The server or router MUST also send the notify +type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all +local clients that are joined on the same channels with the remote +server's or router's clients. + +Router server MUST also check whether some client in the local cell +is watching for the nickname this client has, and send the +SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left +the network has the SILC_UMODE_REJECT_WATCHING user mode set. + + +.ti 0 +4.11 Detaching and Resuming a Session + +SILC protocol provides a possibility for a client to detach itself from +the network without actually signing off from the network. The client +connection to the server is closed but the client remains as valid client +in the network. The client may then later resume its session back from +any server in the network. + +When client wishes to detach from the network it MUST send the +SILC_COMMAND_DETACH command to its server. The server then MUST set +SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE +notify to its primary router, which 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 +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. + + +.ti 0 +5 Security Considerations + +Security is central to the design of this protocol, and these security +considerations permeate the specification. Common security considerations +such as keeping private keys truly private and using adequate lengths for +symmetric and asymmetric keys must be followed in order to maintain the +security of this protocol. + +Special attention must also be paid 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. + + +.ti 0 +6 References + +[SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft, + May 2002. + +[SILC3] Riikonen, P., "SILC Key Exchange and Authentication + Protocols", Internet Draft, May 2002. + +[SILC4] Riikonen, P., "SILC Commands", Internet Draft, May 2002. + +[IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol", + RFC 1459, May 1993. + +[IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810, + April 2000. + +[IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC + 2811, April 2000. + +[IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC + 2812, April 2000. + +[IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC + 2813, April 2000. + +[SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", + Internet Draft. + +[PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440, + November 1998. + +[SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693, + September 1999. + +[PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key + Infrastructure, Certificate and CRL Profile", RFC 2459, + January 1999. + +[Schneier] Schneier, B., "Applied Cryptography Second Edition", + John Wiley & Sons, New York, NY, 1996. + +[Menezes] Menezes, A., et al, "Handbook of Applied Cryptography", + CRC Press 1997. + +[OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", + RFC 2412, November 1998. + +[ISAKMP] Maughan D., et al, "Internet Security Association and + Key Management Protocol (ISAKMP)", RFC 2408, November + 1998. + +[IKE] Harkins D., and Carrel D., "The Internet Key Exchange + (IKE)", RFC 2409, November 1998. + +[HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message + Authentication", RFC 2104, February 1997. + +[PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography + Specifications, Version 2.0", RFC 2437, October 1998. + +[RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + +[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO + 10646", RFC 3629, November 2003. + +[RFC1321] Rivest R., "The MD5 Message-Digest Algorithm", RFC 1321, + April 1992. + +[RFC3174] Eastlake, F., et al., "US Secure Hash Algorithm 1 (SHA1)", + RFC 3174, September 2001. + +[PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax, + Version 1.5", RFC 2315, March 1998. + +[RFC2253] Wahl, M., et al., "Lightweight Directory Access Protocol + (v3): UTF-8 String Representation of Distinguished Names", + RFC 2253, December 1997. + +[RFC3454] Hoffman, P., et al., "Preparation of Internationalized + Strings ("stringprep")", RFC 3454, December 2002. + + +.ti 0 +7 Author's Address + +.nf +Pekka Riikonen +Snellmaninkatu 34 A 15 +70100 Kuopio +Finland + +EMail: priikone@iki.fi + + +.ti 0 +Appendix A + +This appendix defines the stringprep [RFC3454] profile for string +identifiers in SILC protocol. Compliant implementation MUST use this +profile to prepare the identifier strings in the SILC protocol. The +profile defines the following as required by [RFC3454]. + +- Intended applicability of the profile: the following identifiers in + the SILC Protocol; nicknames, usernames, server names, hostnames, + service names, algorithm names and other security property names [SILC3], + and SILC Public Key name. + +- The character repertoire that is the input and output to + stringprep: Unicode 3.2 with the list of unassigned code points + being the Table A.1, as defined in [RFC3454]. + +- The mapping tables used: the following tables are used, in order, + as defined in [RFC3454]. + + Table B.1 + Table B.2 + + The mandatory case folding is done using the Table B.2 which includes + the characters for the normalization form KC. + +- The Unicode normalization used: the Unicode normalization form + KC is used, as defined in [RFC3454]. + +- The prohibited characters as output: the following tables are used + to prohibit characters, as defined in [RFC3454]; + + Table C.1.1 + Table C.1.2 + Table C.2.1 + Table C.2.2 + Table C.3 + Table C.4 + Table C.5 + Table C.6 + Table C.7 + Table C.8 + Table C.9 + +- Additional prohibited characters as output: in addition, the following + tables are used to prohibit characters, as defined in this document; + + Appendix C + Appendix D + +- The bidirectional string testing used: bidirectional string testing + is ignored in this profile. + +This profile is to be maintained in the IANA registry for stringprep +profiles. The name of this profile is "silc-identifier-prep" and this +document defines the profile. This document defines the first version of +this profile. + + +.ti 0 +Appendix B + +This appendix defines the stringprep [RFC3454] profile for channel name +strings in SILC protocol. Compliant implementation MUST use this profile +to prepare the channel name strings in the SILC protocol. The profile +defines the following as required by [RFC3454]. + +- Intended applicability of the profile: channel names. + +- The character repertoire that is the input and output to + stringprep: Unicode 3.2 with the list of unassigned code points + being the Table A.1, as defined in [RFC3454]. + +- The mapping tables used: the following tables are used, in order, + as defined in [RFC3454]. + + Table B.1 + Table B.2 + + The mandatory case folding is done using the Table B.2 which includes + the characters for the normalization form KC. + +- The Unicode normalization used: the Unicode normalization form + KC is used, as defined in [RFC3454]. + +- The prohibited characters as output: the following tables are used + to prohibit characters, as defined in [RFC3454]; + + Table C.1.1 + Table C.1.2 + Table C.2.1 + Table C.2.2 + Table C.3 + Table C.4 + Table C.5 + Table C.6 + Table C.7 + Table C.8 + Table C.9 + +- Additional prohibited characters as output: in addition, the following + tables are used to prohibit characters, as defined in this document; + + Appendix D + +- The bidirectional string testing used: bidirectional string testing + is ignored in this profile. + +This profile is to be maintained in the IANA registry for stringprep +profiles. The name of this profile is "silc-identifier-ch-prep" and this +document defines the profile. This document defines the first version of +this profile. + + +.ti 0 +Appendix C + +This appendix defines additional prohibited characters in the identifier +strings as defined in the stringprep profile in Appendix A. + +Reserved US-ASCII characters +0021 002A 002C 003F 0040 + + +.ti 0 +Appendix D + +This appendix defines additional prohibited characters in the identifier +strings as defined in the stringprep profile in Appendix A and Appendix B. +Note that the prohibited character tables listed in the Appendix A and +Appendix B may include some of the same characters listed in this +appendix as well. + +Symbol characters and other symbol like characters +00A2-00A9 00AC 00AE 00AF 00B0 00B1 00B4 00B6 00B8 00D7 00F7 +02C2-02C5 02D2-02FF 0374 0375 0384 0385 03F6 0482 060E 060F +06E9 06FD 06FE 09F2 09F3 09FA 0AF1 0B70 0BF3-0BFA 0E3F +0F01-0F03 0F13-0F17 0F1A-0F1F 0F34 0F36 0F38 0FBE 0FBF +0FC0-0FC5 0FC7-0FCF 17DB 1940 19E0-19FF 1FBD 1FBF-1FC1 +1FCD-1FCF 1FDD-1FDF 1FED-1FEF 1FFD 1FFE 2044 2052 207A-207C +208A-208C 20A0-20B1 2100-214F 2150-218F 2190-21FF 2200-22FF +2300-23FF 2400-243F 2440-245F 2460-24FF 2500-257F 2580-259F +25A0-25FF 2600-26FF 2700-27BF 27C0-27EF 27F0-27FF 2800-28FF +2900-297F 2980-29FF 2A00-2AFF 2B00-2BFF 2E9A 2EF4-2EFF +2FF0-2FFF 303B-303D 3040 3095-3098 309F-30A0 30FF-3104 +312D-3130 318F 31B8-31FF 321D-321F 3244-325F 327C-327E +32B1-32BF 32CC-32CF 32FF 3377-337A 33DE-33DF 33FF 4DB6-4DFF +9FA6-9FFF A48D-A48F A4A2-A4A3 A4B4 A4C1 A4C5 A4C7-ABFF +D7A4-D7FF FA2E-FAFF FFE0-FFEE FFFC 10000-1007F 10080-100FF +10100-1013F 1D000-1D0FF 1D100-1D1FF 1D300-1D35F 1D400-1D7FF + +Other characters +E0100-E01EF + + +.ti 0 +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 +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 +BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION +HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF +MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.