8 .ds RF FORMFEED[Page %]
11 .ds RH 26 November 2002
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-06.txt 26 November 2002
20 Expires: 26 April 2003
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-06.txt>
32 This document is an Internet-Draft and is in full conformance with
33 all provisions of Section 10 of RFC 2026. Internet-Drafts are
34 working documents of the Internet Engineering Task Force (IETF), its
35 areas, and its working groups. Note that other groups may also
36 distribute working documents as Internet-Drafts.
38 Internet-Drafts are draft documents valid for a maximum of six months
39 and may be updated, replaced, or obsoleted by other documents at any
40 time. It is inappropriate to use Internet-Drafts as reference
41 material or to cite them other than as "work in progress."
43 The list of current Internet-Drafts can be accessed at
44 http://www.ietf.org/ietf/1id-abstracts.txt
46 The list of Internet-Draft Shadow Directories can be accessed at
47 http://www.ietf.org/shadow.html
49 The distribution of this memo is unlimited.
55 This memo describes a Secure Internet Live Conferencing (SILC)
56 protocol which provides secure conferencing services over insecure
57 network channel. SILC is IRC [IRC] like protocol, however, it is
58 not equivalent to IRC and does not support IRC. Strong cryptographic
59 methods are used to protect SILC packets inside the SILC network.
60 Three other Internet Drafts relates very closely to this memo;
61 SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
62 Protocols [SILC3] and SILC Commands [SILC4].
73 1 Introduction .................................................. 3
74 1.1 Requirements Terminology .................................. 4
75 2 SILC Concepts ................................................. 4
76 2.1 SILC Network Topology ..................................... 4
77 2.2 Communication Inside a Cell ............................... 6
78 2.3 Communication in the Network .............................. 6
79 2.4 Channel Communication ..................................... 7
80 2.5 Router Connections ........................................ 8
81 3 SILC Specification ............................................ 8
82 3.1 Client .................................................... 9
83 3.1.1 Client ID ........................................... 9
84 3.2 Server .................................................... 10
85 3.2.1 Server's Local ID List .............................. 10
86 3.2.2 Server ID ........................................... 11
87 3.2.3 SILC Server Ports ................................... 12
88 3.3 Router .................................................... 12
89 3.3.1 Router's Local ID List .............................. 13
90 3.3.2 Router's Global ID List ............................. 13
91 3.3.3 Router's Server ID .................................. 14
92 3.4 Channels .................................................. 14
93 3.4.1 Channel ID .......................................... 15
94 3.5 Operators ................................................. 16
95 3.6 SILC Commands ............................................. 16
96 3.7 SILC Packets .............................................. 17
97 3.8 Packet Encryption ......................................... 17
98 3.8.1 Determination of the Source and the Destination ..... 18
99 3.8.2 Client To Client .................................... 18
100 3.8.3 Client To Channel ................................... 20
101 3.8.4 Server To Server .................................... 20
102 3.9 Key Exchange And Authentication ........................... 20
103 3.9.1 Authentication Payload .............................. 21
104 3.10 Algorithms ............................................... 23
105 3.10.1 Ciphers ............................................ 23
106 3.10.1.1 CBC Mode .................................. 24
107 3.10.1.2 CTR Mode .................................. 24
108 3.10.1.3 Randomized CBC Mode ....................... 25
109 3.10.2 Public Key Algorithms .............................. 26
110 3.10.3 Hash Functions ..................................... 26
111 3.10.4 MAC Algorithms ..................................... 27
112 3.10.5 Compression Algorithms ............................. 27
113 3.11 SILC Public Key .......................................... 28
114 3.12 SILC Version Detection ................................... 30
115 3.13 Backup Routers ........................................... 31
116 3.13.1 Switching to Backup Router ......................... 32
117 3.13.2 Resuming Primary Router ............................ 33
118 3.13.3 Discussion on Backup Router Scheme ................. 36
119 4 SILC Procedures ............................................... 36
120 4.1 Creating Client Connection ................................ 36
121 4.2 Creating Server Connection ................................ 38
122 4.2.1 Announcing Clients, Channels and Servers ............ 38
123 4.3 Joining to a Channel ...................................... 39
124 4.4 Channel Key Generation .................................... 41
125 4.5 Private Message Sending and Reception ..................... 41
126 4.6 Private Message Key Generation ............................ 42
127 4.7 Channel Message Sending and Reception ..................... 43
128 4.8 Session Key Regeneration .................................. 43
129 4.9 Command Sending and Reception ............................. 44
130 4.10 Closing Connection ....................................... 45
131 4.11 Detaching and Resuming a Session ......................... 45
132 5 Security Considerations ....................................... 47
133 6 References .................................................... 48
134 7 Author's Address .............................................. 49
141 Figure 1: SILC Network Topology
142 Figure 2: Communication Inside cell
143 Figure 3: Communication Between Cells
144 Figure 4: Router Connections
145 Figure 5: SILC Public Key
146 Figure 6: Counter Block
152 This document describes a Secure Internet Live Conferencing (SILC)
153 protocol which provides secure conferencing services over insecure
154 network channel. SILC is IRC [IRC] like protocol, however, it is
155 not equivalent to IRC and does not support IRC. Some of the SILC's
156 features are not found in IRC but in traditional Instant Message (IM)
157 protocols. SILC combines features from both of these chat protocol
158 styles, and SILC can be implemented as either IRC-like system or
161 Strong cryptographic methods are used to protect SILC packets inside
162 the SILC network. Three other Internet Drafts relates very closely
163 to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
164 Authentication Protocols [SILC3] and SILC Commands [SILC4].
166 The protocol uses extensively packets as conferencing protocol
167 requires message and command sending. The SILC Packet Protocol is
168 described in [SILC2] and should be read to fully comprehend this
169 document and protocol. [SILC2] also describes the packet encryption
170 and decryption in detail. The SILC Packet Protocol provides secured
171 and authenticated packets, and the protocol is designed to be compact.
172 This makes SILC also suitable in environment of low bandwidth
173 requirements such as mobile networks. All packet payloads in SILC
174 can be also compressed.
176 The security of SILC protocol, and for any security protocol for that
177 matter, is based on strong and secure key exchange protocol. The SILC
178 Key Exchange protocol is described in [SILC3] along with connection
179 authentication protocol and should be read to fully comprehend this
180 document and protocol.
182 The SILC protocol has been developed to work on TCP/IP network
183 protocol, although it could be made to work on other network protocols
184 with only minor changes. However, it is recommended that TCP/IP
185 protocol is used under SILC protocol. Typical implementation would
186 be made in client-server model.
190 1.1 Requirements Terminology
192 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
193 MAY, and OPTIONAL, when they appear in this document, are to be
194 interpreted as described in [RFC2119].
200 This section describes various SILC protocol concepts that forms the
201 actual protocol, and in the end, the actual SILC network. The mission
202 of the protocol is to deliver messages from clients to other clients
203 through routers and servers in secure manner. The messages may also
204 be delivered from one client to many clients forming a group, also
207 This section does not focus to security issues. Instead, basic network
208 concepts are introduced to make the topology of the SILC network
213 2.1 SILC Network Topology
215 SILC network forms a ring as opposed to tree style network topology that
216 conferencing protocols usually have. The network has a cells which are
217 constructed from router and zero or more servers. The servers are
218 connected to the router in a star like network topology. Routers in the
219 network are connected to each other forming a ring. The rationale for
220 this is to have servers that can perform specific kind of tasks what
221 other servers cannot perform. This leads to two kinds of servers; normal
222 SILC servers and SILC routers.
224 A difference between normal server and router server is that routers
225 knows everything about everything in the network. They also do the
226 actual routing of the messages to the correct receiver. Normal servers
227 knows only about local information and nothing about global information.
228 This makes the network faster as there are less servers that needs to
229 keep global information up to date at all time.
231 This, on the other hand, leads to kind of a cellular like network, where
232 routers are in the center of the cell and servers are connected to the
235 The following diagram represents SILC network topology.
239 ---- ---- ---- ---- ---- ----
240 | S8 | S5 | S4 | | S7 | S5 | S6 |
241 ----- ---- ----- ----- ---- -----
242 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
243 ---- ------ ---- ---- ------ ----
244 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
245 ---- ---- ---- ---- ---- ---- | S3 | S1 |
246 Cell 1. \\ Cell 2. | \\____ ----- -----
248 ---- ---- ---- ---- ---- ---- ---- ------
249 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
250 ----- ---- ----- ----- ---- ----- ---- ----
251 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
252 ---- ------ ---- ---- ------ ----
253 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
254 ---- ---- ---- ---- ---- ----
259 Figure 1: SILC Network Topology
262 A cell is formed when a server or servers connect to one router. In
263 SILC network normal server cannot directly connect to other normal
264 server. Normal server may only connect to SILC router which then
265 routes the messages to the other servers in the cell. Router servers
266 on the other hand may connect to other routers to form the actual SILC
267 network, as seen in above figure. However, router is also normal SILC
268 server; clients may connect to it the same way as to normal SILC
269 server. Normal server also cannot have active connections to more
270 than one router. Normal server cannot be connected to two different
271 cells. Router servers, on the other hand, may have as many router to
272 router connections as needed.
274 There are many issues in this network topology that needs to be careful
275 about. Issues like the size of the cells, the number of the routers in
276 the SILC network and the capacity requirements of the routers. These
277 issues should be discussed in the Internet Community and additional
278 documents on the issue may be written.
282 2.2 Communication Inside a Cell
284 It is always guaranteed that inside a cell message is delivered to the
285 recipient with at most two server hops. A client which is connected to
286 server in the cell and is talking on channel to other client connected
287 to other server in the same cell, will have its messages delivered from
288 its local server first to the router of the cell, and from the router
289 to the other server in the cell.
291 The following diagram represents this scenario:
305 Figure 2: Communication Inside cell
308 Example: Client 1. connected to Server 1. send message to
309 Client 4. connected to Server 2. travels from Server 1.
310 first to Router which routes the message to Server 2.
311 which then sends it to the Client 4. All the other
312 servers in the cell will not see the routed message.
315 If the client is connected directly to the router, as router is also normal
316 SILC server, the messages inside the cell are always delivered only with
317 one server hop. If clients communicating with each other are connected
318 to the same server, no router interaction is needed. This is the optimal
319 situation of message delivery in the SILC network.
323 2.3 Communication in the Network
325 If the message is destined to server that does not belong to local cell
326 the message is routed to the router server to which the destination
327 server belongs, if the local router is connected to destination router.
328 If there is no direct connection to the destination router, the local
329 router routes the message to its primary route. The following diagram
330 represents message sending between cells.
336 1 --- S1 S4 --- 5 S2 --- 1
337 S/R - - - - - - - - S/R
347 Figure 3: Communication Between Cells
350 Example: Client 5. connected to Server 4. in Cell 1. sends message
351 to Client 2. connected to Server 1. in Cell 2. travels
352 from Server 4. to Router which routes the message to
353 Router in Cell 2, which then routes the message to
354 Server 1. All the other servers and routers in the
355 network will not see the routed message.
358 The optimal case of message delivery from the client point of view is
359 when clients are connected directly to the routers and the messages
360 are delivered from one router to the other.
364 2.4 Channel Communication
366 Messages may be sent to group of clients as well. Sending messages to
367 many clients works the same way as sending messages point to point, from
368 message delivery point of view. Security issues are another matter
369 which are not discussed in this section.
371 Router server handles the message routing to multiple recipients. If
372 any recipient is not in the same cell as the sender the messages are
375 Server distributes the channel message to its local clients which are
376 joined to the channel. Router also distributes the message to its
377 local clients on the channel.
382 2.5 Router Connections
384 Router connections play very important role in making the SILC like
385 network topology to work. For example, sending broadcast packets in
386 SILC network require special connections between routers; routers must
387 be connected in a specific way.
389 Every router has their primary route which is a connection to another
390 router in the network. Unless there is only two routers in the network
391 must not routers use each other as their primary routes. The router
392 connections in the network must form a ring.
394 Example with three routers in the network:
399 S/R1 - < - < - < - < - < - < - S/R2
402 \\ - > - > - S/R3 - > - > - /
407 Figure 4: Router Connections
410 Example: Network with three routers. Router 1. uses Router 2. as its
411 primary router. Router 2. uses Router 3. as its primary router,
412 and Router 3. uses Router 1. as its primary router. There may
413 be other direct connections between the routers but they must
414 not be used as primary routes.
416 The above example is applicable to any amount of routers in the network
417 except for two routers. If there are only two routers in the network both
418 routers must be able to handle situation where they use each other as their
421 The issue of router connections are very important especially with SILC
422 broadcast packets. Usually all router wide information in the network is
423 distributed by SILC broadcast packets. This sort of ring network, with
424 ability to have other direct routes in the network cause interesting
425 routing problems. The [SILC2] discusses the routing of packets in this
426 sort of network in more detail.
430 3. SILC Specification
432 This section describes the SILC protocol. However, [SILC2] and
433 [SILC3] describes other important protocols that are part of this SILC
434 specification and must be read.
440 A client is a piece of software connecting to SILC server. SILC client
441 cannot be SILC server. Purpose of clients is to provide the user
442 interface of the SILC services for end user. Clients are distinguished
443 from other clients by unique Client ID. Client ID is a 128 bit ID that
444 is used in the communication in the SILC network. The client ID is
445 based on the nickname selected by the user. User uses logical nicknames
446 in communication which are then mapped to the corresponding Client ID.
447 Client ID's are low level identifications and should not be seen by the
450 Clients provide other information about the end user as well. Information
451 such as the nickname of the user, username and the host name of the end
452 user and user's real name. See section 3.2 Server for information of
453 the requirements of keeping this information.
455 The nickname selected by the user is not unique in the SILC network.
456 There can be 2^8 same nicknames for one IP address. As for comparison to
457 IRC [IRC] where nicknames are unique this is a fundamental difference
458 between SILC and IRC. This typically causes the server names or client's
459 host names to be used along with the nicknames on user interface to
460 identify specific users when sending messages. This feature of SILC
461 makes IRC style nickname-wars obsolete as no one owns their nickname;
462 there can always be someone else with the same nickname. The maximum
463 length of nickname is 128 bytes.
469 Client ID is used to identify users in the SILC network. The Client ID
470 is unique to the extent that there can be 2^128 different Client ID's,
471 and ID's based on IPv6 addresses extends this to 2^224 different Client
472 ID's. Collisions are not expected to happen. The Client ID is defined
476 128 bit Client ID based on IPv4 addresses:
478 32 bit Server ID IP address (bits 1-32)
479 8 bit Random number or counter
480 88 bit Truncated MD5 hash value of the nickname
482 224 bit Client ID based on IPv6 addresses:
484 128 bit Server ID IP address (bits 1-128)
485 8 bit Random number or counter
486 88 bit Truncated MD5 hash value of the nickname
488 o Server ID IP address - Indicates the server where this
489 client is coming from. The IP address hence equals the
490 server IP address where to the client has connected.
492 o Random number or counter - Random number to further
493 randomize the Client ID. Another choice is to use
494 a counter starting from the zero (0). This makes it
495 possible to have 2^8 same nicknames from the same
498 o MD5 hash - MD5 hash value of the lowercase nickname is
499 truncated taking 88 bits from the start of the hash value.
500 This hash value is used to search the user's Client ID
501 from the ID lists. Note that the nickname MUST be in
505 Collisions could occur when more than 2^8 clients using same nickname
506 from the same server IP address is connected to the SILC network.
507 Server MUST be able to handle this situation by refusing to accept
508 anymore of that nickname.
510 Another possible collision may happen with the truncated hash value of
511 the nickname. It could be possible to have same truncated hash value for
512 two different nicknames. However, this is not expected to happen nor
513 cause any problems if it would occur. Nicknames are usually logical and
514 it is unlikely to have two distinct logical nicknames produce same
515 truncated hash value.
521 Servers are the most important parts of the SILC network. They form the
522 basis of the SILC, providing a point to which clients may connect to.
523 There are two kinds of servers in SILC; normal servers and router servers.
524 This section focus on the normal server and router server is described
525 in the section 3.3 Router.
527 Normal servers MUST NOT directly connect to other normal server. Normal
528 servers may only directly connect to router server. If the message sent
529 by the client is destined outside the local server it is always sent to
530 the router server for further routing. Server may only have one active
531 connection to router on same port. Normal server MUST NOT connect to other
532 cell's router except in situations where its cell's router is unavailable.
536 3.2.1 Server's Local ID List
538 Normal server keeps various information about the clients and their end
539 users connected to it. Every normal server MUST keep list of all locally
540 connected clients, Client ID's, nicknames, usernames and host names and
541 user's real name. Normal servers only keeps local information and it
542 does not keep any global information. Hence, normal servers knows only
543 about their locally connected clients. This makes servers efficient as
544 they don't have to worry about global clients. Server is also responsible
545 of creating the Client ID's for their clients.
547 Normal server also keeps information about locally created channels and
550 Hence, local list for normal server includes:
553 server list - Router connection
561 client list - All clients in server
571 channel list - All channels in server
574 o Client ID's on channel
575 o Client ID modes on channel
583 Servers are distinguished from other servers by unique 64 bit Server ID
584 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
585 the SILC to route messages to correct servers. Server ID's also provide
586 information for Client ID's, see section 3.1.1 Client ID. Server ID is
590 64 bit Server ID based on IPv4 addresses:
592 32 bit IP address of the server
596 160 bit Server ID based on IPv6 addresses:
598 128 bit IP address of the server
602 o IP address of the server - This is the real IP address of
605 o Port - This is the port the server is bound to.
607 o Random number - This is used to further randomize the Server ID.
610 Collisions are not expected to happen in any conditions. The Server ID
611 is always created by the server itself and server is responsible of
612 distributing it to the router.
616 3.2.3 SILC Server Ports
618 The following ports has been assigned by IANA for the SILC protocol:
626 If there are needs to create new SILC networks in the future the port
627 numbers must be officially assigned by the IANA.
629 Server on network above privileged ports (>1023) SHOULD NOT be trusted
630 as they could have been set up by untrusted party.
636 Router server in SILC network is responsible for keeping the cell together
637 and routing messages to other servers and to other routers. Router server
638 is also a normal server thus clients may connect to it as it would be
639 just normal SILC server.
641 However, router servers has a lot of important tasks that normal servers
642 do not have. Router server knows everything about everything in the SILC.
643 They know all clients currently on SILC, all servers and routers and all
644 channels in SILC. Routers are the only servers in SILC that care about
645 global information and keeping them up to date at all time. And, this
646 is what they must do.
650 3.3.1 Router's Local ID List
652 Router server as well MUST keep local list of connected clients and
653 locally created channels. However, this list is extended to include all
654 the informations of the entire cell, not just the server itself as for
657 However, on router this list is a lot smaller since routers do not need
658 to keep information about user's nickname, username and host name and real
659 name since these are not needed by the router. The router keeps only
660 information that it needs.
662 Hence, local list for router includes:
665 server list - All servers in the cell
672 client list - All clients in the cell
675 channel list - All channels in the cell
677 o Client ID's on channel
678 o Client ID modes on channel
683 Note that locally connected clients and other information include all the
684 same information as defined in section section 3.2.1 Server's Local ID
689 3.3.2 Router's Global ID List
691 Router server MUST also keep global list. Normal servers do not have
692 global list as they know only about local information. Global list
693 includes all the clients on SILC, their Client ID's, all created channels
694 and their Channel ID's and all servers and routers on SILC and their
695 Server ID's. That is said, global list is for global information and the
696 list must not include the local information already on the router's local
699 Note that the global list does not include information like nicknames,
700 usernames and host names or user's real names. Router does not need to
701 keep these informations as they are not needed by the router. This
702 information is available from the client's server which maybe queried
705 Hence, global list includes:
708 server list - All servers in SILC
713 client list - All clients in SILC
716 channel list - All channels in SILC
718 o Client ID's on channel
719 o Client ID modes on channel
725 3.3.3 Router's Server ID
727 Router's Server ID's are equivalent to normal Server ID's. As routers
728 are normal servers as well same types of ID's applies for routers as well.
729 Thus, see section 3.2.2 Server ID.
735 A channel is a named group of one or more clients which will all receive
736 messages addressed to that channel. The channel is created when first
737 client requests JOIN command to the channel, and the channel ceases to
738 exist when the last client has left it. When channel exists, any client
739 can reference it using the name of the channel. If the channel has
740 a founder mode set and last client leaves the channel the channel does
741 not cease to exist. The founder mode can be used to make permanent
742 channels in the network. The founder of the channel can regain the
743 channel founder privileges on the channel later when he joins the
746 Channel names are unique although the real uniqueness comes from 64 bit
747 Channel ID. However, channel names are still unique and no two global
748 channels with same name may exist. The channel name is a string of
749 maximum length of 256 bytes. Channel names MUST NOT contain any
750 whitespaces (` '), any non-printable ASCII characters, commas (`,')
751 and wildcard characters.
753 Channels can have operators that can administrate the channel and
754 operate all of its modes. The following operators on channel exist on
758 o Channel founder - When channel is created the joining client becomes
759 channel founder. Channel founder is channel operator with some more
760 privileges. Basically, channel founder can fully operate the channel
761 and all of its modes. The privileges are limited only to the
762 particular channel. There can be only one channel founder per
763 channel. Channel founder supersedes channel operator's privileges.
765 Channel founder privileges cannot be removed by any other operator on
766 channel. When channel founder leaves the channel there is no channel
767 founder on the channel. However, it is possible to set a mode for
768 the channel which allows the original channel founder to regain the
769 founder privileges even after leaving the channel. Channel founder
770 also cannot be removed by force from the channel.
772 o Channel operator - When client joins to channel that has not existed
773 previously it will become automatically channel operator (and channel
774 founder discussed above). Channel operator is able administrate the
775 channel, set some modes on channel, remove a badly behaving client
776 from the channel and promote other clients to become channel
777 operator. The privileges are limited only to the particular channel.
779 Normal channel user may be promoted (opped) to channel operator
780 gaining channel operator privileges. Channel founder or other
781 channel operator may also demote (deop) channel operator to normal
789 Channels are distinguished from other channels by unique Channel ID.
790 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
791 collisions are not expected to happen in any conditions. Channel names
792 are just for logical use of channels. The Channel ID is created by the
793 server where the channel is created. The Channel ID is defined as
797 64 bit Channel ID based on IPv4 addresses:
799 32 bit Router's Server ID IP address (bits 1-32)
800 16 bit Router's Server ID port (bits 33-48)
803 160 bit Channel ID based on IPv6 addresses:
805 128 bit Router's Server ID IP address (bits 1-128)
806 16 bit Router's Server ID port (bits 129-144)
809 o Router's Server ID IP address - Indicates the IP address of
810 the router of the cell where this channel is created. This is
811 taken from the router's Server ID. This way SILC router knows
812 where this channel resides in the SILC network.
814 o Router's Server ID port - Indicates the port of the channel on
815 the server. This is taken from the router's Server ID.
817 o Random number - To further randomize the Channel ID. This makes
818 sure that there are no collisions. This also means that
819 in a cell there can be 2^16 channels.
826 Operators are normal users with extra privileges to their server or
827 router. Usually these people are SILC server and router administrators
828 that take care of their own server and clients on them. The purpose of
829 operators is to administrate the SILC server or router. However, even
830 an operator with highest privileges is not able to enter invite-only
831 channel, to gain access to the contents of a encrypted and authenticated
832 packets traveling in the SILC network or to gain channel operator
833 privileges on public channels without being promoted. They have the
834 same privileges as everyone else except they are able to administrate
835 their server or router.
841 Commands are very important part on SILC network especially for client
842 which uses commands to operate on the SILC network. Commands are used
843 to set nickname, join to channel, change modes and many other things.
845 Client usually sends the commands and server replies by sending a reply
846 packet to the command. Server MAY also send commands usually to serve
847 the original client's request. Usually server cannot send commands to
848 clients, however there MAY be commands that allow the server to send
849 commands to client. By default servers MAY send commands only to other
852 Note that the command reply is usually sent only after client has sent
853 the command request but server is allowed to send command reply packet
854 to client even if client has not requested the command. Client MAY
855 choose to ignore the command reply.
857 It is expected that some of the commands may be miss-used by clients
858 resulting various problems on the server side. Every implementation
859 SHOULD assure that commands may not be executed more than once, say,
860 in two (2) seconds. However, to keep response rate up, allowing for
861 example five (5) commands before limiting is allowed. It is RECOMMENDED
862 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
863 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
864 as they require heavy operations. This should be sufficient to prevent
865 the miss-use of commands.
867 SILC commands are described in [SILC4].
873 Packets are naturally the most important part of the protocol and the
874 packets are what actually makes the protocol. Packets in SILC network
875 are always encrypted using, usually the shared secret session key
876 or some other key, for example, channel key, when encrypting channel
877 messages. It is not possible to send packet in SILC network without
878 encryption. The SILC Packet Protocol is a wide protocol and is described
879 in [SILC2]. This document does not define or describe details of
884 3.8 Packet Encryption
886 All packets passed in SILC network MUST be encrypted. This section
887 defines how packets must be encrypted in the SILC network. The detailed
888 description of the actual encryption process of the packets are
889 described in [SILC2].
891 Client and its server shares secret symmetric session key which is
892 established by the SILC Key Exchange Protocol, described in [SILC3].
893 Every packet sent from client to server, with exception of packets for
894 channels, are encrypted with this session key.
896 Channels has a channel key that are shared by every client on the channel.
897 However, the channel keys are cell specific thus one cell does not know
898 the channel key of the other cell, even if that key is for same channel.
899 Channel key is also known by the routers and all servers that has clients
900 on the channel. However, channels MAY have channel private keys that
901 are entirely local setting for the client. All clients on the channel
902 MUST know the channel private key before hand to be able to talk on the
903 channel. In this case, no server or router know the key for channel.
905 Server shares secret symmetric session key with router which is
906 established by the SILC Key Exchange Protocol. Every packet passed from
907 server to router, with exception of packets for channels, are encrypted
908 with the shared session key. Same way, router server shares secret
909 symmetric key with its primary route. However, every packet passed
910 from router to other router, including packets for channels, are
911 encrypted with the shared session key. Every router connection has
912 their own session keys.
916 3.8.1 Determination of the Source and the Destination
918 The source and the destination of the packet needs to be determined
919 to be able to route the packets to correct receiver. This information
920 is available in the SILC Packet Header which is included in all packets
921 sent in SILC network. The SILC Packet Header is described in [SILC2].
923 The header MUST be encrypted with the session key who is next receiver
924 of the packet along the route. The receiver of the packet, for example
925 a router along the route, is able to determine the sender and the
926 destination of the packet by decrypting the SILC Packet Header and
927 checking the ID's attached to the header. The ID's in the header will
928 tell to where the packet needs to be sent and where it is coming from.
930 The header in the packet MUST NOT change during the routing of the
931 packet. The original sender, for example client, assembles the packet
932 and the packet header and server or router between the sender and the
933 receiver MUST NOT change the packet header. Note however, that some
934 packets such as commands may be resent by a server to serve the client's
935 original command. In this case the command packet sent by the server
936 includes the server's IDs.
938 Note that the packet and the packet header may be encrypted with
939 different keys. For example, packets to channels are encrypted with
940 the channel key, however, the header is encrypted with the session key
941 as described above. However, the header and the packet may be encrypted
942 with same key. This is the case, for example, with command packets.
946 3.8.2 Client To Client
948 The process of message delivery and encryption from client to another
949 client is as follows.
951 Example: Private message from client to another client on different
952 servers. Clients do not share private message delivery
953 keys; normal session keys are used.
955 o Client 1. sends encrypted packet to its server. The packet is
956 encrypted with the session key shared between client and its
959 o Server determines the destination of the packet and decrypts
960 the packet. Server encrypts the packet with session key shared
961 between the server and its router, and sends the packet to the
964 o Router determines the destination of the packet and decrypts
965 the packet. Router encrypts the packet with session key
966 shared between the router and the destination server, and sends
967 the packet to the server.
969 o Server determines the client to which the packet is destined
970 to and decrypts the packet. Server encrypts the packet with
971 session key shared between the server and the destination client,
972 and sends the packet to the client.
974 o Client 2. decrypts the packet.
977 Example: Private message from client to another client on different
978 servers. Clients has established secret shared private
979 message delivery key with each other and that is used in
980 the message encryption.
982 o Client 1. sends encrypted packet to its server. The packet header
983 is encrypted with the session key shared between the client and
984 server, and the private message is encrypted with the private
985 message delivery key shared between clients.
987 o Server determines the destination of the packet and sends the
988 packet to the router.
990 o Router determines the destination of the packet and sends the
991 packet to the server.
993 o Server determines the client to which the packet is destined
994 to and sends the packet to the client.
996 o Client 2. decrypts the packet with the secret shared key.
998 If clients share secret key with each other the private message
999 delivery is much simpler since servers and routers between the
1000 clients do not need to decrypt and re-encrypt the packet.
1002 The process for clients on same server is much simpler as there are
1003 no need to send the packet to the router. The process for clients
1004 on different cells is same as above except that the packet is routed
1005 outside the cell. The router of the destination cell routes the
1006 packet to the destination same way as described above.
1010 3.8.3 Client To Channel
1012 Process of message delivery from client on channel to all the clients
1015 Example: Channel of four users; two on same server, other two on
1016 different cells. Client sends message to the channel.
1018 o Client 1. encrypts the packet with channel key and sends the
1019 packet to its server.
1021 o Server determines local clients on the channel and sends the
1022 packet to the Client on the same server. Server then sends
1023 the packet to its router for further routing.
1025 o Router determines local clients on the channel, if found
1026 sends packet to the local clients. Router determines global
1027 clients on the channel and sends the packet to its primary
1028 router or fastest route.
1030 o (Other router(s) do the same thing and sends the packet to
1033 o Server determines local clients on the channel and sends the
1034 packet to the client.
1036 o All clients receiving the packet decrypts it.
1040 3.8.4 Server To Server
1042 Server to server packet delivery and encryption is described in above
1043 examples. Router to router packet delivery is analogous to server to
1044 server. However, some packets, such as channel packets, are processed
1045 differently. These cases are described later in this document and
1046 more in detail in [SILC2].
1050 3.9 Key Exchange And Authentication
1052 Key exchange is done always when for example client connects to server
1053 but also when server and router, and router and router connects to each
1054 other. The purpose of key exchange protocol is to provide secure key
1055 material to be used in the communication. The key material is used to
1056 derive various security parameters used to secure SILC packets. The
1057 SILC Key Exchange protocol is described in detail in [SILC3].
1059 Authentication is done after key exchange protocol has been successfully
1060 completed. The purpose of authentication is to authenticate for example
1061 client connecting to the server. However, clients may be accepted
1062 to connect to server without explicit authentication. Servers are
1063 required to use authentication protocol when connecting. The
1064 authentication may be based on passphrase (pre-shared-secret) or public
1065 key based on digital signatures. All passphrases sent in SILC protocol
1066 MUST be UTF-8 [RFC2279] encoded. The connection authentication protocol
1067 is described in detail in [SILC3].
1071 3.9.1 Authentication Payload
1073 Authentication payload is used separately from the SKE and the Connection
1074 Authentication protocol. It can be used during the session to authenticate
1075 with the remote. For example, the client can authenticate itself to the
1076 server to become server operator. In this case, Authentication Payload is
1079 The format of the Authentication Payload is as follows:
1084 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
1085 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1086 | Payload Length | Authentication Method |
1087 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1088 | Public Data Length | |
1089 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1093 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1094 | Authentication Data Length | |
1095 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1097 ~ Authentication Data ~
1099 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1103 Figure 5: Authentication Payload
1107 o Payload Length (2 bytes) - Length of the entire payload.
1109 o Authentication Method (2 bytes) - The method of the
1110 authentication. The authentication methods are defined
1111 in [SILC2] in the Connection Auth Request Payload. The NONE
1112 authentication method SHOULD NOT be used.
1114 o Public Data Length (2 bytes) - Indicates the length of
1115 the Public Data field.
1117 o Public Data (variable length) - This is defined only if
1118 the authentication method is public key. If it is any other
1119 this field MAY include a random data for padding purposes.
1120 However, in this case the field MUST be ignored by the
1123 When the authentication method is public key this includes
1124 128 to 4096 bytes of non-zero random data that is used in
1125 the signature process, described subsequently.
1127 o Authentication Data Length (2 bytes) - Indicates the
1128 length of the Authentication Data field. If zero (0)
1129 value is found in this field the payload MUST be
1132 o Authentication Data (variable length) - Authentication
1133 method dependent authentication data.
1137 If the authentication method is password based, the Authentication
1138 Data field includes the plaintext UTF-8 encoded password. It is safe
1139 to send plaintext password since the entire payload is encrypted. In
1140 this case the Public Data Length is set to zero (0), but MAY also include
1141 random data for padding purposes. It is also RECOMMENDED that maximum
1142 amount of padding is applied to SILC packet when using password based
1143 authentication. This way it is not possible to approximate the length
1144 of the password from the encrypted packet.
1146 If the authentication method is public key based (or certificate)
1147 the Authentication Data is computed as follows:
1149 HASH = hash(random bytes | ID | public key (or certificate));
1150 Authentication Data = sign(HASH);
1152 The hash() and the sign() are the hash function and the public key
1153 cryptography function selected in the SKE protocol, unless otherwise
1154 stated in the context where this payload is used. The public key
1155 is SILC style public key unless certificates are used. The ID is the
1156 entity's ID (Client or Server ID) which is authenticating itself. The
1157 ID encoding is described in [SILC2]. The random bytes are non-zero
1158 random bytes of length between 128 and 4096 bytes, and will be included
1159 into the Public Data field as is.
1161 The receiver will compute the signature using the random data received
1162 in the payload, the ID associated to the connection and the public key
1163 (or certificate) received in the SKE protocol. After computing the
1164 receiver MUST verify the signature. In case of public key authentication
1165 also this payload is encrypted.
1171 This section defines all the allowed algorithms that can be used in
1172 the SILC protocol. This includes mandatory cipher, mandatory public
1173 key algorithm and MAC algorithms.
1179 Cipher is the encryption algorithm that is used to protect the data
1180 in the SILC packets. See [SILC2] of the actual encryption process and
1181 definition of how it must be done. SILC has a mandatory algorithm that
1182 must be supported in order to be compliant with this protocol.
1184 The following ciphers are defined in SILC protocol:
1186 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1187 aes-256-ctr AES in CTR mode, 256 bit key (RECOMMENDED)
1188 aes-256-rcbc AES in randomized CBC mode, 256 bit key (OPTIONAL)
1189 aes-192-<mode> AES in <mode> mode, 192 bit key (OPTIONAL)
1190 aes-128-<mode> AES in <mode> mode, 128 bit key (RECOMMENDED)
1191 twofish-256-<mode> Twofish in <mode> mode, 256 bit key (OPTIONAL)
1192 twofish-192-<mode> Twofish in <mode> mode, 192 bit key (OPTIONAL)
1193 twofish-128-<mode> Twofish in <mode> mode, 128 bit key (OPTIONAL)
1194 cast-256-<mode> CAST-256 in <mode> mode, 256 bit key (OPTIONAL)
1195 cast-192-<mode> CAST-256 in <mode> mode, 192 bit key (OPTIONAL)
1196 cast-128-<mode> CAST-256 in <mode> mode, 128 bit key (OPTIONAL)
1197 serpent-<len>-<mode> Serpent in <mode> mode, <len> bit key (OPTIONAL)
1198 rc6-<len>-<mode> RC6 in <mode> mode, <len> bit key (OPTIONAL)
1199 mars-<len>-<mode> MARS in <mode> mode, <len> bit key (OPTIONAL)
1200 none No encryption (OPTIONAL)
1202 The <mode> is either "cbc", "ctr" or "rcbc". Other encryption modes MAY
1203 be defined as to be used in SILC using the same format. The <len> is
1204 either 256, 192 or 128 bit key length. Also, additional ciphers MAY be
1205 defined to be used in SILC by using the same name format as above.
1207 Algorithm "none" does not perform any encryption process at all and
1208 thus is not recommended to be used. It is recommended that no client
1209 or server implementation would accept none algorithm except in special
1216 The "cbc" encryption mode is CBC mode with inter-packet chaining. This
1217 means that the Initial Vector (IV) for the next encryption block is
1218 the previous ciphertext block. The very first IV MUST be random and is
1219 generated as described in [SILC3].
1225 The "ctr" encryption mode is CTR mode. The CTR mode in SILC is stateful
1226 in encryption and decryption. Both sender and receiver maintain the
1227 counter for the CTR mode and thus can precompute the key stream for
1228 encryption and decryption. By default, CTR mode does not require
1229 plaintext padding, however implementations MAY apply padding to the
1230 packets. If the last key block is larger than the last plaintext block
1231 the resulted value is truncated to the size of the plaintext block and
1232 the most significant bits are used. When sending authentication data
1233 inside packets the maximum amount of padding SHOULD be applied with
1236 In CTR mode only the encryption operation of the cipher is used. The
1237 decryption operation is not needed since both encryption and decryption
1238 process is simple XOR with the plaintext block and the key stream block.
1240 The counter block is used to create the key for the CTR mode. When
1241 SILC specifications refer to Initial Vector (IV) in general cases, in
1242 case of CTR mode it refers to the counter block. The format of the
1243 128 bit counter block is as follows:
1248 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
1249 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1250 | Truncated HASH from SKE |
1251 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1252 | Sending/Receiving IV from SKE |
1254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1260 Figure 6: Counter Block
1263 o Truncated HASH from SKE (4 bytes) - This value is the first 4
1264 bytes from the HASH value that was computed as a result of SKE
1265 protocol. This acts as session identifier and each rekey MUST
1266 produce a new HASH value.
1268 o Sending/Receiving IV from SKE (8 bytes) - This value is the
1269 first 8 bytes from the Sending IV or Receiving IV generated in
1270 the SKE protocol. When this mode is used to encrypt sending
1271 traffic the Sending IV is used, when used to decrypt receiving
1272 traffic the Receiving IV is used. This assures that two parties
1273 of the protocol use different IV for sending traffic. Each rekey
1274 MUST produce a new value.
1276 o Block Counter (4 bytes) - This is the counter value for the
1277 counter block and is MSB ordered number starting from one (1)
1278 value for first block and incrementing for subsequent blocks.
1279 The same value MUST NOT be used twice. The rekey MUST be
1280 performed before this counter value wraps.
1283 CTR mode MUST NOT be used with "none" MAC. Implementations also MUST
1284 assure that the same counter block is not used to encrypt more than
1285 one block. Also, the key material used with CTR mode MUST be fresh
1286 key material. Static keys (pre-shared keys) MUST NOT be used with
1287 CTR mode. For this reason using CTR mode to encrypt for example
1288 channel messages or private messages with a pre-shared key is
1289 inappropriate. For private messages, the Key Agreement could be
1290 performed to produce fresh key material.
1292 If the IV Included flag was negotiated in SKE, implementations SHOULD
1293 still use the same counter block format as defined above. However,
1294 implementations are RECOMMENDED to replace the Truncated HASH field
1295 with a 32 bit random value for each IV (counter block) per encrypted
1296 SILC packet. Also note, that in this case the decryption process is
1297 not stateful and receiver cannot precompute the key stream.
1301 3.10.1.3 Randomized CBC Mode
1303 The "rcbc" encryption mode is CBC mode with randomized IV. This means
1304 that each IV for each packet MUST be chosen randomly. When encrypting
1305 more than one block the normal inter-packet chaining is used, but for
1306 the first block new random IV is selected in each packet. In this mode
1307 the IV is appended at the end of the last ciphertext block and thus
1308 delivered to the recipient. This mode increases the ciphertext size by
1309 one ciphertext block. Note also that some data payloads in SILC are
1310 capable of delivering the IV to the recipient. When explicitly
1311 encrypting these payloads with randomized CBC the IV MUST NOT be appended
1312 at the end of the ciphertext. When encrypting these payloads with
1313 "cbc" mode they implicitly become randomized CBC since the IV is
1314 usually selected random and included in the ciphertext. In these
1315 cases using either CBC or randomized CBC is actually equivalent.
1319 3.10.2 Public Key Algorithms
1321 Public keys are used in SILC to authenticate entities in SILC network
1322 and to perform other tasks related to public key cryptography. The
1323 public keys are also used in the SILC Key Exchange protocol [SILC3].
1325 The following public key algorithms are defined in SILC protocol:
1332 DSS is described in [Menezes]. The RSA MUST be implemented according
1333 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1334 compliant to either PKCS #1 version 1.5 or newer with the following
1335 notes: The signature encoding is always in same format as the encryption
1336 encoding regardless of the PKCS #1 version. The signature with appendix
1337 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1338 rationale for this is that there is no binding between the PKCS #1 OIDs
1339 and the hash algorithms used in the SILC protocol. Hence, the encoding
1340 is always in PKCS #1 version 1.5 format.
1342 Additional public key algorithms MAY be defined to be used in SILC.
1344 When signatures are computed in SILC the computing of the signature is
1345 represented as sign(). The signature computing procedure is dependent
1346 of the public key algorithm, and the public key or certificate encoding.
1347 When using SILC public key the signature is computed as described in
1348 previous paragraph for RSA and DSS keys. If the hash function is not
1349 specified separately for signing process sha1 MUST be used. When using
1350 SSH2 public keys the signature is computed as described in [SSH-TRANS].
1351 When using X.509 version 3 certificates the signature is computed as
1352 described in [PKCS7]. When using OpenPGP certificates the signature is
1353 computed as described in [PGP].
1357 3.10.3 Hash Functions
1359 Hash functions are used as part of MAC algorithms defined in the next
1360 section. They are also used in the SILC Key Exchange protocol defined
1363 The following Hash algorithm are defined in SILC protocol:
1366 sha1 SHA-1, length = 20 (REQUIRED)
1367 md5 MD5, length = 16 (RECOMMENDED)
1373 3.10.4 MAC Algorithms
1375 Data integrity is protected by computing a message authentication code
1376 (MAC) of the packet data. See [SILC2] for details how to compute the
1379 The following MAC algorithms are defined in SILC protocol:
1382 hmac-sha1-96 HMAC-SHA1, length = 12 bytes (REQUIRED)
1383 hmac-md5-96 HMAC-MD5, length = 12 bytes (OPTIONAL)
1384 hmac-sha1 HMAC-SHA1, length = 20 bytes (OPTIONAL)
1385 hmac-md5 HMAC-MD5, length = 16 bytes (OPTIONAL)
1386 none No MAC (OPTIONAL)
1389 The "none" MAC is not recommended to be used as the packet is not
1390 authenticated when MAC is not computed. It is recommended that no
1391 client or server would accept none MAC except in special debugging
1394 The HMAC algorithm is described in [HMAC] and hash algorithms that
1395 are used as part of the HMACs are described in [Scheneir] and in
1398 Additional MAC algorithms MAY be defined to be used in SILC.
1402 3.10.5 Compression Algorithms
1404 SILC protocol supports compression that may be applied to unencrypted
1405 data. It is recommended to use compression on slow links as it may
1406 significantly speed up the data transmission. By default, SILC does not
1407 use compression which is the mode that must be supported by all SILC
1410 The following compression algorithms are defined:
1413 none No compression (REQUIRED)
1414 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1417 Additional compression algorithms MAY be defined to be used in SILC.
1422 3.11 SILC Public Key
1424 This section defines the type and format of the SILC public key. All
1425 implementations MUST support this public key type. See [SILC3] for
1426 other optional public key and certificate types allowed in the SILC
1427 protocol. Public keys in SILC may be used to authenticate entities
1428 and to perform other tasks related to public key cryptography.
1430 The format of the SILC Public Key is as follows:
1436 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
1437 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1438 | Public Key Length |
1439 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1440 | Algorithm Name Length | |
1441 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1445 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1446 | Identifier Length | |
1447 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1459 Figure 5: SILC Public Key
1463 o Public Key Length (4 bytes) - Indicates the full length
1464 of the public key, not including this field.
1466 o Algorithm Name Length (2 bytes) - Indicates the length
1467 of the Algorithm Length field, not including this field.
1469 o Algorithm name (variable length) - Indicates the name
1470 of the public key algorithm that the key is. See the
1471 section 3.10.2 Public Key Algorithms for defined names.
1473 o Identifier Length (2 bytes) - Indicates the length of
1474 the Identifier field, not including this field.
1476 o Identifier (variable length) - Indicates the identifier
1477 of the public key. This data can be used to identify
1478 the owner of the key. The identifier is of the following
1482 HN Host name or IP address
1489 Examples of an identifier:
1491 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1493 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1495 At least user name (UN) and host name (HN) MUST be provided as
1496 identifier. The fields are separated by commas (`,'). If
1497 comma is in the identifier string it must be written as `\\,',
1498 for example, `O=Company XYZ\\, Inc.'.
1500 o Public Data (variable length) - Includes the actual
1501 public data of the public key.
1503 The format of this field for RSA algorithm is
1512 The format of this field for DSS algorithm is
1524 The variable length fields are multiple precession
1525 integers encoded as strings in both examples.
1527 Other algorithms must define their own type of this
1528 field if they are used.
1531 All fields in the public key are in MSB (most significant byte first)
1532 order. All strings in the public key are UTF-8 encoded.
1534 If an external protocol need to refer to SILC Public Key by name, the
1535 name "silc-rsa" and "silc-dss" for SILC Public Key based on RSA algorithm
1536 and SILC Public Key based on DSS algorithm, respectively, are to be used.
1537 However, this SILC specification does not use these names directly, and
1538 they are defined here for external protocols (protocols that may like
1539 to use SILC Public Key).
1543 3.12 SILC Version Detection
1545 The version detection of both client and server is performed at the
1546 connection phase while executing the SILC Key Exchange protocol. The
1547 version identifier is exchanged between initiator and responder. The
1548 version identifier is of the following format:
1551 SILC-<protocol version>-<software version>
1554 The version strings are of the following format:
1557 protocol version = <major>.<minor>
1558 software version = <major>[.<minor>[.<build or vendor string>]]
1561 Protocol version MUST provide both major and minor version. Currently
1562 implementations MUST set the protocol version and accept at least the
1563 protocol version as SILC-1.2-<software version>. If new protocol version
1564 causes incompatibilities with older version the <minor> version number
1565 MUST be incremented. The <major> is incremented if new protocol version
1566 is fully incompatible.
1568 Software version MAY provide major, minor and build (vendor) version.
1569 The software version MAY be freely set and accepted. The version string
1570 MUST consist of printable US-ASCII characters.
1572 Thus, the version strings could be, for example:
1577 SILC-1.2-1.0.VendorXYZ
1578 SILC-1.2-2.4.5 Vendor Limited
1585 Backup routers may exist in the cell in addition of the primary router.
1586 However, they must not be active routers and act as routers in the cell.
1587 Only one router may be acting as primary router in the cell. In the case
1588 of failure of the primary router may one of the backup routers become
1589 active. The purpose of backup routers are in case of failure of the
1590 primary router to maintain working connections inside the cell and outside
1591 the cell and to avoid netsplits.
1593 Backup routers are normal servers in the cell that are prepared to take
1594 over the tasks of the primary router if needed. They need to have at
1595 least one direct and active connection to the primary router of the cell.
1596 This communication channel is used to send the router information to
1597 the backup router. When the backup router connects to the primary router
1598 of the cell it MUST present itself as router server in the Connection
1599 Authentication protocol, even though it is normal server as long as the
1600 primary router is available. Reason for this is that the configuration
1601 needed in the responder end requires usually router connection level
1602 configuration. The responder, however must understand and treat the
1603 connection as normal server (except when feeding router level data to
1606 Backup router must know everything that the primary router knows to be
1607 able to take over the tasks of the primary router. It is the primary
1608 router's responsibility to feed the data to the backup router. If the
1609 backup router does not know all the data in the case of failure some
1610 connections may be lost. The primary router of the cell must consider
1611 the backup router being actual router server when it feeds the data to
1614 In addition of having direct connection to the primary router of the
1615 cell, the backup router must also have connection to the same router
1616 the primary router of the cell is connected. However, it must not be
1617 active router connection meaning that the backup router must not use
1618 that channel as its primary route and it must not notify the router
1619 about having connected servers, channels and clients behind it. It
1620 merely connects to the router. This sort of connection is later
1621 referred as being passive connection. Some keepalive actions may be
1622 needed by the router to keep the connection alive.
1624 It is required that other normal servers have passive connections to
1625 the backup router(s) in the cell. Some keepalive actions may be needed
1626 by the server to keep the connection alive. After they notice the
1627 failure of the primary router they must start using the connection to
1628 the first backup router as their primary route.
1630 Also, if any other router in the network is using the cell's primary
1631 router as its own primary router, it must also have passive connection
1632 to the cell's backup router. It too is prepared to switch to use the
1633 backup router as its new primary router as soon as the original primary
1634 router becomes unresponsive.
1636 All of the parties of this protocol knows which one is the backup router
1637 of the cell from their local configuration. Each of the entity must
1638 be configured accordingly and care must be taken when configuring the
1639 backup routers, servers and other routers in the network.
1641 It must be noted that some of the channel messages and private messages
1642 may be lost during the switch to the backup router. The announcements
1643 assures that the state of the network is not lost during the switch.
1645 It is RECOMMENDED that there would be at least one backup router in
1646 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
1647 as backup routers as it requires establishing several connections to
1648 several servers in the cell. Large cells can easily have several
1649 backup routers in the cell.
1651 The order of the backup routers are decided at the configuration phase.
1652 All the parties of this protocol must be configured accordingly to
1653 understand the order of the backup routers. It is not required that
1654 the backup server is actually active server in the cell. Backup router
1655 may be a spare server in the cell that does not accept normal client
1656 connections at all. It may be reserved purely for the backup purposes.
1657 These, however, are cell management issues.
1659 If also the first backup router is down as well and there is another
1660 backup router in the cell then it will start acting as the primary
1661 router as described above.
1665 3.13.1 Switching to Backup Router
1667 When the primary router of the cell becomes unresponsive, for example
1668 by sending EOF to the connection, all the parties of this protocol MUST
1669 replace the old connection to the primary router with first configured
1670 backup router. The backup router usually needs to do local modifications
1671 to its database in order to update all the information needed to maintain
1672 working routes. The backup router must understand that clients that
1673 were originated from the primary router are now originated from some of
1674 the existing server connections and must update them accordingly. It
1675 must also remove those clients that were owned by the primary router
1676 since those connections were lost when the primary router became
1679 All the other parties of the protocol must also update their local
1680 database to understand that the route to the primary router will now go
1681 to the backup router.
1683 Servers connected to the backup router MUST send SILC_PACKET_RESUME_ROUTER
1684 packet with type number 21, to indicate that the server will start using
1685 the backup router as primary router. The backup router MUST NOT allow
1686 this action if it detects that primary is still up and running. If
1687 backup router knows that primary is up and running it MUST send type
1688 number 22 back to the server. The server then MUST NOT use the backup
1689 as primary router, but must try to establish connection back to the
1690 primary router. If the action is allowed type number 21 is sent back
1691 to the server from the backup router.
1693 The servers connected to the backup router must then announce their
1694 clients, channels, channel users, channel user modes and channel modes
1695 to the backup router. This is to assure that none of the important notify
1696 packets were lost during the switch to the backup router. The backup
1697 router must check which of these announced entities it already have
1698 and distribute the new ones to the primary route.
1700 The backup router too must announce its servers, clients, channels
1701 and other information to the new primary router. The primary router
1702 of the backup router too must announce its informations to the backup
1703 router. Both must process only the ones they do not know about. If
1704 any of the announced modes does not match then they are enforced in
1705 normal manner defined later in this specification.
1709 3.13.2 Resuming Primary Router
1711 Usually the primary router is unresponsive only a short period of time
1712 and it is intended that the original router of the cell will resume
1713 its position as primary router when it comes back online. The backup
1714 router that is now acting as primary router of the cell must constantly
1715 try to connect to the original primary router of the cell. It is
1716 RECOMMENDED that it would try to reconnect in 30 second intervals to
1719 When the connection is established to the primary router the backup
1720 resuming protocol is executed. The protocol is advanced as follows:
1722 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1723 value 1 the primary router that came back online. The packet
1724 will indicate the primary router has been replaced by the backup
1725 router. After sending the packet the backup router will announce
1726 all of its channels, channel users, modes etc. to the primary
1729 If the primary knows that it has not been replaced (for example
1730 the backup itself disconnected from the primary router and thinks
1731 that it is now primary in the cell) the primary router send
1732 SILC_PACKET_FAILURE with the type value 1 back to the backup
1733 router. If backup receives this it MUST NOT continue with the
1734 backup resuming protocol.
1736 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1737 value 2 to its current primary router to indicate that it will
1738 resign as being primary router. Then, backup router sends the
1739 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
1740 connected servers to also indicate that it will resign as being
1743 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
1744 type value 2 to the router that is using the backup router
1745 currently as its primary router.
1747 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1748 with type value 1 or 2 must reconnect immediately to the
1749 primary router of the cell that came back online. After they
1750 have created the connection they MUST NOT use that connection
1751 as active primary route but still route all packets to the
1752 backup router. After the connection is created they MUST send
1753 SILC_PACKET_RESUME_ROUTER with type value 3 back to the
1754 backup router. The session ID value found in the first packet
1755 MUST be set in this packet.
1757 5. Backup router MUST wait for all packets with type value 3 before
1758 it continues with the protocol. It knows from the session ID values
1759 set in the packet when it have received all packets. The session
1760 value should be different in all packets it have sent earlier.
1761 After the packets is received the backup router sends the
1762 SILC_PACKET_RESUME_ROUTER packet with type value 4 to the
1763 primary router that came back online. This packet will indicate
1764 that the backup router is now ready to resign as being primary
1765 router. The session ID value in this packet MUST be the same as
1766 in first packet sent to the primary router. During this time
1767 the backup router must still route all packets it is receiving
1768 from server connections.
1770 6. The primary router receives the packet and send the
1771 SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
1772 including the backup router. It also sends the packet with type
1773 value 6 to its primary router, and to the router that is using
1774 it as its primary router. The Session ID value in this packet
1777 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1778 with type value 5 or 6 must switch their primary route to the
1779 new primary router and remove the route for the backup router, since
1780 it is not anymore the primary router of the cell. They must also
1781 update their local database to understand that the clients are
1782 not originated from the backup router but from the locally connected
1783 servers. After that they MUST announce their channels, channel
1784 users, modes etc. to the primary router. They must not use the
1785 backup router connection after this and the connection is considered
1786 to be passive connection. The implementations SHOULD be able
1787 to disable the connection without closing the actual link.
1789 After this protocol is executed the backup router is now again normal
1790 server in the cell that has the backup link to the primary router. The
1791 primary router feeds the router specific data again to the backup router.
1792 All server connections in the backup router are considered passive
1795 When the primary router of the cell comes back online and connects
1796 to its primary router, the remote primary router must send the
1797 SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
1798 connection is not allowed since the router has been replaced by an
1799 backup router. The session ID value in this packet SHOULD be zero (0).
1800 When the router receives this packet it must not use the connection
1801 as active connection but to understand that it cannot act as primary
1802 router in the cell. It must wait that the backup router connects to
1803 it, and the backup resuming protocol is executed.
1805 The following type values has been defined for SILC_PACKET_RESUME_ROUTER
1808 1 SILC_SERVER_BACKUP_START
1809 2 SILC_SERVER_BACKUP_START_GLOBAL
1810 3 SILC_SERVER_BACKUP_START_CONNECTED
1811 4 SILC_SERVER_BACKUP_START_ENDING
1812 5 SILC_SERVER_BACKUP_START_RESUMED
1813 6 SILC_SERVER_BACKUP_START_RESUMED_GLOBAL
1814 20 SILC_SERVER_BACKUP_START_REPLACED
1815 21 SILC_SERVER_BACKUP_START_USE
1816 22 SILC_SERVER_BACKUP_START_USE_DENIED
1818 If any other value is found in the type field the packet must be
1819 discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
1820 is defined in [SILC2].
1826 3.13.3 Discussion on Backup Router Scheme
1828 It is clear that this backup router support is not able to handle all
1829 possible situations arising in unreliable network environment. This
1830 scheme for example does not handle situation when the router actually
1831 does not go offline but the network link goes down temporarily. It would
1832 require some intelligence to figure out when it is best time to switch
1833 to the backup router. To make it even more complicated it is possible
1834 that the backup router may have not lost the network link to the primary
1837 Other possible situation is when the network link is lost temporarily
1838 between two primary routers in the SILC network. Unless the routers
1839 notice the link going down they cannot perhaps find alternative routes.
1840 Worst situation is when the link goes down only for a short period of
1841 time, thus causing lag. Should the routers or servers find alternative
1842 routes if they cannot get response from the router during the lag?
1843 When alternative routes are being found it must be careful not to
1844 mess up existing primary routes between routers in the network.
1846 It is suggested that the current backup router scheme is only temporary
1847 solution and existing backup router protocols are studied further. It
1848 is also suggested that the backup router specification will be separated
1849 from this SILC specification Internet-Draft and additional specification
1850 is written on the subject.
1856 This section describes various SILC procedures such as how the
1857 connections are created and registered, how channels are created and
1858 so on. The section describes the procedures only generally as details
1859 are described in [SILC2] and [SILC3].
1863 4.1 Creating Client Connection
1865 This section describes the procedure when client connects to SILC server.
1866 When client connects to server the server MUST perform IP address lookup
1867 and reverse IP address lookup to assure that the origin host really is
1868 who it claims to be. Client, host, connecting to server SHOULD have
1869 both valid IP address and fully qualified domain name (FQDN).
1871 After that the client and server performs SILC Key Exchange protocol
1872 which will provide the key material used later in the communication.
1873 The key exchange protocol MUST be completed successfully before the
1874 connection registration may continue. The SILC Key Exchange protocol
1875 is described in [SILC3].
1877 Typical server implementation would keep a list of connections that it
1878 allows to connect to the server. The implementation would check, for
1879 example, the connecting client's IP address from the connection list
1880 before the SILC Key Exchange protocol has been started. Reason for
1881 this is that if the host is not allowed to connect to the server there
1882 is no reason to perform the key exchange protocol.
1884 After successful key exchange protocol the client and server performs
1885 connection authentication protocol. The purpose of the protocol is to
1886 authenticate the client connecting to the server. Flexible
1887 implementation could also accept the client to connect to the server
1888 without explicit authentication. However, if authentication is
1889 desired for a specific client it may be based on passphrase or
1890 public key authentication. If authentication fails the connection
1891 MUST be terminated. The connection authentication protocol is described
1894 After successful key exchange and authentication protocol the client
1895 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1896 server. This packet includes various information about the client
1897 that the server uses to create the client. Server creates the client
1898 and sends SILC_PACKET_NEW_ID to the client which includes the created
1899 Client ID that the client MUST start using after that. After that
1900 all SILC packets from the client MUST have the Client ID as the
1901 Source ID in the SILC Packet Header, described in [SILC2].
1903 Client MUST also get the server's Server ID that is to be used as
1904 Destination ID in the SILC Packet Header when communicating with
1905 the server (for example when sending commands to the server). The
1906 ID may be resolved in two ways. Client can take the ID from an
1907 previously received packet from server that MUST include the ID,
1908 or to send SILC_COMMAND_INFO command and receive the Server ID as
1911 Server MAY choose not to use the information received in the
1912 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1913 certificate were used in the authentication, server MAY use those
1914 informations rather than what it received from client. This is suitable
1915 way to get the true information about client if it is available.
1917 The nickname of client is initially set to the username sent in the
1918 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1919 suitable by sending SILC_COMMAND_NICK command. However, this is not
1920 required as part of registration process.
1922 Server MUST also distribute the information about newly registered
1923 client to its router (or if the server is router, to all routers in
1924 the SILC network). More information about this in [SILC2].
1926 Router server MUST also check whether some client in the local cell
1927 is watching for the nickname this new client has, and send the
1928 SILC_NOTIFY_TYPE_WATCH to the watcher.
1932 4.2 Creating Server Connection
1934 This section describes the procedure when server connects to its
1935 router (or when router connects to other router, the cases are
1936 equivalent). The procedure is very much alike when client connects
1937 to the server thus it is not repeated here.
1939 One difference is that server MUST perform connection authentication
1940 protocol with proper authentication. A proper authentication is based
1941 on passphrase authentication or public key authentication based on
1944 After server and router has successfully performed the key exchange
1945 and connection authentication protocol, the server register itself
1946 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1947 includes the server's Server ID that it has created by itself and
1948 other relevant information about the server.
1950 After router has received the SILC_PACKET_NEW_SERVER packet it
1951 distributes the information about newly registered server to all routers
1952 in the SILC network. More information about this in [SILC2].
1954 As client needed to resolve the destination ID this MUST be done by the
1955 server that connected to the router, as well. The way to resolve it is
1956 to get the ID from previously received packet. The server MAY also
1957 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1958 using its own Server ID as Source ID in SILC Packet Header and the
1959 router's Server ID as Destination when communicating with the router.
1963 4.2.1 Announcing Clients, Channels and Servers
1965 After server or router has connected to the remote router, and it already
1966 has connected clients and channels it MUST announce them to the router.
1967 If the server is router server, also all the local servers in the cell
1970 All clients are announced by compiling a list of ID Payloads into the
1971 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1972 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet.
1973 Channels' mode and founder public key and other channel mode specific
1974 data is announced by sending SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.
1975 Also, the channel users on the channels must be announced by compiling a
1976 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1977 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1978 also be announced by compiling list of Notify Payloads with the
1979 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1982 The router MUST also announce the local servers by compiling list of
1983 ID Payloads into the SILC_PACKET_NEW_ID packet.
1985 Also, clients' modes (user modes in SILC) MUST be announced. This is
1986 done by compiling a list of Notify Payloads with SILC_NOTIFY_UMODE_CHANGE
1987 notify type into the SILC_PACKET_NOTIFY packet. Also, channel's topics
1988 MUST be announced by compiling a list of Notify Payloads with the
1989 SILC_NOTIFY_TOPIC_SET notify type into the SILC_PACKET_NOTIFY packet.
1991 The router which receives these lists MUST process them and broadcast
1992 the packets to its primary route. When processing the announced channels
1993 and channel users the router MUST check whether a channel exists already
1994 with the same name. If channel exists with the same name it MUST check
1995 whether the Channel ID is different. If the Channel ID is different the
1996 router MUST send the notify type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the
1997 server to force the channel ID change to the ID the router has. If the
1998 mode of the channel is different the router MUST send the notify type
1999 SILC_NOTIFY_TYPE_CMODE_CHANGE to the server to force the mode change
2000 to the mode that the router has.
2002 The router MUST also generate new channel key and distribute it to the
2003 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
2006 If the channel has channel founder on the router the router MUST send
2007 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
2008 the mode change for the channel founder on the server. The channel
2009 founder privileges MUST be removed.
2011 The router processing the channels MUST also compile a list of
2012 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
2013 SILC_PACKET_NOTIFY and send the packet to the server. This way the
2014 server (or router) will receive the clients on the channel that
2019 4.3 Joining to a Channel
2021 This section describes the procedure when client joins to a channel.
2022 Client joins to channel by sending command SILC_COMMAND_JOIN to the
2023 server. If the receiver receiving join command is normal server the
2024 server MUST check its local list whether this channel already exists
2025 locally. This would indicate that some client connected to the server
2026 has already joined to the channel. If this is case the client is
2027 joined to the channel, new channel key is created and information about
2028 newly joined channel is sent to the router. The router is informed
2029 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
2030 also be sent to the local clients on the channel. The new channel key
2031 is also sent to the router and to local clients on the channel.
2033 If the channel does not exist in the local list the client's command
2034 MUST be sent to the router which will then perform the actual joining
2035 procedure. When server receives the reply to the command from the
2036 router it MUST be sent to the client which sent the command originally.
2037 Server will also receive the channel key from the server that it MUST
2038 send to the client which originally requested the join command. The
2039 server MUST also save the channel key.
2041 If the receiver of the join command is router it MUST first check its
2042 local list whether anyone in the cell has already joined to the channel.
2043 If this is the case the client is joined to the channel and reply is
2044 sent to the client. If the command was sent by server the command reply
2045 is sent to the server which sent it. Then the router MUST also create
2046 new channel key and distribute it to all clients on the channel and
2047 all servers that has clients on the channel. Router MUST also send
2048 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
2049 and to local servers that has clients on the channel.
2051 If the channel does not exist on the router's local list it MUST
2052 check the global list whether the channel exists at all. If it does
2053 the client is joined to the channel as described previously. If
2054 the channel does not exist the channel is created and the client
2055 is joined to the channel. The channel key is also created and
2056 distributed as previously described. The client joining to the created
2057 channel is made automatically channel founder and both channel founder
2058 and channel operator privileges is set for the client.
2060 If the router created the channel in the process, information about the
2061 new channel MUST be broadcasted to all routers. This is done by
2062 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
2063 route. When the router joins the client to the channel it MUST also
2064 send information about newly joined client to all routers in the SILC
2065 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
2066 type to the router's primary route.
2068 It is important to note that new channel key is created always when
2069 new client joins to channel, whether the channel has existed previously
2070 or not. This way the new client on the channel is not able to decrypt
2071 any of the old traffic on the channel. Client which receives the reply to
2072 the join command MUST start using the received Channel ID in the channel
2073 message communication thereafter. Client also receives the key for the
2074 channel in the command reply. Note that the channel key is never
2075 generated if the SILC_CMODE_PRIVKEY mode is set.
2079 4.4 Channel Key Generation
2081 Channel keys are created by router which creates the channel by taking
2082 enough randomness from cryptographically strong random number generator.
2083 The key is generated always when channel is created, when new client
2084 joins a channel and after the key has expired. Key could expire for
2087 The key MUST also be re-generated whenever some client leaves a channel.
2088 In this case the key is created from scratch by taking enough randomness
2089 from the random number generator. After that the key is distributed to
2090 all clients on the channel. However, channel keys are cell specific thus
2091 the key is created only on the cell where the client, which left the
2092 channel, exists. While the server or router is creating the new channel
2093 key, no other client may join to the channel. Messages that are sent
2094 while creating the new key are still processed with the old key. After
2095 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
2096 using the new key. If server creates the new key the server MUST also
2097 send the new key to its router. See [SILC2] on more information about
2098 how channel messages must be encrypted and decrypted when router is
2101 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
2102 Channel Key Payload it MUST process the key data to create encryption
2103 and decryption key, and to create the HMAC key that is used to compute
2104 the MACs of the channel messages. The processing is as follows:
2106 channel_key = raw key data
2107 HMAC key = hash(raw key data)
2109 The raw key data is the key data received in the Channel Key Payload.
2110 The hash() function is the hash function used in the HMAC of the channel.
2111 Note that the server also MUST save the channel key.
2115 4.5 Private Message Sending and Reception
2117 Private messages are sent point to point. Client explicitly destine
2118 a private message to specific client that is delivered to only to that
2119 client. No other client may receive the private message. The receiver
2120 of the private message is destined in the SILC Packet Header as any
2121 other packet as well.
2123 If the sender of a private message does not know the receiver's Client
2124 ID, it MUST resolve it from server. There are two ways to resolve the
2125 client ID from server; it is RECOMMENDED that client implementations
2126 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
2127 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
2128 If the sender has received earlier a private message from the receiver
2129 it should have cached the Client ID from the SILC Packet Header.
2131 If server receives a private message packet which includes invalid
2132 destination Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2133 notify to the client with error status indicating that such Client ID
2136 See [SILC2] for description of private message encryption and decryption
2141 4.6 Private Message Key Generation
2143 Private message MAY be protected with a key generated by the client.
2144 The key may be generated and sent to the other client by sending packet
2145 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
2146 and is secured by session keys. After that the private message key
2147 is used in the private message communication between those clients.
2148 The key sent inside the payload SHOULD be randomly generated. This
2149 packet MUST NOT be used to send pre-shared keys.
2151 Other choice is to entirely use keys that are not sent through
2152 the SILC network at all. This significantly adds security. This key
2153 could be a pre-shared-key that is known by both of the clients. Both
2154 agree about using the key and starts sending packets that indicate
2155 that the private message is secured using private message key. In
2156 case of pre-shared keys (static keys) the IV used in encryption SHOULD
2159 It is also possible to negotiate fresh key material by performing
2160 Key Agreement. The SILC_PACKET_KEY_AGREEMENT packet MAY be used to
2161 negotiate the fresh key material. In this case the resulted key
2162 material is used to secure the private messages. Also, the IV used
2163 in encryption is used as defined in [SILC3], unless otherwise stated
2164 by the encryption mode used. By performing Key Agreement the clients
2165 may negotiate the cipher and HMAC to be used in the private message
2166 encryption and to negotiate additional security parameters.
2168 If the key is pre-shared key or other key material not generated by
2169 Key Agreement, then the key material SHOULD be processed as defined
2170 in [SILC3]. In the processing, however, the HASH, as defined in
2171 [SILC3] MUST be ignored. After processing the key material it is
2172 employed as defined in [SILC3]. In this case also, implementations
2173 SHOULD use the SILC protocol's mandatory cipher and HMAC in private
2178 4.7 Channel Message Sending and Reception
2180 Channel messages are delivered to group of users. The group forms a
2181 channel and all clients on the channel receives messages sent to the
2184 Channel messages are destined to channel by specifying the Channel ID
2185 as Destination ID in the SILC Packet Header. The server MUST then
2186 distribute the message to all clients on the channel by sending the
2187 channel message destined explicitly to a client on the channel.
2189 If server receives a channel message packet which includes invalid
2190 destination Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2191 notify to the sender with error status indicating that such Channel ID
2194 See the [SILC2] for description of channel message routing for router
2195 servers, and channel message encryption and decryption process.
2199 4.8 Session Key Regeneration
2201 Session keys MUST be regenerated periodically, say, once in an hour.
2202 The re-key process is started by sending SILC_PACKET_REKEY packet to
2203 other end, to indicate that re-key must be performed. The initiator
2204 of the connection SHOULD initiate the re-key.
2206 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2207 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2208 SKE protocol. In this case the protocol is secured with the old key
2209 and the protocol results to new key material. See [SILC3] for more
2210 information. After the SILC_PACKET_REKEY packet is sent the sender
2211 will perform the SKE protocol.
2213 If PFS flag was set the resulted key material is processed as described
2214 in the section Processing the Key Material in [SILC3]. The difference
2215 with re-key in the processing is that the initial data for the hash
2216 function is just the resulted key material and not the HASH as it
2217 is not computed at all with re-key. Other than that, the key processing
2218 it equivalent to normal SKE negotiation.
2220 If PFS flag was not set, which is the default case, then re-key is done
2221 without executing SKE protocol. In this case, the new key is created by
2222 providing the current sending encryption key to the SKE protocol's key
2223 processing function. The process is described in the section Processing
2224 the Key Material in [SILC3]. The difference in the processing is that
2225 the initial data for the hash function is the current sending encryption
2226 key and not the SKE's KEY and HASH values. Other than that, the key
2227 processing is equivalent to normal SKE negotiation.
2229 After both parties has regenerated the session key, both MUST send
2230 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2231 secured with the old key. After these packets, the subsequent packets
2232 MUST be protected with the new key.
2236 4.9 Command Sending and Reception
2238 Client usually sends the commands in the SILC network. In this case
2239 the client simply sends the command packet to server and the server
2240 processes it and replies with command reply packet. See the [SILC3]
2241 for detailed description of all commands.
2243 However, if the server is not able to process the command, it is sent
2244 to the server's router. This is case for example with commands such
2245 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2246 are other commands as well. For example, if client sends the WHOIS
2247 command requesting specific information about some client the server must
2248 send the WHOIS command to router so that all clients in SILC network
2249 are searched. The router, on the other hand, sends the WHOIS command
2250 further to receive the exact information about the requested client.
2251 The WHOIS command travels all the way to the server which owns the client
2252 and it replies with command reply packet. Finally, the server which
2253 sent the command receives the command reply and it must be able to
2254 determine which client sent the original command. The server then
2255 sends command reply to the client. Implementations should have some
2256 kind of cache to handle, for example, WHOIS information. Servers
2257 and routers along the route could all cache the information for faster
2258 referencing in the future.
2260 The commands sent by server may be sent hop by hop until someone is able
2261 to process the command. However, it is preferred to destine the command
2262 as precisely as it is possible. In this case, other routers en route
2263 MUST route the command packet by checking the true sender and true
2264 destination of the packet. However, servers and routers MUST NOT route
2265 command reply packets to clients coming from other server. Client
2266 MUST NOT accept command reply packet originated from anyone else but
2267 from its own server.
2272 4.10 Closing Connection
2274 When remote client connection is closed the server MUST send the notify
2275 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2276 the client was joined. The server MUST also save the client's information
2277 for a period of time for history purposes.
2279 When remote server or router connection is closed the server or router
2280 MUST also remove all the clients that was behind the server or router
2281 from the SILC Network. The server or router MUST also send the notify
2282 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2283 local clients that are joined on the same channels with the remote
2284 server's or router's clients.
2286 Router server MUST also check whether some client in the local cell
2287 is watching for the nickname this client has, and send the
2288 SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
2289 the network has the SILC_UMODE_REJECT_WATCHING user mode set.
2293 4.11 Detaching and Resuming a Session
2295 SILC protocol provides a possibility for a client to detach itself from
2296 the network without actually signing off from the network. The client
2297 connection to the server is closed but the client remains as valid client
2298 in the network. The client may then later resume its session back from
2299 any server in the network.
2301 When client wishes to detach from the network it MUST send the
2302 SILC_COMMAND_DETACH command to its server. The server then MUST set
2303 SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
2304 notify to its primary router, which will then MUST broadcast it further
2305 to other routers in the network. This user mode indicates that the
2306 client is detached from the network. Implementations MUST NOT use
2307 the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
2308 to the client. All packets MUST still be sent to the client even if
2309 client is detached from the network. Only the server that originally
2310 had the active client connection is able to make the decision after it
2311 notices that the network connection is not active. In this case the
2312 default case is to discard the packet.
2314 The SILC_UMODE_DETACHED flag cannot be set by client itself directly
2315 with SILC_COMMAND_UMODE command, but only implicitly by sending the
2316 SILC_COMMAND_DETACH command. The flag also cannot be unset by the
2317 client, server or router with SILC_COMMAND_UMODE command, but only
2318 implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
2321 When the client wishes to resume its session in the SILC Network it
2322 connects to a server in the network, which MAY also be a different
2323 from the original server, and performs normal procedures regarding
2324 creating a connection as described in section 4.1. After the SKE
2325 and the Connection Authentication protocols has been successfully
2326 completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
2327 MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
2328 perform the resuming procedure. The packet MUST include the detached
2329 client's Client ID, which the client must know. It also includes
2330 Authentication Payload which includes signature made with the client's
2331 private key. The signature is computed as defined in the section
2332 3.9.1. Thus, the authentication method MUST be based in public key
2335 When server receives the SILC_PACKET_RESUME_CLIENT packet it MUST
2336 do the following: Server checks that the Client ID is valid client
2337 and that it has the SILC_UMODE_DETACHED mode set. Then it verifies
2338 the Authentication Payload with the detached client's public key.
2339 If it does not have the public key it retrieves it by sending
2340 SILC_COMMAND_GETKEY command to the server that has the public key from
2341 the original client connection. The server MUST NOT use the public
2342 key received in the SKE protocol for this connection. If the
2343 signature is valid the server unsets the SILC_UMODE_DETACHED flag,
2344 and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
2345 The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
2346 flag when the packet is received. If the server is router server it
2347 also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
2348 server whom owned the detached client.
2350 The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
2351 packet MUST know whether the packet already has been received for
2352 the client. It is protocol error to attempt to resume the client
2353 session from more than one server. The implementations could set
2354 internal flag that indicates that the client is resumed. If router
2355 receive SILC_PACKET_RESUME_CLIENT packet for client that is already
2356 resumed the client MUST be killed from the network. This would
2357 indicate that the client is attempting to resume the session more
2358 than once which is protocol error. In this case the router sends
2359 SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
2360 the same situation MUST also send the notify for the client.
2362 The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
2363 must also understand that the client may not be found behind the
2364 same server that it originally came from. They must update their
2365 caches according this. The server that now owns the client session
2366 MUST check whether the Client ID of the resumed client is based
2367 on the server's Server ID. If it is not it creates a new Client
2368 ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
2369 also send the channel keys of all channels that the client is
2370 joined to the client since it does not have them. Whether the
2371 Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
2372 packet to the client. Only after this the client is resumed back
2373 to the network and may start sending packets and messages.
2375 It is also possible that the server does not know about the channels
2376 that the client has joined. In this case it join the client internally
2377 to the channels, generate new channel keys and distribute the keys
2378 to the channels as described in section 4.4.
2380 It is implementation issue for how long servers keep detached client
2381 sessions. It is RECOMMENDED that the detached sessions would be
2382 persistent as long as the server is running.
2386 5 Security Considerations
2388 Security is central to the design of this protocol, and these security
2389 considerations permeate the specification. Common security considerations
2390 such as keeping private keys truly private and using adequate lengths for
2391 symmetric and asymmetric keys must be followed in order to maintain the
2392 security of this protocol.
2394 Special attention must also be paid on the servers and routers that are
2395 running the SILC service. The SILC protocol's security depends greatly
2396 on the security and the integrity of the servers and administrators that
2397 are running the service. It is recommended that some form of registration
2398 is required by the server and router administrator prior acceptance to
2399 the SILC Network. Even though, the SILC protocol is secure in a network
2400 of mutual distrust between clients, servers, routers and administrators
2401 of the servers, the client should be able to trust the servers they are
2402 using if they wish to do so.
2404 It however must be noted that if the client requires absolute security
2405 by not trusting any of the servers or routers in the SILC Network, it can
2406 be accomplished by negotiating private keys outside the SILC Network,
2407 either using SKE or some other key exchange protocol, or to use some
2408 other external means for distributing the keys. This applies for all
2409 messages, private messages and channel messages.
2411 It is important to note that SILC, like any other security protocol is
2412 not full proof system; the SILC servers and routers could very well be
2413 compromised. However, to provide acceptable level of security and
2414 usability for end user the protocol use many times session keys or other
2415 keys generated by the servers to secure the messages. This is
2416 intentional design feature to allow ease of use for end user. This way
2417 the network is still usable, and remains encrypted even if the external
2418 means of distributing the keys is not working. The implementation,
2419 however, may like to not follow this design feature, and always negotiate
2420 the keys outside SILC network. This is acceptable solution and many times
2421 recommended. The implementation still must be able to work with the
2422 server generated keys.
2424 If this is unacceptable for the client or end user, the private keys
2425 negotiated outside the SILC Network should always be used. In the end
2426 it is always implementor's choice whether to negotiate private keys by
2427 default or whether to use the keys generated by the servers.
2429 It is also recommended that router operators in the SILC Network would
2430 form a joint forum to discuss the router and SILC Network management
2431 issues. Also, router operators along with the cell's server operators
2432 should have a forum to discuss the cell management issues.
2438 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2441 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2442 Protocols", Internet Draft, May 2002.
2444 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, May 2002.
2446 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2449 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2452 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2455 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2458 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2461 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2464 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2467 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2470 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2471 Infrastructure, Certificate and CRL Profile", RFC 2459,
2474 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2475 John Wiley & Sons, New York, NY, 1996.
2477 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2480 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2481 RFC 2412, November 1998.
2483 [ISAKMP] Maughan D., et al, "Internet Security Association and
2484 Key Management Protocol (ISAKMP)", RFC 2408, November
2487 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2488 (IKE)", RFC 2409, November 1998.
2490 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2491 Authentication", RFC 2104, February 1997.
2493 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2494 Specifications, Version 2.0", RFC 2437, October 1998.
2496 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2497 Requirement Levels", BCP 14, RFC 2119, March 1997.
2499 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
2500 10646", RFC 2279, January 1998.
2502 [PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
2503 Version 1.5", RFC 2315, March 1998.
2511 Snellmaninkatu 34 A 15
2515 EMail: priikone@iki.fi
2517 This Internet-Draft expires 26 April 2003