8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-02.txt XX April 2001
20 Expires: XX October 2001
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-02.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 ............................... 5
78 2.3 Communication in the Network .............................. 6
79 2.4 Channel Communication ..................................... 7
80 2.5 Router Connections ........................................ 7
81 2.6 Backup Routers ............................................ 8
82 3 SILC Specification ............................................ 10
83 3.1 Client .................................................... 10
84 3.1.1 Client ID ........................................... 10
85 3.2 Server .................................................... 11
86 3.2.1 Server's Local ID List .............................. 12
87 3.2.2 Server ID ........................................... 13
88 3.2.3 SILC Server Ports ................................... 14
89 3.3 Router .................................................... 14
90 3.3.1 Router's Local ID List .............................. 14
91 3.3.2 Router's Global ID List ............................. 15
92 3.3.3 Router's Server ID .................................. 15
93 3.4 Channels .................................................. 16
94 3.4.1 Channel ID .......................................... 17
95 3.5 Operators ................................................. 17
96 3.6 SILC Commands ............................................. 18
97 3.7 SILC Packets .............................................. 18
98 3.8 Packet Encryption ......................................... 19
99 3.8.1 Determination of the Source and the Destination ..... 19
100 3.8.2 Client To Client .................................... 20
101 3.8.3 Client To Channel ................................... 21
102 3.8.4 Server To Server .................................... 22
103 3.9 Key Exchange And Authentication ........................... 22
104 3.9.1 Authentication Payload .............................. 22
105 3.10 Algorithms ............................................... 24
106 3.10.1 Ciphers ............................................ 24
107 3.10.2 Public Key Algorithms .............................. 25
108 3.10.3 Hash Functions ..................................... 26
109 3.10.4 MAC Algorithms ..................................... 26
110 3.10.5 Compression Algorithms ............................. 26
111 3.11 SILC Public Key .......................................... 27
112 3.12 SILC Version Detection ................................... 29
113 4 SILC Procedures ............................................... 30
114 4.1 Creating Client Connection ................................ 30
115 4.2 Creating Server Connection ................................ 31
116 4.2.1 Announcing Clients, Channels and Servers ............ 32
117 4.3 Joining to a Channel ...................................... 33
118 4.4 Channel Key Generation .................................... 34
119 4.5 Private Message Sending and Reception ..................... 34
120 4.6 Private Message Key Generation ............................ 35
121 4.7 Channel Message Sending and Reception ..................... 36
122 4.8 Session Key Regeneration .................................. 36
123 4.9 Command Sending and Reception ............................. 37
124 4.10 Closing Connection ....................................... 37
125 5 Security Considerations ....................................... XXX
126 6 References .................................................... XXX
127 7 Author's Address .............................................. XXX
134 Figure 1: SILC Network Topology
135 Figure 2: Communication Inside cell
136 Figure 3: Communication Between Cells
137 Figure 4: Router Connections
138 Figure 5: SILC Public Key
144 This document describes a Secure Internet Live Conferencing (SILC)
145 protocol which provides secure conferencing services over insecure
146 network channel. SILC is IRC [IRC] like protocol, however, it is
147 not equivalent to IRC and does not support IRC.
149 Strong cryptographic methods are used to protect SILC packets inside
150 the SILC network. Three other Internet Drafts relates very closely
151 to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
152 Authentication Protocols [SILC3] and SILC Commands [SILC4].
154 The protocol uses extensively packets as conferencing protocol
155 requires message and command sending. The SILC Packet Protocol is
156 described in [SILC2] and should be read to fully comprehend this
157 document and protocol. [SILC2] also describes the packet encryption
158 and decryption in detail.
160 The security of SILC protocol, and for any security protocol for that
161 matter, is based on strong and secure key exchange protocol. The SILC
162 Key Exchange protocol is described in [SILC3] along with connection
163 authentication protocol and should be read to fully comprehend this
164 document and protocol.
166 The SILC protocol has been developed to work on TCP/IP network
167 protocol, although it could be made to work on other network protocols
168 with only minor changes. However, it is recommended that TCP/IP
169 protocol is used under SILC protocol. Typical implementation would
170 be made in client-server model.
174 1.1 Requirements Terminology
176 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
177 MAY, and OPTIONAL, when they appear in this document, are to be
178 interpreted as described in [RFC2119].
184 This section describes various SILC protocol concepts that forms the
185 actual protocol, and in the end, the actual SILC network. The mission
186 of the protocol is to deliver messages from clients to other clients
187 through routers and servers in secure manner. The messages may also
188 be delivered from one client to many clients forming a group, also
191 This section does not focus to security issues. Instead, basic network
192 concepts are introduced to make the topology of the SILC network
197 2.1 SILC Network Topology
199 SILC network is a cellular network as opposed to tree style network
200 topology. The rationale for this is to have servers that can perform
201 specific kind of tasks what other servers cannot perform. This leads
202 to two kinds of servers; normal SILC servers and SILC routers.
204 A difference between normal server and router server is that routers
205 knows everything about everything in the network. They also do the
206 actual routing of the messages to the correct receiver. Normal servers
207 knows only about local information and nothing about global information.
208 This makes the network faster as there are less servers that needs to
209 keep global information up to date at all time.
211 This, on the other hand, leads to cellular like network, where routers
212 are in the center of the cell and servers are connected to the router.
214 The following diagram represents SILC network topology.
219 ---- ---- ---- ---- ---- ----
220 | S8 | S5 | S4 | | S7 | S5 | S6 |
221 ----- ---- ----- ----- ---- -----
222 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
223 ---- ------ ---- ---- ------ ----
224 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
225 ---- ---- ---- ---- ---- ---- | S3 | S1 |
226 Cell 1. \\ Cell 2. | \\____ ----- -----
228 ---- ---- ---- ---- ---- ---- ---- ------
229 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
230 ----- ---- ----- ----- ---- ----- ---- ----
231 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
232 ---- ------ ---- ---- ------ ----
233 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
234 ---- ---- ---- ---- ---- ----
239 Figure 1: SILC Network Topology
242 A cell is formed when a server or servers connect to one router. In
243 SILC network normal server cannot directly connect to other normal
244 server. Normal server may only connect to SILC router which then
245 routes the messages to the other servers in the cell. Router servers
246 on the other hand may connect to other routers to form the actual SILC
247 network, as seen in above figure. However, router is also normal SILC
248 server; clients may connect to it the same way as to normal SILC
249 server. Normal server also cannot have active connections to more
250 than one router. Normal server cannot be connected to two different
251 cells. Router servers, on the other hand, may have as many router to
252 router connections as needed.
254 There are many issues in this network topology that needs to be careful
255 about. Issues like the size of the cells, the number of the routers in
256 the SILC network and the capacity requirements of the routers. These
257 issues should be discussed in the Internet Community and additional
258 documents on the issue may be written.
262 2.2 Communication Inside a Cell
264 It is always guaranteed that inside a cell message is delivered to the
265 recipient with at most two server hops. A client which is connected to
266 server in the cell and is talking on channel to other client connected
267 to other server in the same cell, will have its messages delivered from
268 its local server first to the router of the cell, and from the router
269 to the other server in the cell.
271 The following diagram represents this scenario:
285 Figure 2: Communication Inside cell
288 Example: Client 1. connected to Server 1. send message to
289 Client 4. connected to Server 2. travels from Server 1.
290 first to Router which routes the message to Server 2.
291 which then sends it to the Client 4. All the other
292 servers in the cell will not see the routed message.
295 If the client is connected directly to the router, as router is also normal
296 SILC server, the messages inside the cell are always delivered only with
297 one server hop. If clients communicating with each other are connected
298 to the same server, no router interaction is needed. This is the optimal
299 situation of message delivery in the SILC network.
303 2.3 Communication in the Network
305 If the message is destined to server that does not belong to local cell
306 the message is routed to the router server to which the destination
307 server belongs, if the local router is connected to destination router.
308 If there is no direct connection to the destination router, the local
309 router routes the message to its primary route. The following diagram
310 represents message sending between cells.
315 1 --- S1 S4 --- 5 S2 --- 1
316 S/R - - - - - - - - S/R
326 Figure 3: Communication Between Cells
329 Example: Client 5. connected to Server 4. in Cell 1. sends message
330 to Client 2. connected to Server 1. in Cell 2. travels
331 from Server 4. to Router which routes the message to
332 Router in Cell 2, which then routes the message to
333 Server 1. All the other servers and routers in the
334 network will not see the routed message.
337 The optimal case of message delivery from the client point of view is
338 when clients are connected directly to the routers and the messages
339 are delivered from one router to the other.
343 2.4 Channel Communication
345 Messages may be sent to group of clients as well. Sending messages to
346 many clients works the same way as sending messages point to point, from
347 message delivery point of view. Security issues are another matter
348 which are not discussed in this section.
350 Router server handles the message routing to multiple recipients. If
351 any recipient is not in the same cell as the sender the messages are
354 Server distributes the channel message to its local clients which are
355 joined to the channel. Router also distributes the message to its
356 local clients on the channel.
360 2.5 Router Connections
362 Router connections play very important role in making the SILC like
363 network topology to work. For example, sending broadcast packets in
364 SILC network require special connections between routers; routers must
365 be connected in a specific way.
367 Every router has their primary route which is a connection to another
368 router in the network. Unless there is only two routers in the network
369 must not routers use each other as their primary routes. The router
370 connections in the network must form a circular.
378 Example with three routers in the network:
383 S/R1 - > - > - > - > - > - > - S/R2
386 \\ - < - < - S/R3 - < - < - /
391 Figure 4: Router Connections
394 Example: Network with three routers. Router 1. uses Router 2. as its
395 primary router. Router 2. uses Router 3. as its primary router,
396 and Router 3. uses Router 1. as its primary router. There may
397 be other direct connections between the routers but they must
398 not be used as primary routes.
400 The above example is applicable to any amount of routers in the network
401 except for two routers. If there are only two routers in the network both
402 routers must be able to handle situation where they use each other as their
405 The issue of router connections are very important especially with SILC
406 broadcast packets. Usually all router wide information in the network is
407 distributed by SILC broadcast packets.
413 Backup routers may exist in the cell in addition of the primary router.
414 However, they must not be active routers and act as routers in the cell.
415 Only one router may be acting as primary router in the cell. In the case
416 of failure of the primary router may one of the backup routers become
417 active. The purpose of backup routers are in case of failure of the
418 primary router to maintain working connections inside the cell and outside
419 the cell and to avoid netsplits.
421 Backup routers are normal servers in the cell that are prepared to take
422 over the tasks of the primary router if needed. They need to have at
423 least one direct and active connection to the primary router of the cell.
424 This communication channel is used to send the router information to
427 Backup router must know everything that the primary router knows to be
428 able to take over the tasks of the primary router. It is the primary
429 router's responsibility to feed the data to the backup router. If the
430 backup router does not know all the data in the case of failure some
431 connections may be lost. The primary router of the cell must consider
432 the backup router being normal router server and feed the data
435 In addition of having direct connection to the primary router of the
436 cell the backup router must also have connection to the same router
437 the primary router of the cell is connected. However, it must not be
438 active router connection meaning that the backup router must not use
439 that channel as its primary route and it must not notify the router
440 about having connected servers, channels and clients behind it. It
441 merely connects to the router. This sort of connection is later
442 referred as being passive connection. Some keepalive actions may be
443 needed by the router to keep the connection alive.
445 The primary router notifies its primary router about having backup
446 routers in the cell by sending SILC_PACKET_CELL_ROUTERS packet. If
447 and when the primary router of the cell becomes unresponsive, its
448 primary router knows that there exists backup routers in the cell.
449 After that it will start using the first backup router sent in the
450 packet as router of that cell.
452 In this case the backup router must notify its new primary router about
453 the servers, channels and clients it has connected to it. The primary
454 router knows that this server has become a router of the cell because
455 of failure of the primary router in the cell. It must also cope with
456 the fact that the servers, channels and clients that the new backup
457 router announces are not really new, since they used to exist in the
458 primary router of the cell.
460 It is required that other normal servers has passive connections to
461 the backup router(s) in the cell. Some keepalive actions may be needed
462 by the server to keep the connection alive. After they notice the
463 failure of the primary router they must start using the connection to
464 the first backup router as their primary route.
466 It is RECOMMENDED that there would be at least one backup router in
467 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
468 as backup routers as it requires establishing several connections to
469 several servers in the cell. Large cells can easily have several
470 backup routers in the cell.
472 The order of the backup routers are decided at the primary router of the
473 cell and servers and backup routers in the cell must be configured
474 accordingly. It is not required that the backup server is actually
475 active server in the cell. Backup router may be a spare server in the
476 cell that does not accept normal client connections at all. It may be
477 reserved purely for the backup purposes. These, however, are cell
480 If also the first backup router is down as well and there is another
481 backup router in the cell then it will start acting as the primary
482 router as described above.
486 3. SILC Specification
488 This section describes the SILC protocol. However, [SILC2] and
489 [SILC3] describes other important protocols that are part of this SILC
490 specification and must be read.
496 A client is a piece of software connecting to SILC server. SILC client
497 cannot be SILC server. Purpose of clients is to provide the user
498 interface of the SILC services for end user. Clients are distinguished
499 from other clients by unique Client ID. Client ID is a 128 bit ID that
500 is used in the communication in the SILC network. The client ID is
501 based on the nickname selected by the user. User uses logical nicknames
502 in communication which are then mapped to the corresponding Client ID.
503 Client ID's are low level identifications and must not be seen by the
506 Clients provide other information about the end user as well. Information
507 such as the nickname of the user, username and the host name of the end
508 user and user's real name. See section 3.2 Server for information of
509 the requirements of keeping this information.
511 The nickname selected by the user is not unique in the SILC network.
512 There can be 2^8 same nicknames for one IP address. As for comparison
513 to IRC [IRC] where nicknames are unique this is a fundamental difference
514 between SILC and IRC. This causes the server names or client's host names
515 to be used along with the nicknames to identify specific users when sending
516 messages. This feature of SILC makes IRC style nickname-wars obsolete as
517 no one owns their nickname; there can always be someone else with the same
518 nickname. The maximum length of nickname is 128 characters.
524 Client ID is used to identify users in the SILC network. The Client ID
525 is unique to the extent that there can be 2^128 different Client ID's,
526 and ID's based on IPv6 addresses extends this to 2^224 different Client
527 ID's. Collisions are not expected to happen. The Client ID is defined
533 128 bit Client ID based on IPv4 addresses:
535 32 bit Server ID IP address (bits 1-32)
536 8 bit Random number or counter
537 88 bit Truncated MD5 hash value of the nickname
539 224 bit Client ID based on IPv6 addresses:
541 128 bit Server ID IP address (bits 1-128)
542 8 bit Random number or counter
543 88 bit Truncated MD5 hash value of the nickname
545 o Server ID IP address - Indicates the server where this
546 client is coming from. The IP address hence equals the
547 server IP address where to the client has connected.
549 o Random number or counter - Random number to further
550 randomize the Client ID. Another choice is to use
551 a counter starting from the zero (0). This makes it
552 possible to have 2^8 same nicknames from the same
555 o MD5 hash - MD5 hash value of the nickname is truncated
556 taking 88 bits from the start of the hash value. This
557 hash value is used to search the user's Client ID from
561 Collisions could occur when more than 2^8 clients using same nickname
562 from the same server IP address is connected to the SILC network.
563 Server MUST be able to handle this situation by refusing to accept
564 anymore of that nickname.
566 Another possible collision may happen with the truncated hash value of
567 the nickname. It could be possible to have same truncated hash value for
568 two different nicknames. However, this is not expected to happen nor
569 cause any problems if it would occur. Nicknames are usually logical and
570 it is unlikely to have two distinct logical nicknames produce same
571 truncated hash value.
577 Servers are the most important parts of the SILC network. They form the
578 basis of the SILC, providing a point to which clients may connect to.
579 There are two kinds of servers in SILC; normal servers and router servers.
580 This section focus on the normal server and router server is described
581 in the section 3.3 Router.
583 Normal servers MUST NOT directly connect to other normal server. Normal
584 servers may only directly connect to router server. If the message sent
585 by the client is destined outside the local server it is always sent to
586 the router server for further routing. Server may only have one active
587 connection to router on same port. Normal server MUST NOT connect to other
588 cell's router except in situations where its cell's router is unavailable.
590 Servers and routers in the SILC network are considered to be trusted.
591 With out a doubt, servers that are set to work on ports above 1023 are
592 not considered to be trusted. Also, the service provider acts important
593 role in the server's trustworthy.
597 3.2.1 Server's Local ID List
599 Normal server keeps various information about the clients and their end
600 users connected to it. Every normal server MUST keep list of all locally
601 connected clients, Client ID's, nicknames, usernames and host names and
602 user's real name. Normal servers only keeps local information and it
603 does not keep any global information. Hence, normal servers knows only
604 about their locally connected clients. This makes servers efficient as
605 they don't have to worry about global clients. Server is also responsible
606 of creating the Client ID's for their clients.
608 Normal server also keeps information about locally created channels and
612 Hence, local list for normal server includes:
615 server list - Router connection
623 client list - All clients in server
633 channel list - All channels in server
636 o Client ID's on channel
637 o Client ID modes on channel
645 Servers are distinguished from other servers by unique 64 bit Server ID
646 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
647 the SILC to route messages to correct servers. Server ID's also provide
648 information for Client ID's, see section 3.1.1 Client ID. Server ID is
652 64 bit Server ID based on IPv4 addresses:
654 32 bit IP address of the server
658 160 bit Server ID based on IPv6 addresses:
660 128 bit IP address of the server
664 o IP address of the server - This is the real IP address of
667 o Port - This is the port the server is bound to.
669 o Random number - This is used to further randomize the Server ID.
672 Collisions are not expected to happen in any conditions. The Server ID
673 is always created by the server itself and server is responsible of
674 distributing it to the router.
678 3.2.3 SILC Server Ports
680 The following ports has been assigned by IANA for the SILC protocol:
688 If there are needs to create new SILC networks in the future the port
689 numbers must be officially assigned by the IANA.
691 Server on network above privileged ports (>1023) SHOULD NOT be trusted
692 as they could have been set up by untrusted party.
698 Router server in SILC network is responsible for keeping the cell together
699 and routing messages to other servers and to other routers. Router server
700 is also a normal server thus clients may connect to it as it would be
701 just normal SILC server.
703 However, router servers has a lot of important tasks that normal servers
704 do not have. Router server knows everything about everything in the SILC.
705 They know all clients currently on SILC, all servers and routers and all
706 channels in SILC. Routers are the only servers in SILC that care about
707 global information and keeping them up to date at all time. And, this
708 is what they must do.
712 3.3.1 Router's Local ID List
714 Router server as well MUST keep local list of connected clients and
715 locally created channels. However, this list is extended to include all
716 the informations of the entire cell, not just the server itself as for
719 However, on router this list is a lot smaller since routers do not need
720 to keep information about user's nickname, username and host name and real
721 name since these are not needed by the router. The router keeps only
722 information that it needs.
725 Hence, local list for router includes:
728 server list - All servers in the cell
735 client list - All clients in the cell
739 channel list - All channels in the cell
741 o Client ID's on channel
742 o Client ID modes on channel
747 Note that locally connected clients and other information include all the
748 same information as defined in section section 3.2.1 Server's Local ID
753 3.3.2 Router's Global ID List
755 Router server MUST also keep global list. Normal servers do not have
756 global list as they know only about local information. Global list
757 includes all the clients on SILC, their Client ID's, all created channels
758 and their Channel ID's and all servers and routers on SILC and their
759 Server ID's. That is said, global list is for global information and the
760 list must not include the local information already on the router's local
763 Note that the global list does not include information like nicknames,
764 usernames and host names or user's real names. Router does not need to
765 keep these informations as they are not needed by the router. This
766 information is available from the client's server which maybe queried
769 Hence, global list includes:
772 server list - All servers in SILC
777 client list - All clients in SILC
780 channel list - All channels in SILC
782 o Client ID's on channel
783 o Client ID modes on channel
794 3.3.3 Router's Server ID
796 Router's Server ID's are equivalent to normal Server ID's. As routers
797 are normal servers as well same types of ID's applies for routers as well.
798 Thus, see section 3.2.2 Server ID.
804 A channel is a named group of one or more clients which will all receive
805 messages addressed to that channel. The channel is created when first
806 client requests JOIN command to the channel, and the channel ceases to
807 exist when the last client has left it. When channel exists, any client
808 can reference it using the name of the channel.
810 Channel names are unique although the real uniqueness comes from 64 bit
811 Channel ID. However, channel names are still unique and no two global
812 channels with same name may exist. The Channel name is a string of
813 maximum length of 256 characters. Channel names MUST NOT contain any
814 spaces (` '), any non-printable ASCII characters, commas (`,') and
817 Channels can have operators that can administrate the channel and
818 operate all of its modes. The following operators on channel exist on
822 o Channel founder - When channel is created the joining client becomes
823 channel founder. Channel founder is channel operator with some more
824 privileges. Basically, channel founder can fully operate the channel
825 and all of its modes. The privileges are limited only to the
826 particular channel. There can be only one channel founder per
827 channel. Channel founder supersedes channel operator's privileges.
829 Channel founder privileges cannot be removed by any other operator on
830 channel. When channel founder leaves the channel there is no channel
831 founder on the channel. However, it is possible to set a mode for
832 the channel which allows the original channel founder to regain the
833 founder privileges even after leaving the channel. Channel founder
834 also cannot be removed by force from the channel.
836 o Channel operator - When client joins to channel that has not existed
837 previously it will become automatically channel operator (and channel
838 founder discussed above). Channel operator is able administrate the
839 channel, set some modes on channel, remove a badly behaving client
840 from the channel and promote other clients to become channel
841 operator. The privileges are limited only to the particular channel.
843 Normal channel user may be promoted (opped) to channel operator
844 gaining channel operator privileges. Channel founder or other
845 channel operator may also demote (deop) channel operator to normal
853 Channels are distinguished from other channels by unique Channel ID.
854 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
855 collisions are not expected to happen in any conditions. Channel names
856 are just for logical use of channels. The Channel ID is created by the
857 server where the channel is created. The Channel ID is defined as
861 64 bit Channel ID based on IPv4 addresses:
863 32 bit Router's Server ID IP address (bits 1-32)
864 16 bit Router's Server ID port (bits 33-48)
867 160 bit Channel ID based on IPv6 addresses:
869 128 bit Router's Server ID IP address (bits 1-128)
870 16 bit Router's Server ID port (bits 129-144)
873 o Router's Server ID IP address - Indicates the IP address of
874 the router of the cell where this channel is created. This is
875 taken from the router's Server ID. This way SILC router knows
876 where this channel resides in the SILC network.
878 o Router's Server ID port - Indicates the port of the channel on
879 the server. This is taken from the router's Server ID.
881 o Random number - To further randomize the Channel ID. This makes
882 sure that there are no collisions. This also means that
883 in a cell there can be 2^16 channels.
890 Operators are normal users with extra privileges to their server or
891 router. Usually these people are SILC server and router administrators
892 that take care of their own server and clients on them. The purpose of
893 operators is to administrate the SILC server or router. However, even
894 an operator with highest privileges is not able to enter invite-only
895 channel, to gain access to the contents of a encrypted and authenticated
896 packets traveling in the SILC network or to gain channel operator
897 privileges on public channels without being promoted. They have the
898 same privileges as everyone else except they are able to administrate
899 their server or router.
905 Commands are very important part on SILC network especially for client
906 which uses commands to operate on the SILC network. Commands are used
907 to set nickname, join to channel, change modes and many other things.
909 Client usually sends the commands and server replies by sending a reply
910 packet to the command. Server MAY also send commands usually to serve
911 the original client's request. However, server MUST NOT send commands
912 to client and there are some commands that server must not send.
914 Note that the command reply is usually sent only after client has sent
915 the command request but server is allowed to send command reply packet
916 to client even if client has not requested the command. Client MAY,
917 choose ignore the command reply.
919 It is expected that some of the commands may be miss-used by clients
920 resulting various problems on the server side. Every implementation
921 SHOULD assure that commands may not be executed more than once, say,
922 in two (2) seconds. However, to keep response rate up, allowing for
923 example five (5) commands before limiting is allowed. It is RECOMMENDED
924 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
925 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
926 as they require heavy operations. This should be sufficient to prevent
927 the miss-use of commands.
929 SILC commands are described in [SILC4].
935 Packets are naturally the most important part of the protocol and the
936 packets are what actually makes the protocol. Packets in SILC network
937 are always encrypted using, usually the shared secret session key
938 or some other key, for example, channel key, when encrypting channel
939 messages. The SILC Packet Protocol is a wide protocol and is described
940 in [SILC2]. This document does not define or describe details of
948 3.8 Packet Encryption
950 All packets passed in SILC network MUST be encrypted. This section
951 defines how packets must be encrypted in the SILC network. The detailed
952 description of the actual encryption process of the packets are
953 described in [SILC2].
955 Client and its server shares secret symmetric session key which is
956 established by the SILC Key Exchange Protocol, described in [SILC3].
957 Every packet sent from client to server, with exception of packets for
958 channels, are encrypted with this session key.
960 Channels has their own key that are shared by every client on the channel.
961 However, the channel keys are cell specific thus one cell does not know
962 the channel key of the other cell, even if that key is for same channel.
963 Channel key is also known by the routers and all servers that has clients
964 on the channel. However, channels MAY have channel private keys that
965 are entirely local setting for the client. All clients on the channel
966 MUST know the channel private key before hand to be able to talk on the
967 channel. In this case, no server or router knows the key for channel.
969 Server shares secret symmetric session key with router which is
970 established by the SILC Key Exchange Protocol. Every packet passed from
971 server to router, with exception of packets for channels, are encrypted
972 with the shared session key. Same way, router server shares secret
973 symmetric key with its primary route. However, every packet passed
974 from router to other router, including packets for channels, are
975 encrypted with the shared session key. Every router connection has
976 their own session keys.
980 3.8.1 Determination of the Source and the Destination
982 The source and the destination of the packet needs to be determined
983 to be able to route the packets to correct receiver. This information
984 is available in the SILC Packet Header which is included in all packets
985 sent in SILC network. The SILC Packet Header is described in [SILC2].
987 The header MUST be encrypted with the session key who is next receiver
988 of the packet along the route. The receiver of the packet, for example
989 a router along the route, is able to determine the sender and the
990 destination of the packet by decrypting the SILC Packet Header and
991 checking the ID's attached to the header. The ID's in the header will
992 tell to where the packet needs to be sent and where it is coming from.
994 The header in the packet MUST NOT change during the routing of the
995 packet. The original sender, for example client, assembles the packet
996 and the packet header and server or router between the sender and the
997 receiver MUST NOT change the packet header.
999 Note that the packet and the packet header may be encrypted with
1000 different keys. For example, packets to channels are encrypted with
1001 the channel key, however, the header is encrypted with the session key
1002 as described above. However, the header and the packet may be encrypted
1003 with same key. This is the case, for example, with command packets.
1007 3.8.2 Client To Client
1009 The process of message delivery and encryption from client to another
1010 client is as follows.
1012 Example: Private message from client to another client on different
1013 servers. Clients do not share private message delivery
1014 keys; normal session keys are used.
1016 o Client 1. sends encrypted packet to its server. The packet is
1017 encrypted with the session key shared between client and its
1020 o Server determines the destination of the packet and decrypts
1021 the packet. Server encrypts the packet with session key shared
1022 between the server and its router, and sends the packet to the
1025 o Router determines the destination of the packet and decrypts
1026 the packet. Router encrypts the packet with session key
1027 shared between the router and the destination server, and sends
1028 the packet to the server.
1030 o Server determines the client to which the packet is destined
1031 to and decrypts the packet. Server encrypts the packet with
1032 session key shared between the server and the destination client,
1033 and sends the packet to the client.
1035 o Client 2. decrypts the packet.
1038 Example: Private message from client to another client on different
1039 servers. Clients has established secret shared private
1040 message delivery key with each other and that is used in
1041 the message encryption.
1043 o Client 1. sends encrypted packet to its server. The packet is
1044 encrypted with the private message delivery key shared between
1047 o Server determines the destination of the packet and sends the
1048 packet to the router.
1050 o Router determines the destination of the packet and sends the
1051 packet to the server.
1053 o Server determines the client to which the packet is destined
1054 to and sends the packet to the client.
1056 o Client 2. decrypts the packet with the secret shared key.
1059 If clients share secret key with each other the private message
1060 delivery is much simpler since servers and routers between the
1061 clients do not need to decrypt and re-encrypt the packet.
1063 The process for clients on same server is much simpler as there are
1064 no need to send the packet to the router. The process for clients
1065 on different cells is same as above except that the packet is routed
1066 outside the cell. The router of the destination cell routes the
1067 packet to the destination same way as described above.
1071 3.8.3 Client To Channel
1073 Process of message delivery from client on channel to all the clients
1076 Example: Channel of four users; two on same server, other two on
1077 different cells. Client sends message to the channel.
1079 o Client 1. encrypts the packet with channel key and sends the
1080 packet to its server.
1082 o Server determines local clients on the channel and sends the
1083 packet to the Client on the same server. Server then sends
1084 the packet to its router for further routing.
1086 o Router determines local clients on the channel, if found
1087 sends packet to the local clients. Router determines global
1088 clients on the channel and sends the packet to its primary
1089 router or fastest route.
1091 o (Other router(s) do the same thing and sends the packet to
1094 o Server determines local clients on the channel and sends the
1095 packet to the client.
1097 o All clients receiving the packet decrypts the packet.
1101 3.8.4 Server To Server
1103 Server to server packet delivery and encryption is described in above
1104 examples. Router to router packet delivery is analogous to server to
1105 server. However, some packets, such as channel packets, are processed
1106 differently. These cases are described later in this document and
1107 more in detail in [SILC2].
1111 3.9 Key Exchange And Authentication
1113 Key exchange is done always when for example client connects to server
1114 but also when server and router, and router and router connects to each
1115 other. The purpose of key exchange protocol is to provide secure key
1116 material to be used in the communication. The key material is used to
1117 derive various security parameters used to secure SILC packets. The
1118 SILC Key Exchange protocol is described in detail in [SILC3].
1120 Authentication is done after key exchange protocol has been successfully
1121 completed. The purpose of authentication is to authenticate for example
1122 client connecting to the server. However, Usually clients are accepted
1123 to connect to server without explicit authentication. Servers are
1124 required use authentication protocol when connecting. The authentication
1125 may be based on passphrase (pre-shared-secret) or public key. The
1126 connection authentication protocol is described in detail in [SILC3].
1130 3.9.1 Authentication Payload
1132 Authentication payload is used separately from the SKE and the Connection
1133 Authentication protocol. It is used during the session to authenticate
1134 with the remote. For example, the client can authenticate itself to the
1135 server to become server operator. In this case, Authentication Payload is
1148 The format of the Authentication Payload is as follows:
1154 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
1155 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1156 | Payload Length | Authentication Method |
1157 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1158 | Public Data Length | |
1159 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1163 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1164 | Authentication Data Length | |
1165 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1167 ~ Authentication Data ~
1169 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|
1173 Figure 5: Authentication Payload
1177 o Payload Length (2 bytes) - Length of the entire payload.
1179 o Authentication Type (2) - The method of the authentication.
1180 The authentication methods are defined in [SILC2] in the
1181 Connection Auth Request Payload. The NONE authentication
1182 method SHOULD NOT be used.
1184 o Public Data Length (2 bytes) - Indicates the length of
1185 the Public Data field.
1187 o Public Data (variable length) - This is defined only if
1188 the authentication method is public key. If it is any other
1189 this field does not exist and the Public Data Length field
1192 When the authentication method is public key this includes
1193 128 to 4096 bytes of non-zero random data that is used in
1194 the signature process, described subsequently.
1196 o Authentication Data Length (2 bytes) - Indicates the
1197 length of the Authentication Data field.
1199 o Authentication Data (variable length) - Authentication
1200 method dependent authentication data.
1204 If the authentication method is password based, the Authentication
1205 Data field includes the plaintext password. It is safe to send
1206 plaintext password since the entire payload is encrypted. In this
1207 case the Public Data Length is set to zero (0).
1209 If the authentication method is public key based (or certificate)
1210 the Authentication Data is computed as follows:
1212 HASH = hash(random bytes | ID | public key (or certificate));
1213 Authentication Data = sign(HASH);
1215 The hash() and the sign() are the hash function and the public key
1216 cryptography function selected in the SKE protocol. The public key
1217 is SILC style public key unless certificates are used. The ID is the
1218 entity's ID (Client or Server ID) which is authenticating itself. The
1219 ID is raw ID data. The random bytes are non-zero random bytes of
1220 length between 128 and 4096 bytes, and will be included into the
1221 Public Data field as is.
1223 The receiver will compute the signature using the random data received
1224 in the payload, the ID associated to the connection and the public key
1225 (or certificate) received in the SKE protocol. After computing the
1226 receiver MUST verify the signature. In this case also, the entire
1227 payload is encrypted.
1233 This section defines all the allowed algorithms that can be used in
1234 the SILC protocol. This includes mandatory cipher, mandatory public
1235 key algorithm and MAC algorithms.
1241 Cipher is the encryption algorithm that is used to protect the data
1242 in the SILC packets. See [SILC2] of the actual encryption process and
1243 definition of how it must be done. SILC has a mandatory algorithm that
1244 must be supported in order to be compliant with this protocol.
1246 The following ciphers are defined in SILC protocol:
1249 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1250 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1251 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1252 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1253 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1254 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1255 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1256 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1257 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1258 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1259 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1260 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1261 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1262 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1263 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1264 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1265 none No encryption (OPTIONAL)
1269 Algorithm none does not perform any encryption process at all and
1270 thus is not recommended to be used. It is recommended that no client
1271 or server implementation would accept none algorithms except in special
1274 Additional ciphers MAY be defined to be used in SILC by using the
1275 same name format as above.
1279 3.10.2 Public Key Algorithms
1281 Public keys are used in SILC to authenticate entities in SILC network
1282 and to perform other tasks related to public key cryptography. The
1283 public keys are also used in the SILC Key Exchange protocol [SILC3].
1285 The following public key algorithms are defined in SILC protocol:
1292 DSS is described in [Menezes]. The RSA MUST be implemented according
1293 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1294 compliant to either PKCS #1 version 1.5 or newer with the the following
1295 notes: The signature encoding is always in same format as the encryption
1296 encoding regardless of the PKCS #1 version. The signature with appendix
1297 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1298 rationale for this is that there is no binding between the PKCS #1 OIDs
1299 and the hash algorithms used in the SILC protocol. Hence, the encoding
1300 is always in PKCS #1 version 1.5 format.
1302 Additional public key algorithms MAY be defined to be used in SILC.
1308 3.10.3 Hash Functions
1310 Hash functions are used as part of MAC algorithms defined in the next
1311 section. They are also used in the SILC Key Exchange protocol defined
1314 The following Hash algorithm are defined in SILC protocol:
1317 sha1 SHA-1, length = 20 (REQUIRED)
1318 md5 MD5, length = 16 (OPTIONAL)
1323 3.10.4 MAC Algorithms
1325 Data integrity is protected by computing a message authentication code
1326 (MAC) of the packet data. See [SILC2] for details how to compute the
1329 The following MAC algorithms are defined in SILC protocol:
1332 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1333 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1334 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1335 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1336 none No MAC (OPTIONAL)
1339 The none MAC is not recommended to be used as the packet is not
1340 authenticated when MAC is not computed. It is recommended that no
1341 client or server would accept none MAC except in special debugging
1344 The HMAC algorithm is described in [HMAC] and hash algorithms that
1345 are used as part of the HMACs are described in [Scheneir] and in
1348 Additional MAC algorithms MAY be defined to be used in SILC.
1352 3.10.5 Compression Algorithms
1354 SILC protocol supports compression that may be applied to unencrypted
1355 data. It is recommended to use compression on slow links as it may
1356 significantly speed up the data transmission. By default, SILC does not
1357 use compression which is the mode that must be supported by all SILC
1362 The following compression algorithms are defined:
1365 none No compression (REQUIRED)
1366 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1369 Additional compression algorithms MAY be defined to be used in SILC.
1373 3.11 SILC Public Key
1375 This section defines the type and format of the SILC public key. All
1376 implementations MUST support this public key type. See [SILC3] for
1377 other optional public key and certificate types allowed in the SILC
1378 protocol. Public keys in SILC may be used to authenticate entities
1379 and to perform other tasks related to public key cryptography.
1381 The format of the SILC Public Key is as follows:
1387 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
1388 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1389 | Public Key Length |
1390 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1391 | Algorithm Name Length | |
1392 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1396 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1397 | Identifier Length | |
1398 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1402 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1406 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1410 Figure 5: SILC Public Key
1414 o Public Key Length (4 bytes) - Indicates the full length
1415 of the public key, not including this field.
1417 o Algorithm Name Length (2 bytes) - Indicates the length
1418 of the Algorithm Length field, not including this field.
1420 o Algorithm name (variable length) - Indicates the name
1421 of the public key algorithm that the key is. See the
1422 section 3.10.2 Public Key Algorithms for defined names.
1424 o Identifier Length (2 bytes) - Indicates the length of
1425 the Identifier field, not including this field.
1427 o Identifier (variable length) - Indicates the identifier
1428 of the public key. This data can be used to identify
1429 the owner of the key. The identifier is of the following
1433 HN Host name or IP address
1440 Examples of an identifier:
1442 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1444 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1446 At least user name (UN) and host name (HN) MUST be provided as
1447 identifier. The fields are separated by commas (`,'). If
1448 comma is in the identifier string it must be written as `\\,',
1449 for example, `O=Company XYZ\\, Inc.'.
1451 o Public Data (variable length) - Includes the actual
1452 public data of the public key.
1454 The format of this field for RSA algorithm is
1463 The format of this field for DSS algorithm is
1475 The variable length fields are multiple precession
1476 integers encoded as strings in both examples.
1478 Other algorithms must define their own type of this
1479 field if they are used.
1482 All fields in the public key are in MSB (most significant byte first)
1487 3.12 SILC Version Detection
1489 The version detection of both client and server is performed at the
1490 connection phase while executing the SILC Key Exchange protocol. The
1491 version identifier is exchanged between initiator and responder. The
1492 version identifier is of the following format:
1495 SILC-<protocol version>-<software version>
1498 The version strings are of the following format:
1501 protocol version = <major>.<minor>
1502 software version = <major>[.<minor>[.<build>]]
1505 Protocol version MAY provide both major and minor version. Currently
1506 implementations MUST set the protocol version and accept the protocol
1507 version as SILC-1.0-<software version>.
1509 Software version MAY provide major, minor and build version. The
1510 software version MAY be freely set and accepted.
1513 Thus, the version string could be, for example:
1525 This section describes various SILC procedures such as how the
1526 connections are created and registered, how channels are created and
1527 so on. The section describes the procedures only generally as details
1528 are described in [SILC2] and [SILC3].
1532 4.1 Creating Client Connection
1534 This section describes the procedure when client connects to SILC server.
1535 When client connects to server the server MUST perform IP address lookup
1536 and reverse IP address lookup to assure that the origin host really is
1537 who it claims to be. Client, host, connecting to server MUST have
1538 both valid IP address and fully qualified domain name (FQDN).
1540 After that the client and server performs SILC Key Exchange protocol
1541 which will provide the key material used later in the communication.
1542 The key exchange protocol MUST be completed successfully before the
1543 connection registration may continue. The SILC Key Exchange protocol
1544 is described in [SILC3].
1546 Typical server implementation would keep a list of connections that it
1547 allows to connect to the server. The implementation would check, for
1548 example, the connecting client's IP address from the connection list
1549 before the SILC Key Exchange protocol has been started. Reason for
1550 this is that if the host is not allowed to connect to the server there
1551 is no reason to perform the key exchange protocol.
1553 After successful key exchange protocol the client and server performs
1554 connection authentication protocol. The purpose of the protocol is to
1555 authenticate the client connecting to the server. Flexible
1556 implementation could also accept the client to connect to the server
1557 without explicit authentication. However, if authentication is
1558 desired for a specific client it may be based on passphrase or
1559 public key authentication. If authentication fails the connection
1560 MUST be terminated. The connection authentication protocol is described
1563 After successful key exchange and authentication protocol the client
1564 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1565 server. This packet includes various information about the client
1566 that the server uses to create the client. Server creates the client
1567 and sends SILC_PACKET_NEW_ID to the client which includes the created
1568 Client ID that the client MUST start using after that. After that
1569 all SILC packets from the client MUST have the Client ID as the
1570 Source ID in the SILC Packet Header, described in [SILC2].
1572 Client MUST also get the server's Server ID that is to be used as
1573 Destination ID in the SILC Packet Header when communicating with
1574 the server (for example when sending commands to the server). The
1575 ID may be resolved in two ways. Client can take the ID from an
1576 previously received packet from server that MUST include the ID,
1577 or to send SILC_COMMAND_INFO command and receive the Server ID as
1580 Server MAY choose not to use the information received in the
1581 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1582 certificate were used in the authentication, server MAY use those
1583 informations rather than what it received from client. This is suitable
1584 way to get the true information about client if it is available.
1586 The nickname of client is initially set to the username sent in the
1587 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1588 suitable by sending SILC_COMMAND_NICK command. However, this is not
1589 required as part of registration process.
1591 Server MUST also distribute the information about newly registered
1592 client to its router (or if the server is router, to all routers in
1593 the SILC network). More information about this in [SILC2].
1597 4.2 Creating Server Connection
1599 This section describes the procedure when server connects to its
1600 router (or when router connects to other router, the cases are
1601 equivalent). The procedure is very much alike when client connects
1602 to the server thus it is not repeated here.
1604 One difference is that server MUST perform connection authentication
1605 protocol with proper authentication. A proper authentication is based
1606 on passphrase or public key authentication.
1608 After server and router has successfully performed the key exchange
1609 and connection authentication protocol, the server register itself
1610 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1611 includes the server's Server ID that it has created by itself and
1612 other relevant information about the server.
1614 After router has received the SILC_PACKET_NEW_SERVER packet it
1615 distributes the information about newly registered server to all routers
1616 in the SILC network. More information about this in [SILC2].
1618 As client needed to resolve the destination ID this MUST be done by the
1619 server that connected to the router, as well. The way to resolve it is
1620 to get the ID from previously received packet. The server MAY also
1621 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1622 using its own Server ID as Source ID in SILC Packet Header and the
1623 router's Server ID as Destination when communicating with the router.
1627 4.2.1 Announcing Clients, Channels and Servers
1629 After server or router has connected to the remote router, and it already
1630 has connected clients and channels it MUST announce them to the router.
1631 If the server is router server, also all the local servers in the cell
1634 All clients are announced by compiling a list of ID Payloads into the
1635 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1636 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also,
1637 the channel users on the channels must be announced by compiling a
1638 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1639 the SILC_PACKET_NOTIFY packet.
1641 The router MUST also announce the local servers by compiling list of
1642 ID Payloads into the SILC_PACKET_NEW_ID packet.
1644 The router which receives these lists MUST process them and broadcast
1645 the packets to its primary route.
1647 When processing the announced channels and channel users the router MUST
1648 check whether a channel exists already with the same name. If channel
1649 exists with the same name it MUST check whether the Channel ID is
1650 different. If the Channel ID is different the router MUST send the notify
1651 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1652 change to the ID the router has. If the mode of the channel is different
1653 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1654 server to force the mode change to the mode that the router has.
1656 The router MUST also generate new channel key and distribute it to the
1657 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1660 If the channel has channel founder on the router the router MUST send
1661 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1662 the mode change for the channel founder on the server. The channel
1663 founder privileges MUST be removed.
1665 The router processing the channels MUST also compile a list of
1666 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1667 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1668 server (or router) will receive the clients on the channel that
1673 4.3 Joining to a Channel
1675 This section describes the procedure when client joins to a channel.
1676 Client may join to channel by sending command SILC_COMMAND_JOIN to the
1677 server. If the receiver receiving join command is normal server the
1678 server MUST check its local list whether this channel already exists
1679 locally. This would indicate that some client connected to the server
1680 has already joined to the channel. If this is case the client is
1681 joined to the client, new channel key is created and information about
1682 newly joined channel is sent to the router. The router is informed
1683 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1684 also be sent to the local clients on the channel. The new channel key
1685 is also sent to the router and to local clients on the channel.
1687 If the channel does not exist in the local list the client's command
1688 MUST be sent to the router which will then perform the actual joining
1689 procedure. When server receives the reply to the command from the
1690 router it MUST be sent to the client which sent the command originally.
1691 Server will also receive the channel key from the server that it MUST
1692 send to the client which originally requested the join command. The
1693 server MUST also save the channel key.
1695 If the receiver of the join command is router it MUST first check its
1696 local list whether anyone in the cell has already joined to the channel.
1697 If this is the case the client is joined to the channel and reply is
1698 sent to the client. If the command was sent by server the command reply
1699 is sent to the server which sent it. Then the router MUST also create
1700 new channel key and distribute it to all clients on the channel and
1701 all servers that has clients on the channel. Router MUST also send
1702 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1703 and to local servers that has clients on the channel.
1705 If the channel does not exist on the router's local list it MUST
1706 check the global list whether the channel exists at all. If it does
1707 the client is joined to the channel as described previously. If
1708 the channel does not exist the channel is created and the client
1709 is joined to the channel. The channel key is also created and
1710 distributed as previously described. The client joining to the created
1711 channel is made automatically channel founder and both channel founder
1712 and channel operator privileges is set for the client.
1714 If the router created the channel in the process, information about the
1715 new channel MUST be broadcasted to all routers. This is done by
1716 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1717 route. When the router joins the client to the channel it MUST also
1718 send information about newly joined client to all routers in the SILC
1719 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1720 type to the router's primary route.
1722 It is important to note that new channel key is created always when
1723 new client joins to channel, whether the channel has existed previously
1724 or not. This way the new client on the channel is not able to decrypt
1725 any of the old traffic on the channel. Client which receives the reply to
1726 the join command MUST start using the received Channel ID in the channel
1727 message communication thereafter. Client also receives the key for the
1728 channel in the command reply. Note that the channel key is never
1729 generated if the SILC_CMODE_PRIVKEY mode is set.
1733 4.4 Channel Key Generation
1735 Channel keys are created by router which creates the channel by taking
1736 enough randomness from cryptographically strong random number generator.
1737 The key is generated always when channel is created, when new client
1738 joins a channel and after the key has expired. Key could expire for
1741 The key MUST also be re-generated whenever some client leaves a channel.
1742 In this case the key is created from scratch by taking enough randomness
1743 from the random number generator. After that the key is distributed to
1744 all clients on the channel. However, channel keys are cell specific thus
1745 the key is created only on the cell where the client, which left the
1746 channel, exists. While the server or router is creating the new channel
1747 key, no other client may join to the channel. Messages that are sent
1748 while creating the new key are still processed with the old key. After
1749 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1750 using the new key. If server creates the new key the server MUST also
1751 send the new key to its router. See [SILC2] on more information about
1752 how channel messages must be encrypted and decrypted when router is
1755 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1756 Channel Key Payload it MUST process the key data to create encryption
1757 and decryption key, and to create the HMAC key that is used to compute
1758 the MACs of the channel messages. The processing is as follows:
1760 channel_key = raw key data
1761 HMAC key = hash(raw key data)
1763 The raw key data is the key data received in the Channel Key Payload.
1764 The hash() function is the hash function used in the HMAC of the channel.
1765 Note that the server MUST also save the channel key.
1769 4.5 Private Message Sending and Reception
1771 Private messages are sent point to point. Client explicitly destines
1772 a private message to specific client that is delivered to only to that
1773 client. No other client may receive the private message. The receiver
1774 of the private message is destined in the SILC Packet Header as any
1775 other packet as well.
1777 If the sender of a private message does not know the receiver's Client
1778 ID, it MUST resolve it from server. There are two ways to resolve the
1779 client ID from server; it is RECOMMENDED that client implementations
1780 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
1781 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
1782 If the sender has received earlier a private message from the receiver
1783 it should have cached the Client ID from the SILC Packet Header.
1785 Receiver of a private message SHOULD NOT explicitly trust the nickname
1786 that it receives in the Private Message Payload, described in [SILC2].
1787 Implementations could resolve the nickname from server, as described
1788 previously, and compare the received Client ID and the SILC Packet
1789 Header's Client ID. The nickname in the payload is merely provided
1790 to be displayed for end user.
1792 See [SILC2] for description of private message encryption and decryption
1797 4.6 Private Message Key Generation
1799 Private message MAY be protected by the key generated by theclient.
1800 The key may be generated and sent to the other client by sending packet
1801 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
1802 and is secured by session keys. After that the private message key
1803 is used in the private message communication between those clients.
1805 Other choice is to entirely use keys that are not sent through
1806 the SILC network at all. This significantly adds security. This key
1807 would be pre-shared-key that is known by both of the clients. Both
1808 agree about using the key and starts sending packets that indicate
1809 that the private message is secured using private message key.
1811 The key material used as private message key is implementation issue.
1812 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
1813 the key material. If the key is normal pre-shared-key or randomly
1814 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
1815 the key material SHOULD be processed as defined in the [SILC3]. In
1816 the processing, however, the HASH, as defined in [SILC3] MUST be
1817 ignored. After processing the key material it is employed as defined
1818 in [SILC3], however, the HMAC key material MUST be discarded.
1820 If the key is pre-shared-key or randomly generated the implementations
1821 should use the SILC protocol's mandatory cipher as the cipher. If the
1822 SKE was used to negotiate key material the cipher was negotiated as well.
1825 4.7 Channel Message Sending and Reception
1827 Channel messages are delivered to group of users. The group forms a
1828 channel and all clients on the channel receives messages sent to the
1831 Channel messages are destined to channel by specifying the Channel ID
1832 as Destination ID in the SILC Packet Header. The server MUST then
1833 distribute the message to all clients on the channel by sending the
1834 channel message destined explicitly to a client on the channel.
1836 See [SILC2] for description of channel message encryption and decryption
1841 4.8 Session Key Regeneration
1843 Session keys MUST be regenerated periodically, say, once in an hour.
1844 The re-key process is started by sending SILC_PACKET_REKEY packet to
1845 other end, to indicate that re-key must be performed. The initiator
1846 of the connection SHOULD initiate the re-key.
1848 If perfect forward secrecy (PFS) flag was selected in the SILC Key
1849 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
1850 SKE protocol. In this case the protocol is secured with the old key
1851 and the protocol results to new key material. See [SILC3] for more
1852 information. After the SILC_PACKET_REKEY packet is sent the sender
1853 will perform the SKE protocol.
1855 If PFS flag was set the resulted key material is processed as described
1856 in the section Processing the Key Material in [SILC3]. The difference
1857 with re-key in the processing is that the initial data for the hash
1858 function is just the resulted key material and not the HASH as it
1859 is not computed at all with re-key. Other than that, the key processing
1860 it equivalent to normal SKE negotiation.
1862 If PFS flag was not set, which is the default case, then re-key is done
1863 without executing SKE protocol. In this case, the new key is created by
1864 providing the current sending encryption key to the SKE protocol's key
1865 processing function. The process is described in the section Processing
1866 the Key Material in [SILC3]. The difference in the processing is that
1867 the initial data for the hash function is the current sending encryption
1868 key and not the SKE's KEY and HASH values. Other than that, the key
1869 processing is equivalent to normal SKE negotiation.
1871 After both parties has regenerated the session key, both MUST send
1872 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
1873 secured with the old key. After these packets, the subsequent packets
1874 MUST be protected with the new key.
1878 4.9 Command Sending and Reception
1880 Client usually sends the commands in the SILC network. In this case
1881 the client simply sends the command packet to server and the server
1882 processes it and replies with command reply packet.
1884 However, if the server is not able to process the command, it is sent
1885 to the server's router. This is case for example with commands such
1886 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
1887 are other commands as well. For example, if client sends the WHOIS
1888 command requesting specific information about some client the server must
1889 send the WHOIS command to router so that all clients in SILC network
1890 are searched. The router, on the other hand, sends the WHOIS command
1891 further to receive the exact information about the requested client.
1892 The WHOIS command travels all the way to the server which owns the client
1893 and it replies with command reply packet. Finally, the server which
1894 sent the command receives the command reply and it must be able to
1895 determine which client sent the original command. The server then
1896 sends command reply to the client. Implementations should have some
1897 kind of cache to handle, for example, WHOIS information. Servers
1898 and routers along the route could all cache the information for faster
1899 referencing in the future.
1901 The commands sent by server may be sent hop by hop until someone is able
1902 to process the command. However, it is preferred to destine the command
1903 as precisely as it is possible. In this case, other routers en route
1904 MUST route the command packet by checking the true sender and true
1905 destination of the packet. However, servers and routers MUST NOT route
1906 command reply packets to clients coming from other server. Client
1907 MUST NOT accept command reply packet originated from anyone else but
1908 from its own server.
1912 4.10 Closing Connection
1914 When remote client connection is closed the server MUST send the notify
1915 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
1916 the client was joined. The server MUST also save the client's information
1917 for a period of time for history purposes.
1919 When remote server or router connection is closed the server or router
1920 MUST also remove all the clients that was behind the server or router
1921 from the SILC Network. The server or router MUST also send the notify
1922 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
1923 local clients that are joined on the same channels with the remote
1924 server's or router's clients.
1928 5 Security Considerations
1930 Security is central to the design of this protocol, and these security
1931 considerations permeate the specification. Common security considerations
1932 such as keeping private keys truly private and using adequate lengths for
1933 symmetric and asymmetric keys must be followed in order to maintain the
1934 security of this protocol.
1936 Special attention must also be paid on the servers and routers that are
1937 running the SILC service. The SILC protocol's security depends greatly
1938 on the security and the integrity of the servers and administrators that
1939 are running the service. It is recommended that some form of registration
1940 is required by the server and router administrator prior acceptance to
1941 the SILC Network. The clients must be able to trust the servers they
1944 It is also recommended that router operators in the SILC Network would
1945 form a joint forum to discuss the router and SILC Network management
1946 issues. Also, router operators along with the cell's server operators
1947 should have a forum to discuss the cell management issues.
1953 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
1956 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
1957 Protocols", Internet Draft, April 2001.
1959 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001.
1961 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
1964 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
1967 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
1970 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
1973 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
1976 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
1979 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
1982 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
1985 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
1986 Infrastructure, Certificate and CRL Profile", RFC 2459,
1989 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
1990 John Wiley & Sons, New York, NY, 1996.
1992 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
1995 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
1996 RFC 2412, November 1998.
1998 [ISAKMP] Maughan D., et al, "Internet Security Association and
1999 Key Management Protocol (ISAKMP)", RFC 2408, November
2002 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2003 (IKE)", RFC 2409, November 1998.
2005 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2006 Authentication", RFC 2104, February 1997.
2008 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2009 Specifications, Version 2.0", RFC 2437, October 1998.
2011 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2012 Requirement Levels", BCP 14, RFC 2119, March 1997.
2024 EMail: priikone@poseidon.pspt.fi
2026 This Internet-Draft expires XX October 2001