8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-04.txt XX XXXXXXX 2001
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-04.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 3 SILC Specification ............................................ 10
82 3.1 Client .................................................... 10
83 3.1.1 Client ID ........................................... 10
84 3.2 Server .................................................... 11
85 3.2.1 Server's Local ID List .............................. 12
86 3.2.2 Server ID ........................................... 13
87 3.2.3 SILC Server Ports ................................... 14
88 3.3 Router .................................................... 14
89 3.3.1 Router's Local ID List .............................. 14
90 3.3.2 Router's Global ID List ............................. 15
91 3.3.3 Router's Server ID .................................. 15
92 3.4 Channels .................................................. 16
93 3.4.1 Channel ID .......................................... 17
94 3.5 Operators ................................................. 17
95 3.6 SILC Commands ............................................. 18
96 3.7 SILC Packets .............................................. 18
97 3.8 Packet Encryption ......................................... 19
98 3.8.1 Determination of the Source and the Destination ..... 19
99 3.8.2 Client To Client .................................... 20
100 3.8.3 Client To Channel ................................... 21
101 3.8.4 Server To Server .................................... 22
102 3.9 Key Exchange And Authentication ........................... 22
103 3.9.1 Authentication Payload .............................. 22
104 3.10 Algorithms ............................................... 24
105 3.10.1 Ciphers ............................................ 24
106 3.10.2 Public Key Algorithms .............................. 25
107 3.10.3 Hash Functions ..................................... 26
108 3.10.4 MAC Algorithms ..................................... 26
109 3.10.5 Compression Algorithms ............................. 26
110 3.11 SILC Public Key .......................................... 27
111 3.12 SILC Version Detection ................................... 29
112 3.13 Backup Routers ........................................... 8
113 3.13.1 Switching to Backup Router ......................... 8
114 3.13.2 Resuming Primary Router ............................ 8
115 3.13.3 Discussion on Backup Router Scheme ................. 8
116 4 SILC Procedures ............................................... 30
117 4.1 Creating Client Connection ................................ 30
118 4.2 Creating Server Connection ................................ 31
119 4.2.1 Announcing Clients, Channels and Servers ............ 32
120 4.3 Joining to a Channel ...................................... 33
121 4.4 Channel Key Generation .................................... 34
122 4.5 Private Message Sending and Reception ..................... 34
123 4.6 Private Message Key Generation ............................ 35
124 4.7 Channel Message Sending and Reception ..................... 35
125 4.8 Session Key Regeneration .................................. 36
126 4.9 Command Sending and Reception ............................. 37
127 4.10 Closing Connection ....................................... 37
128 5 Security Considerations ....................................... 38
129 6 References .................................................... 38
130 7 Author's Address .............................................. 40
138 Figure 1: SILC Network Topology
139 Figure 2: Communication Inside cell
140 Figure 3: Communication Between Cells
141 Figure 4: Router Connections
142 Figure 5: SILC Public Key
148 This document describes a Secure Internet Live Conferencing (SILC)
149 protocol which provides secure conferencing services over insecure
150 network channel. SILC is IRC [IRC] like protocol, however, it is
151 not equivalent to IRC and does not support IRC.
153 Strong cryptographic methods are used to protect SILC packets inside
154 the SILC network. Three other Internet Drafts relates very closely
155 to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
156 Authentication Protocols [SILC3] and SILC Commands [SILC4].
158 The protocol uses extensively packets as conferencing protocol
159 requires message and command sending. The SILC Packet Protocol is
160 described in [SILC2] and should be read to fully comprehend this
161 document and protocol. [SILC2] also describes the packet encryption
162 and decryption in detail.
164 The security of SILC protocol, and for any security protocol for that
165 matter, is based on strong and secure key exchange protocol. The SILC
166 Key Exchange protocol is described in [SILC3] along with connection
167 authentication protocol and should be read to fully comprehend this
168 document and protocol.
170 The SILC protocol has been developed to work on TCP/IP network
171 protocol, although it could be made to work on other network protocols
172 with only minor changes. However, it is recommended that TCP/IP
173 protocol is used under SILC protocol. Typical implementation would
174 be made in client-server model.
178 1.1 Requirements Terminology
180 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
181 MAY, and OPTIONAL, when they appear in this document, are to be
182 interpreted as described in [RFC2119].
188 This section describes various SILC protocol concepts that forms the
189 actual protocol, and in the end, the actual SILC network. The mission
190 of the protocol is to deliver messages from clients to other clients
191 through routers and servers in secure manner. The messages may also
192 be delivered from one client to many clients forming a group, also
195 This section does not focus to security issues. Instead, basic network
196 concepts are introduced to make the topology of the SILC network
201 2.1 SILC Network Topology
203 SILC network is a cellular network as opposed to tree style network
204 topology. The rationale for this is to have servers that can perform
205 specific kind of tasks what other servers cannot perform. This leads
206 to two kinds of servers; normal SILC servers and SILC routers.
208 A difference between normal server and router server is that routers
209 knows everything about everything in the network. They also do the
210 actual routing of the messages to the correct receiver. Normal servers
211 knows only about local information and nothing about global information.
212 This makes the network faster as there are less servers that needs to
213 keep global information up to date at all time.
215 This, on the other hand, leads to cellular like network, where routers
216 are in the center of the cell and servers are connected to the router.
224 The following diagram represents SILC network topology.
228 ---- ---- ---- ---- ---- ----
229 | S8 | S5 | S4 | | S7 | S5 | S6 |
230 ----- ---- ----- ----- ---- -----
231 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
232 ---- ------ ---- ---- ------ ----
233 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
234 ---- ---- ---- ---- ---- ---- | S3 | S1 |
235 Cell 1. \\ Cell 2. | \\____ ----- -----
237 ---- ---- ---- ---- ---- ---- ---- ------
238 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
239 ----- ---- ----- ----- ---- ----- ---- ----
240 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
241 ---- ------ ---- ---- ------ ----
242 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
243 ---- ---- ---- ---- ---- ----
248 Figure 1: SILC Network Topology
251 A cell is formed when a server or servers connect to one router. In
252 SILC network normal server cannot directly connect to other normal
253 server. Normal server may only connect to SILC router which then
254 routes the messages to the other servers in the cell. Router servers
255 on the other hand may connect to other routers to form the actual SILC
256 network, as seen in above figure. However, router is also normal SILC
257 server; clients may connect to it the same way as to normal SILC
258 server. Normal server also cannot have active connections to more
259 than one router. Normal server cannot be connected to two different
260 cells. Router servers, on the other hand, may have as many router to
261 router connections as needed.
263 There are many issues in this network topology that needs to be careful
264 about. Issues like the size of the cells, the number of the routers in
265 the SILC network and the capacity requirements of the routers. These
266 issues should be discussed in the Internet Community and additional
267 documents on the issue may be written.
271 2.2 Communication Inside a Cell
273 It is always guaranteed that inside a cell message is delivered to the
274 recipient with at most two server hops. A client which is connected to
275 server in the cell and is talking on channel to other client connected
276 to other server in the same cell, will have its messages delivered from
277 its local server first to the router of the cell, and from the router
278 to the other server in the cell.
280 The following diagram represents this scenario:
294 Figure 2: Communication Inside cell
297 Example: Client 1. connected to Server 1. send message to
298 Client 4. connected to Server 2. travels from Server 1.
299 first to Router which routes the message to Server 2.
300 which then sends it to the Client 4. All the other
301 servers in the cell will not see the routed message.
304 If the client is connected directly to the router, as router is also normal
305 SILC server, the messages inside the cell are always delivered only with
306 one server hop. If clients communicating with each other are connected
307 to the same server, no router interaction is needed. This is the optimal
308 situation of message delivery in the SILC network.
312 2.3 Communication in the Network
314 If the message is destined to server that does not belong to local cell
315 the message is routed to the router server to which the destination
316 server belongs, if the local router is connected to destination router.
317 If there is no direct connection to the destination router, the local
318 router routes the message to its primary route. The following diagram
319 represents message sending between cells.
324 1 --- S1 S4 --- 5 S2 --- 1
325 S/R - - - - - - - - S/R
335 Figure 3: Communication Between Cells
338 Example: Client 5. connected to Server 4. in Cell 1. sends message
339 to Client 2. connected to Server 1. in Cell 2. travels
340 from Server 4. to Router which routes the message to
341 Router in Cell 2, which then routes the message to
342 Server 1. All the other servers and routers in the
343 network will not see the routed message.
346 The optimal case of message delivery from the client point of view is
347 when clients are connected directly to the routers and the messages
348 are delivered from one router to the other.
352 2.4 Channel Communication
354 Messages may be sent to group of clients as well. Sending messages to
355 many clients works the same way as sending messages point to point, from
356 message delivery point of view. Security issues are another matter
357 which are not discussed in this section.
359 Router server handles the message routing to multiple recipients. If
360 any recipient is not in the same cell as the sender the messages are
363 Server distributes the channel message to its local clients which are
364 joined to the channel. Router also distributes the message to its
365 local clients on the channel.
369 2.5 Router Connections
371 Router connections play very important role in making the SILC like
372 network topology to work. For example, sending broadcast packets in
373 SILC network require special connections between routers; routers must
374 be connected in a specific way.
376 Every router has their primary route which is a connection to another
377 router in the network. Unless there is only two routers in the network
378 must not routers use each other as their primary routes. The router
379 connections in the network must form a ring.
387 Example with three routers in the network:
392 S/R1 - > - > - > - > - > - > - S/R2
395 \\ - < - < - S/R3 - < - < - /
400 Figure 4: Router Connections
403 Example: Network with three routers. Router 1. uses Router 2. as its
404 primary router. Router 2. uses Router 3. as its primary router,
405 and Router 3. uses Router 1. as its primary router. There may
406 be other direct connections between the routers but they must
407 not be used as primary routes.
409 The above example is applicable to any amount of routers in the network
410 except for two routers. If there are only two routers in the network both
411 routers must be able to handle situation where they use each other as their
414 The issue of router connections are very important especially with SILC
415 broadcast packets. Usually all router wide information in the network is
416 distributed by SILC broadcast packets.
420 3. SILC Specification
422 This section describes the SILC protocol. However, [SILC2] and
423 [SILC3] describes other important protocols that are part of this SILC
424 specification and must be read.
430 A client is a piece of software connecting to SILC server. SILC client
431 cannot be SILC server. Purpose of clients is to provide the user
432 interface of the SILC services for end user. Clients are distinguished
433 from other clients by unique Client ID. Client ID is a 128 bit ID that
434 is used in the communication in the SILC network. The client ID is
435 based on the nickname selected by the user. User uses logical nicknames
436 in communication which are then mapped to the corresponding Client ID.
437 Client ID's are low level identifications and must not be seen by the
440 Clients provide other information about the end user as well. Information
441 such as the nickname of the user, username and the host name of the end
442 user and user's real name. See section 3.2 Server for information of
443 the requirements of keeping this information.
445 The nickname selected by the user is not unique in the SILC network.
446 There can be 2^8 same nicknames for one IP address. As for comparison
447 to IRC [IRC] where nicknames are unique this is a fundamental difference
448 between SILC and IRC. This causes the server names or client's host names
449 to be used along with the nicknames to identify specific users when sending
450 messages. This feature of SILC makes IRC style nickname-wars obsolete as
451 no one owns their nickname; there can always be someone else with the same
452 nickname. The maximum length of nickname is 128 characters.
458 Client ID is used to identify users in the SILC network. The Client ID
459 is unique to the extent that there can be 2^128 different Client ID's,
460 and ID's based on IPv6 addresses extends this to 2^224 different Client
461 ID's. Collisions are not expected to happen. The Client ID is defined
467 128 bit Client ID based on IPv4 addresses:
469 32 bit Server ID IP address (bits 1-32)
470 8 bit Random number or counter
471 88 bit Truncated MD5 hash value of the nickname
473 224 bit Client ID based on IPv6 addresses:
475 128 bit Server ID IP address (bits 1-128)
476 8 bit Random number or counter
477 88 bit Truncated MD5 hash value of the nickname
479 o Server ID IP address - Indicates the server where this
480 client is coming from. The IP address hence equals the
481 server IP address where to the client has connected.
483 o Random number or counter - Random number to further
484 randomize the Client ID. Another choice is to use
485 a counter starting from the zero (0). This makes it
486 possible to have 2^8 same nicknames from the same
489 o MD5 hash - MD5 hash value of the nickname is truncated
490 taking 88 bits from the start of the hash value. This
491 hash value is used to search the user's Client ID from
495 Collisions could occur when more than 2^8 clients using same nickname
496 from the same server IP address is connected to the SILC network.
497 Server MUST be able to handle this situation by refusing to accept
498 anymore of that nickname.
500 Another possible collision may happen with the truncated hash value of
501 the nickname. It could be possible to have same truncated hash value for
502 two different nicknames. However, this is not expected to happen nor
503 cause any problems if it would occur. Nicknames are usually logical and
504 it is unlikely to have two distinct logical nicknames produce same
505 truncated hash value.
511 Servers are the most important parts of the SILC network. They form the
512 basis of the SILC, providing a point to which clients may connect to.
513 There are two kinds of servers in SILC; normal servers and router servers.
514 This section focus on the normal server and router server is described
515 in the section 3.3 Router.
517 Normal servers MUST NOT directly connect to other normal server. Normal
518 servers may only directly connect to router server. If the message sent
519 by the client is destined outside the local server it is always sent to
520 the router server for further routing. Server may only have one active
521 connection to router on same port. Normal server MUST NOT connect to other
522 cell's router except in situations where its cell's router is unavailable.
524 Servers and routers in the SILC network are considered to be trusted.
525 With out a doubt, servers that are set to work on ports above 1023 are
526 not considered to be trusted. Also, the service provider acts important
527 role in the server's trustworthy.
531 3.2.1 Server's Local ID List
533 Normal server keeps various information about the clients and their end
534 users connected to it. Every normal server MUST keep list of all locally
535 connected clients, Client ID's, nicknames, usernames and host names and
536 user's real name. Normal servers only keeps local information and it
537 does not keep any global information. Hence, normal servers knows only
538 about their locally connected clients. This makes servers efficient as
539 they don't have to worry about global clients. Server is also responsible
540 of creating the Client ID's for their clients.
542 Normal server also keeps information about locally created channels and
546 Hence, local list for normal server includes:
549 server list - Router connection
557 client list - All clients in server
567 channel list - All channels in server
570 o Client ID's on channel
571 o Client ID modes on channel
579 Servers are distinguished from other servers by unique 64 bit Server ID
580 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
581 the SILC to route messages to correct servers. Server ID's also provide
582 information for Client ID's, see section 3.1.1 Client ID. Server ID is
586 64 bit Server ID based on IPv4 addresses:
588 32 bit IP address of the server
592 160 bit Server ID based on IPv6 addresses:
594 128 bit IP address of the server
598 o IP address of the server - This is the real IP address of
601 o Port - This is the port the server is bound to.
603 o Random number - This is used to further randomize the Server ID.
606 Collisions are not expected to happen in any conditions. The Server ID
607 is always created by the server itself and server is responsible of
608 distributing it to the router.
612 3.2.3 SILC Server Ports
614 The following ports has been assigned by IANA for the SILC protocol:
622 If there are needs to create new SILC networks in the future the port
623 numbers must be officially assigned by the IANA.
625 Server on network above privileged ports (>1023) SHOULD NOT be trusted
626 as they could have been set up by untrusted party.
632 Router server in SILC network is responsible for keeping the cell together
633 and routing messages to other servers and to other routers. Router server
634 is also a normal server thus clients may connect to it as it would be
635 just normal SILC server.
637 However, router servers has a lot of important tasks that normal servers
638 do not have. Router server knows everything about everything in the SILC.
639 They know all clients currently on SILC, all servers and routers and all
640 channels in SILC. Routers are the only servers in SILC that care about
641 global information and keeping them up to date at all time. And, this
642 is what they must do.
646 3.3.1 Router's Local ID List
648 Router server as well MUST keep local list of connected clients and
649 locally created channels. However, this list is extended to include all
650 the informations of the entire cell, not just the server itself as for
653 However, on router this list is a lot smaller since routers do not need
654 to keep information about user's nickname, username and host name and real
655 name since these are not needed by the router. The router keeps only
656 information that it needs.
659 Hence, local list for router includes:
662 server list - All servers in the cell
669 client list - All clients in the cell
673 channel list - All channels in the cell
675 o Client ID's on channel
676 o Client ID modes on channel
681 Note that locally connected clients and other information include all the
682 same information as defined in section section 3.2.1 Server's Local ID
687 3.3.2 Router's Global ID List
689 Router server MUST also keep global list. Normal servers do not have
690 global list as they know only about local information. Global list
691 includes all the clients on SILC, their Client ID's, all created channels
692 and their Channel ID's and all servers and routers on SILC and their
693 Server ID's. That is said, global list is for global information and the
694 list must not include the local information already on the router's local
697 Note that the global list does not include information like nicknames,
698 usernames and host names or user's real names. Router does not need to
699 keep these informations as they are not needed by the router. This
700 information is available from the client's server which maybe queried
703 Hence, global list includes:
706 server list - All servers in SILC
711 client list - All clients in SILC
714 channel list - All channels in SILC
716 o Client ID's on channel
717 o Client ID modes on channel
728 3.3.3 Router's Server ID
730 Router's Server ID's are equivalent to normal Server ID's. As routers
731 are normal servers as well same types of ID's applies for routers as well.
732 Thus, see section 3.2.2 Server ID.
738 A channel is a named group of one or more clients which will all receive
739 messages addressed to that channel. The channel is created when first
740 client requests JOIN command to the channel, and the channel ceases to
741 exist when the last client has left it. When channel exists, any client
742 can reference it using the name of the channel.
744 Channel names are unique although the real uniqueness comes from 64 bit
745 Channel ID. However, channel names are still unique and no two global
746 channels with same name may exist. The Channel name is a string of
747 maximum length of 256 characters. Channel names MUST NOT contain any
748 spaces (` '), any non-printable ASCII characters, commas (`,') and
751 Channels can have operators that can administrate the channel and
752 operate all of its modes. The following operators on channel exist on
756 o Channel founder - When channel is created the joining client becomes
757 channel founder. Channel founder is channel operator with some more
758 privileges. Basically, channel founder can fully operate the channel
759 and all of its modes. The privileges are limited only to the
760 particular channel. There can be only one channel founder per
761 channel. Channel founder supersedes channel operator's privileges.
763 Channel founder privileges cannot be removed by any other operator on
764 channel. When channel founder leaves the channel there is no channel
765 founder on the channel. However, it is possible to set a mode for
766 the channel which allows the original channel founder to regain the
767 founder privileges even after leaving the channel. Channel founder
768 also cannot be removed by force from the channel.
770 o Channel operator - When client joins to channel that has not existed
771 previously it will become automatically channel operator (and channel
772 founder discussed above). Channel operator is able administrate the
773 channel, set some modes on channel, remove a badly behaving client
774 from the channel and promote other clients to become channel
775 operator. The privileges are limited only to the particular channel.
777 Normal channel user may be promoted (opped) to channel operator
778 gaining channel operator privileges. Channel founder or other
779 channel operator may also demote (deop) channel operator to normal
787 Channels are distinguished from other channels by unique Channel ID.
788 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
789 collisions are not expected to happen in any conditions. Channel names
790 are just for logical use of channels. The Channel ID is created by the
791 server where the channel is created. The Channel ID is defined as
795 64 bit Channel ID based on IPv4 addresses:
797 32 bit Router's Server ID IP address (bits 1-32)
798 16 bit Router's Server ID port (bits 33-48)
801 160 bit Channel ID based on IPv6 addresses:
803 128 bit Router's Server ID IP address (bits 1-128)
804 16 bit Router's Server ID port (bits 129-144)
807 o Router's Server ID IP address - Indicates the IP address of
808 the router of the cell where this channel is created. This is
809 taken from the router's Server ID. This way SILC router knows
810 where this channel resides in the SILC network.
812 o Router's Server ID port - Indicates the port of the channel on
813 the server. This is taken from the router's Server ID.
815 o Random number - To further randomize the Channel ID. This makes
816 sure that there are no collisions. This also means that
817 in a cell there can be 2^16 channels.
824 Operators are normal users with extra privileges to their server or
825 router. Usually these people are SILC server and router administrators
826 that take care of their own server and clients on them. The purpose of
827 operators is to administrate the SILC server or router. However, even
828 an operator with highest privileges is not able to enter invite-only
829 channel, to gain access to the contents of a encrypted and authenticated
830 packets traveling in the SILC network or to gain channel operator
831 privileges on public channels without being promoted. They have the
832 same privileges as everyone else except they are able to administrate
833 their server or router.
839 Commands are very important part on SILC network especially for client
840 which uses commands to operate on the SILC network. Commands are used
841 to set nickname, join to channel, change modes and many other things.
843 Client usually sends the commands and server replies by sending a reply
844 packet to the command. Server MAY also send commands usually to serve
845 the original client's request. However, server MUST NOT send commands
846 to client and there are some commands that server must not send.
848 Note that the command reply is usually sent only after client has sent
849 the command request but server is allowed to send command reply packet
850 to client even if client has not requested the command. Client MAY,
851 choose to ignore the command reply.
853 It is expected that some of the commands may be miss-used by clients
854 resulting various problems on the server side. Every implementation
855 SHOULD assure that commands may not be executed more than once, say,
856 in two (2) seconds. However, to keep response rate up, allowing for
857 example five (5) commands before limiting is allowed. It is RECOMMENDED
858 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
859 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
860 as they require heavy operations. This should be sufficient to prevent
861 the miss-use of commands.
863 SILC commands are described in [SILC4].
869 Packets are naturally the most important part of the protocol and the
870 packets are what actually makes the protocol. Packets in SILC network
871 are always encrypted using, usually the shared secret session key
872 or some other key, for example, channel key, when encrypting channel
873 messages. The SILC Packet Protocol is a wide protocol and is described
874 in [SILC2]. This document does not define or describe details of
882 3.8 Packet Encryption
884 All packets passed in SILC network MUST be encrypted. This section
885 defines how packets must be encrypted in the SILC network. The detailed
886 description of the actual encryption process of the packets are
887 described in [SILC2].
889 Client and its server shares secret symmetric session key which is
890 established by the SILC Key Exchange Protocol, described in [SILC3].
891 Every packet sent from client to server, with exception of packets for
892 channels, are encrypted with this session key.
894 Channels has their own key that are shared by every client on the channel.
895 However, the channel keys are cell specific thus one cell does not know
896 the channel key of the other cell, even if that key is for same channel.
897 Channel key is also known by the routers and all servers that has clients
898 on the channel. However, channels MAY have channel private keys that
899 are entirely local setting for the client. All clients on the channel
900 MUST know the channel private key before hand to be able to talk on the
901 channel. In this case, no server or router know the key for channel.
903 Server shares secret symmetric session key with router which is
904 established by the SILC Key Exchange Protocol. Every packet passed from
905 server to router, with exception of packets for channels, are encrypted
906 with the shared session key. Same way, router server shares secret
907 symmetric key with its primary route. However, every packet passed
908 from router to other router, including packets for channels, are
909 encrypted with the shared session key. Every router connection has
910 their own session keys.
914 3.8.1 Determination of the Source and the Destination
916 The source and the destination of the packet needs to be determined
917 to be able to route the packets to correct receiver. This information
918 is available in the SILC Packet Header which is included in all packets
919 sent in SILC network. The SILC Packet Header is described in [SILC2].
921 The header MUST be encrypted with the session key who is next receiver
922 of the packet along the route. The receiver of the packet, for example
923 a router along the route, is able to determine the sender and the
924 destination of the packet by decrypting the SILC Packet Header and
925 checking the ID's attached to the header. The ID's in the header will
926 tell to where the packet needs to be sent and where it is coming from.
928 The header in the packet MUST NOT change during the routing of the
929 packet. The original sender, for example client, assembles the packet
930 and the packet header and server or router between the sender and the
931 receiver MUST NOT change the packet header.
933 Note that the packet and the packet header may be encrypted with
934 different keys. For example, packets to channels are encrypted with
935 the channel key, however, the header is encrypted with the session key
936 as described above. However, the header and the packet may be encrypted
937 with same key. This is the case, for example, with command packets.
941 3.8.2 Client To Client
943 The process of message delivery and encryption from client to another
944 client is as follows.
946 Example: Private message from client to another client on different
947 servers. Clients do not share private message delivery
948 keys; normal session keys are used.
950 o Client 1. sends encrypted packet to its server. The packet is
951 encrypted with the session key shared between client and its
954 o Server determines the destination of the packet and decrypts
955 the packet. Server encrypts the packet with session key shared
956 between the server and its router, and sends the packet to the
959 o Router determines the destination of the packet and decrypts
960 the packet. Router encrypts the packet with session key
961 shared between the router and the destination server, and sends
962 the packet to the server.
964 o Server determines the client to which the packet is destined
965 to and decrypts the packet. Server encrypts the packet with
966 session key shared between the server and the destination client,
967 and sends the packet to the client.
969 o Client 2. decrypts the packet.
972 Example: Private message from client to another client on different
973 servers. Clients has established secret shared private
974 message delivery key with each other and that is used in
975 the message encryption.
977 o Client 1. sends encrypted packet to its server. The packet is
978 encrypted with the private message delivery key shared between
981 o Server determines the destination of the packet and sends the
982 packet to the router.
984 o Router determines the destination of the packet and sends the
985 packet to the server.
987 o Server determines the client to which the packet is destined
988 to and sends the packet to the client.
990 o Client 2. decrypts the packet with the secret shared key.
993 If clients share secret key with each other the private message
994 delivery is much simpler since servers and routers between the
995 clients do not need to decrypt and re-encrypt the packet.
997 The process for clients on same server is much simpler as there are
998 no need to send the packet to the router. The process for clients
999 on different cells is same as above except that the packet is routed
1000 outside the cell. The router of the destination cell routes the
1001 packet to the destination same way as described above.
1005 3.8.3 Client To Channel
1007 Process of message delivery from client on channel to all the clients
1010 Example: Channel of four users; two on same server, other two on
1011 different cells. Client sends message to the channel.
1013 o Client 1. encrypts the packet with channel key and sends the
1014 packet to its server.
1016 o Server determines local clients on the channel and sends the
1017 packet to the Client on the same server. Server then sends
1018 the packet to its router for further routing.
1020 o Router determines local clients on the channel, if found
1021 sends packet to the local clients. Router determines global
1022 clients on the channel and sends the packet to its primary
1023 router or fastest route.
1025 o (Other router(s) do the same thing and sends the packet to
1028 o Server determines local clients on the channel and sends the
1029 packet to the client.
1031 o All clients receiving the packet decrypts the packet.
1035 3.8.4 Server To Server
1037 Server to server packet delivery and encryption is described in above
1038 examples. Router to router packet delivery is analogous to server to
1039 server. However, some packets, such as channel packets, are processed
1040 differently. These cases are described later in this document and
1041 more in detail in [SILC2].
1045 3.9 Key Exchange And Authentication
1047 Key exchange is done always when for example client connects to server
1048 but also when server and router, and router and router connects to each
1049 other. The purpose of key exchange protocol is to provide secure key
1050 material to be used in the communication. The key material is used to
1051 derive various security parameters used to secure SILC packets. The
1052 SILC Key Exchange protocol is described in detail in [SILC3].
1054 Authentication is done after key exchange protocol has been successfully
1055 completed. The purpose of authentication is to authenticate for example
1056 client connecting to the server. However, usually clients are accepted
1057 to connect to server without explicit authentication. Servers are
1058 required use authentication protocol when connecting. The authentication
1059 may be based on passphrase (pre-shared-secret) or public key. The
1060 connection authentication protocol is described in detail in [SILC3].
1064 3.9.1 Authentication Payload
1066 Authentication payload is used separately from the SKE and the Connection
1067 Authentication protocol. It is used during the session to authenticate
1068 with the remote. For example, the client can authenticate itself to the
1069 server to become server operator. In this case, Authentication Payload is
1082 The format of the Authentication Payload is as follows:
1088 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
1089 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1090 | Payload Length | Authentication Method |
1091 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1092 | Public Data Length | |
1093 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1097 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1098 | Authentication Data Length | |
1099 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1101 ~ Authentication Data ~
1103 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1107 Figure 5: Authentication Payload
1111 o Payload Length (2 bytes) - Length of the entire payload.
1113 o Authentication Method (2) - The method of the authentication.
1114 The authentication methods are defined in [SILC2] in the
1115 Connection Auth Request Payload. The NONE authentication
1116 method SHOULD NOT be used.
1118 o Public Data Length (2 bytes) - Indicates the length of
1119 the Public Data field.
1121 o Public Data (variable length) - This is defined only if
1122 the authentication method is public key. If it is any other
1123 this field MAY include a random data for padding purposes.
1124 However, in this case the field MUST be ignored by the
1127 When the authentication method is public key this includes
1128 128 to 4096 bytes of non-zero random data that is used in
1129 the signature process, described subsequently.
1131 o Authentication Data Length (2 bytes) - Indicates the
1132 length of the Authentication Data field.
1134 o Authentication Data (variable length) - Authentication
1135 method dependent authentication data.
1139 If the authentication method is password based, the Authentication
1140 Data field includes the plaintext password. It is safe to send
1141 plaintext password since the entire payload is encrypted. In this
1142 case the Public Data Length is set to zero (0), but MAY also include
1143 random data for padding purposes. It is also RECOMMENDED that maximum
1144 amount of padding is applied to SILC packet when using password based
1145 authentication. This way it is not possible to approximate the length
1146 of the password from the encrypted packet.
1148 If the authentication method is public key based (or certificate)
1149 the Authentication Data is computed as follows:
1151 HASH = hash(random bytes | ID | public key (or certificate));
1152 Authentication Data = sign(HASH);
1154 The hash() and the sign() are the hash function and the public key
1155 cryptography function selected in the SKE protocol. The public key
1156 is SILC style public key unless certificates are used. The ID is the
1157 entity's ID (Client or Server ID) which is authenticating itself. The
1158 ID is raw ID data. The random bytes are non-zero random bytes of
1159 length between 128 and 4096 bytes, and will be included into the
1160 Public Data field as is.
1162 The receiver will compute the signature using the random data received
1163 in the payload, the ID associated to the connection and the public key
1164 (or certificate) received in the SKE protocol. After computing the
1165 receiver MUST verify the signature. In this case also, the entire
1166 payload is encrypted.
1172 This section defines all the allowed algorithms that can be used in
1173 the SILC protocol. This includes mandatory cipher, mandatory public
1174 key algorithm and MAC algorithms.
1180 Cipher is the encryption algorithm that is used to protect the data
1181 in the SILC packets. See [SILC2] of the actual encryption process and
1182 definition of how it must be done. SILC has a mandatory algorithm that
1183 must be supported in order to be compliant with this protocol.
1185 The following ciphers are defined in SILC protocol:
1188 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1189 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1190 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1191 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1192 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1193 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1194 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1195 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1196 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1197 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1198 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1199 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1200 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1201 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1202 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1203 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1204 none No encryption (OPTIONAL)
1208 Algorithm none does not perform any encryption process at all and
1209 thus is not recommended to be used. It is recommended that no client
1210 or server implementation would accept none algorithms except in special
1213 Additional ciphers MAY be defined to be used in SILC by using the
1214 same name format as above.
1218 3.10.2 Public Key Algorithms
1220 Public keys are used in SILC to authenticate entities in SILC network
1221 and to perform other tasks related to public key cryptography. The
1222 public keys are also used in the SILC Key Exchange protocol [SILC3].
1224 The following public key algorithms are defined in SILC protocol:
1231 DSS is described in [Menezes]. The RSA MUST be implemented according
1232 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1233 compliant to either PKCS #1 version 1.5 or newer with the following
1234 notes: The signature encoding is always in same format as the encryption
1235 encoding regardless of the PKCS #1 version. The signature with appendix
1236 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1237 rationale for this is that there is no binding between the PKCS #1 OIDs
1238 and the hash algorithms used in the SILC protocol. Hence, the encoding
1239 is always in PKCS #1 version 1.5 format.
1241 Additional public key algorithms MAY be defined to be used in SILC.
1247 3.10.3 Hash Functions
1249 Hash functions are used as part of MAC algorithms defined in the next
1250 section. They are also used in the SILC Key Exchange protocol defined
1253 The following Hash algorithm are defined in SILC protocol:
1256 sha1 SHA-1, length = 20 (REQUIRED)
1257 md5 MD5, length = 16 (OPTIONAL)
1262 3.10.4 MAC Algorithms
1264 Data integrity is protected by computing a message authentication code
1265 (MAC) of the packet data. See [SILC2] for details how to compute the
1268 The following MAC algorithms are defined in SILC protocol:
1271 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1272 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1273 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1274 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1275 none No MAC (OPTIONAL)
1278 The none MAC is not recommended to be used as the packet is not
1279 authenticated when MAC is not computed. It is recommended that no
1280 client or server would accept none MAC except in special debugging
1283 The HMAC algorithm is described in [HMAC] and hash algorithms that
1284 are used as part of the HMACs are described in [Scheneir] and in
1287 Additional MAC algorithms MAY be defined to be used in SILC.
1291 3.10.5 Compression Algorithms
1293 SILC protocol supports compression that may be applied to unencrypted
1294 data. It is recommended to use compression on slow links as it may
1295 significantly speed up the data transmission. By default, SILC does not
1296 use compression which is the mode that must be supported by all SILC
1301 The following compression algorithms are defined:
1304 none No compression (REQUIRED)
1305 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1308 Additional compression algorithms MAY be defined to be used in SILC.
1312 3.11 SILC Public Key
1314 This section defines the type and format of the SILC public key. All
1315 implementations MUST support this public key type. See [SILC3] for
1316 other optional public key and certificate types allowed in the SILC
1317 protocol. Public keys in SILC may be used to authenticate entities
1318 and to perform other tasks related to public key cryptography.
1320 The format of the SILC Public Key is as follows:
1326 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
1327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1328 | Public Key Length |
1329 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1330 | Algorithm Name Length | |
1331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1336 | Identifier Length | |
1337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1341 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1345 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1349 Figure 5: SILC Public Key
1353 o Public Key Length (4 bytes) - Indicates the full length
1354 of the public key, not including this field.
1356 o Algorithm Name Length (2 bytes) - Indicates the length
1357 of the Algorithm Length field, not including this field.
1359 o Algorithm name (variable length) - Indicates the name
1360 of the public key algorithm that the key is. See the
1361 section 3.10.2 Public Key Algorithms for defined names.
1363 o Identifier Length (2 bytes) - Indicates the length of
1364 the Identifier field, not including this field.
1366 o Identifier (variable length) - Indicates the identifier
1367 of the public key. This data can be used to identify
1368 the owner of the key. The identifier is of the following
1372 HN Host name or IP address
1379 Examples of an identifier:
1381 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1383 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1385 At least user name (UN) and host name (HN) MUST be provided as
1386 identifier. The fields are separated by commas (`,'). If
1387 comma is in the identifier string it must be written as `\\,',
1388 for example, `O=Company XYZ\\, Inc.'.
1390 o Public Data (variable length) - Includes the actual
1391 public data of the public key.
1393 The format of this field for RSA algorithm is
1402 The format of this field for DSS algorithm is
1414 The variable length fields are multiple precession
1415 integers encoded as strings in both examples.
1417 Other algorithms must define their own type of this
1418 field if they are used.
1421 All fields in the public key are in MSB (most significant byte first)
1426 3.12 SILC Version Detection
1428 The version detection of both client and server is performed at the
1429 connection phase while executing the SILC Key Exchange protocol. The
1430 version identifier is exchanged between initiator and responder. The
1431 version identifier is of the following format:
1434 SILC-<protocol version>-<software version>
1437 The version strings are of the following format:
1440 protocol version = <major>.<minor>
1441 software version = <major>[.<minor>[.<build>]]
1444 Protocol version MAY provide both major and minor version. Currently
1445 implementations MUST set the protocol version and accept the protocol
1446 version as SILC-1.0-<software version>.
1448 Software version MAY provide major, minor and build version. The
1449 software version MAY be freely set and accepted.
1452 Thus, the version string could be, for example:
1462 Backup routers may exist in the cell in addition of the primary router.
1463 However, they must not be active routers and act as routers in the cell.
1464 Only one router may be acting as primary router in the cell. In the case
1465 of failure of the primary router may one of the backup routers become
1466 active. The purpose of backup routers are in case of failure of the
1467 primary router to maintain working connections inside the cell and outside
1468 the cell and to avoid netsplits.
1470 Backup routers are normal servers in the cell that are prepared to take
1471 over the tasks of the primary router if needed. They need to have at
1472 least one direct and active connection to the primary router of the cell.
1473 This communication channel is used to send the router information to
1474 the backup router. When the backup router connects to the primary router
1475 of the cell it MUST present itself as router server in the Connection
1476 Authentication protocol, even though it is normal server as long as the
1477 primary router is available. Reason for this is that the configuration
1478 needed in the responder end requires usually router connection level
1479 configuration. The responder, however must understand and treat the
1480 connection as normal server (except when feeding router level data to
1483 Backup router must know everything that the primary router knows to be
1484 able to take over the tasks of the primary router. It is the primary
1485 router's responsibility to feed the data to the backup router. If the
1486 backup router does not know all the data in the case of failure some
1487 connections may be lost. The primary router of the cell must consider
1488 the backup router being actual router server when it feeds the data to
1491 In addition of having direct connection to the primary router of the
1492 cell, the backup router must also have connection to the same router
1493 the primary router of the cell is connected. However, it must not be
1494 active router connection meaning that the backup router must not use
1495 that channel as its primary route and it must not notify the router
1496 about having connected servers, channels and clients behind it. It
1497 merely connects to the router. This sort of connection is later
1498 referred as being passive connection. Some keepalive actions may be
1499 needed by the router to keep the connection alive.
1501 It is required that other normal servers have passive connections to
1502 the backup router(s) in the cell. Some keepalive actions may be needed
1503 by the server to keep the connection alive. After they notice the
1504 failure of the primary router they must start using the connection to
1505 the first backup router as their primary route.
1507 Also, if any other router in the network is using the cell's primary
1508 router as its own primary router, it must also have passive connection
1509 to the cell's backup router. It too is prepared to switch to use the
1510 backup router as its new primary router as soon as the orignal primary
1511 router becomes unresponsive.
1513 All of the parties of this protocol knows which one is the backup router
1514 of the cell from their local configuration. Each of the entity must
1515 be configured accordingly and care must be taken when configuring the
1516 backup routers, servers and other routers in the network.
1518 It must be noted that some of the channel messages and private messages
1519 may be lost during the switch to the backup router. The announcements
1520 assures that the state of the network is not lost during the switch.
1522 It is RECOMMENDED that there would be at least one backup router in
1523 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
1524 as backup routers as it requires establishing several connections to
1525 several servers in the cell. Large cells can easily have several
1526 backup routers in the cell.
1528 The order of the backup routers are decided at the configuration phase.
1529 All the parties of this protocol must be configured accordingly to
1530 understand the order of the backup routers. It is not required that
1531 the backup server is actually active server in the cell. Backup router
1532 may be a spare server in the cell that does not accept normal client
1533 connections at all. It may be reserved purely for the backup purposes.
1534 These, however, are cell management issues.
1536 If also the first backup router is down as well and there is another
1537 backup router in the cell then it will start acting as the primary
1538 router as described above.
1542 3.13.1 Switching to Backup Router
1544 When the primary router of the cell becomes unresponsive, for example
1545 by sending EOF to the connection, all the parties of this protocol MUST
1546 replace the old connection to the primary router with first configured
1547 backup router. The backup router usually needs to do local modifications
1548 to its database in order to update all the information needed to maintain
1549 working routes. The backup router must understand that clients that
1550 were orignated from the primary router are now originated from some of
1551 the existing server connections and must update them accordingly. It
1552 must also remove those clients that were owned by the primary router
1553 since those connections were lost when the primary router became
1556 All the other parties of the protocol must also update their local
1557 database to understand that the route to the primary router will now go
1558 to the backup router.
1560 The servers connected to the backup router must announce their clients,
1561 channels, channel users, channel user modes and channel modes to the
1562 backup router. This is to assure that none of the important notify
1563 packets were lost during the switch to the backup router. The backup
1564 router must check which of these announced entities it already have
1565 and distribute the new ones to the primary route.
1567 The backup router too must announce its servers, clients, channels
1568 and other information to the new primary router. The primary router
1569 of the backup router too must announce its informations to the backup
1570 router. Both must process only the ones they do not know about. If
1571 any of the announced modes does not match then they are enforced in
1572 normal manner defined later in this specification.
1576 3.13.2 Resuming Primary Router
1578 Usually the primary router is unresponsive only a short period of time
1579 and it is intended that the original router of the cell will reassume
1580 its position as primary router when it comes back online. The backup
1581 router that is now acting as primary router of the cell must constantly
1582 try to connect to the original primary router of the cell. It is
1583 RECOMMENDED that it would try to reconnect in 30 second intervals to
1586 When the connection is established to the primary router the backup
1587 resuming protocol is executed. The protocol is advanced as follows:
1589 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1590 value 1 the primary router that came back online. The packet
1591 will indicate the primary router has been replaced by the backup
1592 router. After sending the packet the backup router will announce
1593 all of its channels, channel users, modes etc. to the primary
1596 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1597 value 2 to its current primary router to indicate that it will
1598 resign as being primary router. Then, backup router sends the
1599 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
1600 connected servers to also indicate that it will resign as being
1603 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
1604 type value 2 to the router that is using the backup router
1605 currently as its primary router.
1607 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1608 with type value 1 or 2 must reconnect immediately to the
1609 primary router of the cell that came back online. After they
1610 have created the connection they MUST NOT use that connection
1611 as active primary route but still route all packets to the
1612 backup router. After the connection is created they MUST send
1613 SILC_PACKET_RESUME_ROUTER with type value 3 back to the
1614 backup router. The session ID value found in the first packet
1615 MUST be set in this packet.
1617 5. Backup router MUST wait for all packets with type value 3 before
1618 it continues with the protocol. It knows from the session ID values
1619 set in the packet when it have received all packets. The session
1620 value should be different in all packets it have send earlier.
1621 After the packets is received the backup router sends the
1622 SILC_PACKET_RESUME_ROUTER packet with type value 4 to the
1623 primary router that came back online. This packet will indicate
1624 that the backup router is now ready to resign as being primary
1625 router. The session ID value in this packet MUST be the same as
1626 in first packet sent to the primary router. During this time
1627 the backup router should still route all packets it is receiving
1628 from server connections.
1630 6. The primary router receives the packet and send the
1631 SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
1632 including the backup router. It also sends the packet with type
1633 value 6 to its primary router, and to the router that is using
1634 it as its primary router. The Session ID value in this packet
1637 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1638 with type value 5 or 6 must switch their primary route to the
1639 new primary router and remove the route for the backup router, since
1640 it is not anymore the primary router of the cell. They must also
1641 update their local database to understand that the clients are
1642 not originated from the backup router but from the locally connected
1643 servers. After that they MUST announce their channels, channel
1644 users, modes etc. to the primary router. They must not use the
1645 backup router connection after this and the connection is considered
1646 to be passive connection. The implementations SHOULD be able
1647 to disable the connection without closing the actual link.
1649 After this protocol is executed the backup router is now again normal
1650 server in the cell that has the backup link to the primary router. The
1651 primary router feeds the router specific data again to the backup router.
1652 All server connections in the backup router are considered passive
1655 When the primary router of the cell comes back online and connects
1656 to its primary router, the remote primary router must send the
1657 SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
1658 connection is not allowed since the router has been replaced by an
1659 backup router. The session ID value in this packet SHOULD be zero (0).
1660 When the router receives this packet it must not use the connection
1661 as active connection but to understand that it cannot act as primary
1662 router in the cell. It must wait that the backup router connects to
1663 it, and the backup resuming protocol is executed.
1665 The following type values has been defined for SILC_PACKET_RESUME_ROUTER
1668 1 SILC_SERVER_BACKUP_START
1669 2 SILC_SERVER_BACKUP_START_GLOBAL
1670 3 SILC_SERVER_BACKUP_START_CONNECTED
1671 4 SILC_SERVER_BACKUP_START_ENDING
1672 5 SILC_SERVER_BACKUP_START_RESUMED
1673 6 SILC_SERVER_BACKUP_START_GLOBAL
1674 20 SILC_SERVER_BACKUP_START_REPLACED
1676 If any other value is found in the type field the packet must be
1677 discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
1678 is defined in [SILC2].
1682 3.13.3 Discussion on Backup Router Scheme
1684 It is clear that this backup router support is not able to handle all
1685 possible situations arrising in unreliable network environment. This
1686 scheme for example does not handle situation when the router actually
1687 does not go offline but the network link goes down temporarily. It would
1688 require some intelligence to figure out when it is best time to switch
1689 to the backup router. To make it even more complicated it is possible
1690 that the backup router may have not lost the network link to the primary
1693 Other possible situation is when the network link is lost temporarily
1694 between two primary routers in the SILC network. Unless the routers
1695 notice the link going down they cannot perhaps find alternative routes.
1696 Worst situation is when the link goes down only for a short period of
1697 time, thus causing lag. Should the routers or servers find alternative
1698 routes if they cannot get response from the router during the lag?
1699 When alternative routes are being found it must be careful not to
1700 mess up existing primary routes between routers in the network.
1702 It is suggested that the current backup router scheme is only temporary
1703 solution and existing backup router protocols are studied further. It
1704 is also suggested that the backup router specification will be separated
1705 from this SILC specification Internet-Draft and additional specification
1706 is written on the subject.
1713 This section describes various SILC procedures such as how the
1714 connections are created and registered, how channels are created and
1715 so on. The section describes the procedures only generally as details
1716 are described in [SILC2] and [SILC3].
1720 4.1 Creating Client Connection
1722 This section describes the procedure when client connects to SILC server.
1723 When client connects to server the server MUST perform IP address lookup
1724 and reverse IP address lookup to assure that the origin host really is
1725 who it claims to be. Client, host, connecting to server SHOULD have
1726 both valid IP address and fully qualified domain name (FQDN).
1728 After that the client and server performs SILC Key Exchange protocol
1729 which will provide the key material used later in the communication.
1730 The key exchange protocol MUST be completed successfully before the
1731 connection registration may continue. The SILC Key Exchange protocol
1732 is described in [SILC3].
1734 Typical server implementation would keep a list of connections that it
1735 allows to connect to the server. The implementation would check, for
1736 example, the connecting client's IP address from the connection list
1737 before the SILC Key Exchange protocol has been started. Reason for
1738 this is that if the host is not allowed to connect to the server there
1739 is no reason to perform the key exchange protocol.
1741 After successful key exchange protocol the client and server performs
1742 connection authentication protocol. The purpose of the protocol is to
1743 authenticate the client connecting to the server. Flexible
1744 implementation could also accept the client to connect to the server
1745 without explicit authentication. However, if authentication is
1746 desired for a specific client it may be based on passphrase or
1747 public key authentication. If authentication fails the connection
1748 MUST be terminated. The connection authentication protocol is described
1751 After successful key exchange and authentication protocol the client
1752 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1753 server. This packet includes various information about the client
1754 that the server uses to create the client. Server creates the client
1755 and sends SILC_PACKET_NEW_ID to the client which includes the created
1756 Client ID that the client MUST start using after that. After that
1757 all SILC packets from the client MUST have the Client ID as the
1758 Source ID in the SILC Packet Header, described in [SILC2].
1760 Client MUST also get the server's Server ID that is to be used as
1761 Destination ID in the SILC Packet Header when communicating with
1762 the server (for example when sending commands to the server). The
1763 ID may be resolved in two ways. Client can take the ID from an
1764 previously received packet from server that MUST include the ID,
1765 or to send SILC_COMMAND_INFO command and receive the Server ID as
1768 Server MAY choose not to use the information received in the
1769 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1770 certificate were used in the authentication, server MAY use those
1771 informations rather than what it received from client. This is suitable
1772 way to get the true information about client if it is available.
1774 The nickname of client is initially set to the username sent in the
1775 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1776 suitable by sending SILC_COMMAND_NICK command. However, this is not
1777 required as part of registration process.
1779 Server MUST also distribute the information about newly registered
1780 client to its router (or if the server is router, to all routers in
1781 the SILC network). More information about this in [SILC2].
1785 4.2 Creating Server Connection
1787 This section describes the procedure when server connects to its
1788 router (or when router connects to other router, the cases are
1789 equivalent). The procedure is very much alike when client connects
1790 to the server thus it is not repeated here.
1792 One difference is that server MUST perform connection authentication
1793 protocol with proper authentication. A proper authentication is based
1794 on passphrase or public key authentication.
1796 After server and router has successfully performed the key exchange
1797 and connection authentication protocol, the server register itself
1798 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1799 includes the server's Server ID that it has created by itself and
1800 other relevant information about the server.
1802 After router has received the SILC_PACKET_NEW_SERVER packet it
1803 distributes the information about newly registered server to all routers
1804 in the SILC network. More information about this in [SILC2].
1806 As client needed to resolve the destination ID this MUST be done by the
1807 server that connected to the router, as well. The way to resolve it is
1808 to get the ID from previously received packet. The server MAY also
1809 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1810 using its own Server ID as Source ID in SILC Packet Header and the
1811 router's Server ID as Destination when communicating with the router.
1815 4.2.1 Announcing Clients, Channels and Servers
1817 After server or router has connected to the remote router, and it already
1818 has connected clients and channels it MUST announce them to the router.
1819 If the server is router server, also all the local servers in the cell
1822 All clients are announced by compiling a list of ID Payloads into the
1823 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1824 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also,
1825 the channel users on the channels must be announced by compiling a
1826 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1827 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1828 also be announced by compiling list of Notify Payloads with the
1829 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1832 The router MUST also announce the local servers by compiling list of
1833 ID Payloads into the SILC_PACKET_NEW_ID packet.
1835 The router which receives these lists MUST process them and broadcast
1836 the packets to its primary route.
1838 When processing the announced channels and channel users the router MUST
1839 check whether a channel exists already with the same name. If channel
1840 exists with the same name it MUST check whether the Channel ID is
1841 different. If the Channel ID is different the router MUST send the notify
1842 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1843 change to the ID the router has. If the mode of the channel is different
1844 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1845 server to force the mode change to the mode that the router has.
1847 The router MUST also generate new channel key and distribute it to the
1848 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1851 If the channel has channel founder on the router the router MUST send
1852 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1853 the mode change for the channel founder on the server. The channel
1854 founder privileges MUST be removed.
1856 The router processing the channels MUST also compile a list of
1857 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1858 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1859 server (or router) will receive the clients on the channel that
1864 4.3 Joining to a Channel
1866 This section describes the procedure when client joins to a channel.
1867 Client joins to channel by sending command SILC_COMMAND_JOIN to the
1868 server. If the receiver receiving join command is normal server the
1869 server MUST check its local list whether this channel already exists
1870 locally. This would indicate that some client connected to the server
1871 has already joined to the channel. If this is case the client is
1872 joined to the channel, new channel key is created and information about
1873 newly joined channel is sent to the router. The router is informed
1874 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1875 also be sent to the local clients on the channel. The new channel key
1876 is also sent to the router and to local clients on the channel.
1878 If the channel does not exist in the local list the client's command
1879 MUST be sent to the router which will then perform the actual joining
1880 procedure. When server receives the reply to the command from the
1881 router it MUST be sent to the client which sent the command originally.
1882 Server will also receive the channel key from the server that it MUST
1883 send to the client which originally requested the join command. The
1884 server MUST also save the channel key.
1886 If the receiver of the join command is router it MUST first check its
1887 local list whether anyone in the cell has already joined to the channel.
1888 If this is the case the client is joined to the channel and reply is
1889 sent to the client. If the command was sent by server the command reply
1890 is sent to the server which sent it. Then the router MUST also create
1891 new channel key and distribute it to all clients on the channel and
1892 all servers that has clients on the channel. Router MUST also send
1893 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1894 and to local servers that has clients on the channel.
1896 If the channel does not exist on the router's local list it MUST
1897 check the global list whether the channel exists at all. If it does
1898 the client is joined to the channel as described previously. If
1899 the channel does not exist the channel is created and the client
1900 is joined to the channel. The channel key is also created and
1901 distributed as previously described. The client joining to the created
1902 channel is made automatically channel founder and both channel founder
1903 and channel operator privileges is set for the client.
1905 If the router created the channel in the process, information about the
1906 new channel MUST be broadcasted to all routers. This is done by
1907 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1908 route. When the router joins the client to the channel it MUST also
1909 send information about newly joined client to all routers in the SILC
1910 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1911 type to the router's primary route.
1913 It is important to note that new channel key is created always when
1914 new client joins to channel, whether the channel has existed previously
1915 or not. This way the new client on the channel is not able to decrypt
1916 any of the old traffic on the channel. Client which receives the reply to
1917 the join command MUST start using the received Channel ID in the channel
1918 message communication thereafter. Client also receives the key for the
1919 channel in the command reply. Note that the channel key is never
1920 generated if the SILC_CMODE_PRIVKEY mode is set.
1924 4.4 Channel Key Generation
1926 Channel keys are created by router which creates the channel by taking
1927 enough randomness from cryptographically strong random number generator.
1928 The key is generated always when channel is created, when new client
1929 joins a channel and after the key has expired. Key could expire for
1932 The key MUST also be re-generated whenever some client leaves a channel.
1933 In this case the key is created from scratch by taking enough randomness
1934 from the random number generator. After that the key is distributed to
1935 all clients on the channel. However, channel keys are cell specific thus
1936 the key is created only on the cell where the client, which left the
1937 channel, exists. While the server or router is creating the new channel
1938 key, no other client may join to the channel. Messages that are sent
1939 while creating the new key are still processed with the old key. After
1940 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1941 using the new key. If server creates the new key the server MUST also
1942 send the new key to its router. See [SILC2] on more information about
1943 how channel messages must be encrypted and decrypted when router is
1946 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1947 Channel Key Payload it MUST process the key data to create encryption
1948 and decryption key, and to create the HMAC key that is used to compute
1949 the MACs of the channel messages. The processing is as follows:
1951 channel_key = raw key data
1952 HMAC key = hash(raw key data)
1954 The raw key data is the key data received in the Channel Key Payload.
1955 The hash() function is the hash function used in the HMAC of the channel.
1956 Note that the server MUST also save the channel key.
1960 4.5 Private Message Sending and Reception
1962 Private messages are sent point to point. Client explicitly destines
1963 a private message to specific client that is delivered to only to that
1964 client. No other client may receive the private message. The receiver
1965 of the private message is destined in the SILC Packet Header as any
1966 other packet as well.
1968 If the sender of a private message does not know the receiver's Client
1969 ID, it MUST resolve it from server. There are two ways to resolve the
1970 client ID from server; it is RECOMMENDED that client implementations
1971 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
1972 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
1973 If the sender has received earlier a private message from the receiver
1974 it should have cached the Client ID from the SILC Packet Header.
1976 See [SILC2] for description of private message encryption and decryption
1981 4.6 Private Message Key Generation
1983 Private message MAY be protected by the key generated by the client.
1984 The key may be generated and sent to the other client by sending packet
1985 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
1986 and is secured by session keys. After that the private message key
1987 is used in the private message communication between those clients.
1989 Other choice is to entirely use keys that are not sent through
1990 the SILC network at all. This significantly adds security. This key
1991 would be pre-shared-key that is known by both of the clients. Both
1992 agree about using the key and starts sending packets that indicate
1993 that the private message is secured using private message key.
1995 The key material used as private message key is implementation issue.
1996 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
1997 the key material. If the key is normal pre-shared-key or randomly
1998 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
1999 the key material SHOULD be processed as defined in the [SILC3]. In
2000 the processing, however, the HASH, as defined in [SILC3] MUST be
2001 ignored. After processing the key material it is employed as defined
2002 in [SILC3], however, the HMAC key material MUST be discarded.
2004 If the key is pre-shared-key or randomly generated the implementations
2005 should use the SILC protocol's mandatory cipher as the cipher. If the
2006 SKE was used to negotiate key material the cipher was negotiated as well.
2009 4.7 Channel Message Sending and Reception
2011 Channel messages are delivered to group of users. The group forms a
2012 channel and all clients on the channel receives messages sent to the
2015 Channel messages are destined to channel by specifying the Channel ID
2016 as Destination ID in the SILC Packet Header. The server MUST then
2017 distribute the message to all clients on the channel by sending the
2018 channel message destined explicitly to a client on the channel.
2020 See the [SILC2] for description of channel messege routing for router
2023 See [SILC2] for description of channel message encryption and decryption
2028 4.8 Session Key Regeneration
2030 Session keys MUST be regenerated periodically, say, once in an hour.
2031 The re-key process is started by sending SILC_PACKET_REKEY packet to
2032 other end, to indicate that re-key must be performed. The initiator
2033 of the connection SHOULD initiate the re-key.
2035 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2036 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2037 SKE protocol. In this case the protocol is secured with the old key
2038 and the protocol results to new key material. See [SILC3] for more
2039 information. After the SILC_PACKET_REKEY packet is sent the sender
2040 will perform the SKE protocol.
2042 If PFS flag was set the resulted key material is processed as described
2043 in the section Processing the Key Material in [SILC3]. The difference
2044 with re-key in the processing is that the initial data for the hash
2045 function is just the resulted key material and not the HASH as it
2046 is not computed at all with re-key. Other than that, the key processing
2047 it equivalent to normal SKE negotiation.
2049 If PFS flag was not set, which is the default case, then re-key is done
2050 without executing SKE protocol. In this case, the new key is created by
2051 providing the current sending encryption key to the SKE protocol's key
2052 processing function. The process is described in the section Processing
2053 the Key Material in [SILC3]. The difference in the processing is that
2054 the initial data for the hash function is the current sending encryption
2055 key and not the SKE's KEY and HASH values. Other than that, the key
2056 processing is equivalent to normal SKE negotiation.
2058 After both parties has regenerated the session key, both MUST send
2059 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2060 secured with the old key. After these packets, the subsequent packets
2061 MUST be protected with the new key.
2067 4.9 Command Sending and Reception
2069 Client usually sends the commands in the SILC network. In this case
2070 the client simply sends the command packet to server and the server
2071 processes it and replies with command reply packet.
2073 However, if the server is not able to process the command, it is sent
2074 to the server's router. This is case for example with commands such
2075 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2076 are other commands as well. For example, if client sends the WHOIS
2077 command requesting specific information about some client the server must
2078 send the WHOIS command to router so that all clients in SILC network
2079 are searched. The router, on the other hand, sends the WHOIS command
2080 further to receive the exact information about the requested client.
2081 The WHOIS command travels all the way to the server which owns the client
2082 and it replies with command reply packet. Finally, the server which
2083 sent the command receives the command reply and it must be able to
2084 determine which client sent the original command. The server then
2085 sends command reply to the client. Implementations should have some
2086 kind of cache to handle, for example, WHOIS information. Servers
2087 and routers along the route could all cache the information for faster
2088 referencing in the future.
2090 The commands sent by server may be sent hop by hop until someone is able
2091 to process the command. However, it is preferred to destine the command
2092 as precisely as it is possible. In this case, other routers en route
2093 MUST route the command packet by checking the true sender and true
2094 destination of the packet. However, servers and routers MUST NOT route
2095 command reply packets to clients coming from other server. Client
2096 MUST NOT accept command reply packet originated from anyone else but
2097 from its own server.
2101 4.10 Closing Connection
2103 When remote client connection is closed the server MUST send the notify
2104 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2105 the client was joined. The server MUST also save the client's information
2106 for a period of time for history purposes.
2108 When remote server or router connection is closed the server or router
2109 MUST also remove all the clients that was behind the server or router
2110 from the SILC Network. The server or router MUST also send the notify
2111 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2112 local clients that are joined on the same channels with the remote
2113 server's or router's clients.
2117 5 Security Considerations
2119 Security is central to the design of this protocol, and these security
2120 considerations permeate the specification. Common security considerations
2121 such as keeping private keys truly private and using adequate lengths for
2122 symmetric and asymmetric keys must be followed in order to maintain the
2123 security of this protocol.
2125 Special attention must also be paid on the servers and routers that are
2126 running the SILC service. The SILC protocol's security depends greatly
2127 on the security and the integrity of the servers and administrators that
2128 are running the service. It is recommended that some form of registration
2129 is required by the server and router administrator prior acceptance to
2130 the SILC Network. The clients should be able to trust the servers they
2133 It however must be noted that if the client requires absolute security
2134 by not trusting any of the servers or routers in the SILC Network, can
2135 be accomplished by negotiating private keys outside the SILC Network,
2136 either using SKE or some other key negotiation protocol, or to use some
2137 other external means for distributing the keys. This applies for all
2138 messages, private messages and channel messages. It is important to note
2139 that SILC, like any other security protocol is not full proof system and
2140 cannot secure from insecure environment; the SILC servers and routers could
2141 very well be compromised. However, to provide acceptable level of security
2142 and usability for end user the protocol uses many times session keys or
2143 other keys generated by the servers to secure the messages. If this is
2144 unacceptable for the client or end user, the private keys negotiatied
2145 outside the SILC Network should always be used. In the end it is always
2146 implementor's choice whether to negotiate private keys by default or
2147 whether to use the keys generated by the servers.
2149 It is also recommended that router operators in the SILC Network would
2150 form a joint forum to discuss the router and SILC Network management
2151 issues. Also, router operators along with the cell's server operators
2152 should have a forum to discuss the cell management issues.
2158 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2161 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2162 Protocols", Internet Draft, April 2001.
2164 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001.
2166 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2169 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2172 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2175 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2178 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2181 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2184 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2187 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2190 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2191 Infrastructure, Certificate and CRL Profile", RFC 2459,
2194 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2195 John Wiley & Sons, New York, NY, 1996.
2197 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2200 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2201 RFC 2412, November 1998.
2203 [ISAKMP] Maughan D., et al, "Internet Security Association and
2204 Key Management Protocol (ISAKMP)", RFC 2408, November
2207 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2208 (IKE)", RFC 2409, November 1998.
2210 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2211 Authentication", RFC 2104, February 1997.
2213 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2214 Specifications, Version 2.0", RFC 2437, October 1998.
2216 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2217 Requirement Levels", BCP 14, RFC 2119, March 1997.
2225 Snellmanninkatu 34 A 15
2229 EMail: priikone@silcnet.org
2231 This Internet-Draft expires XXX