8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-05.txt XXX
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-05.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 ............................................ 8
82 3.1 Client .................................................... 8
83 3.1.1 Client ID ........................................... 9
84 3.2 Server .................................................... 10
85 3.2.1 Server's Local ID List .............................. 10
86 3.2.2 Server ID ........................................... 11
87 3.2.3 SILC Server Ports ................................... 12
88 3.3 Router .................................................... 12
89 3.3.1 Router's Local ID List .............................. 12
90 3.3.2 Router's Global ID List ............................. 13
91 3.3.3 Router's Server ID .................................. 14
92 3.4 Channels .................................................. 14
93 3.4.1 Channel ID .......................................... 16
94 3.5 Operators ................................................. 16
95 3.6 SILC Commands ............................................. 16
96 3.7 SILC Packets .............................................. 17
97 3.8 Packet Encryption ......................................... 17
98 3.8.1 Determination of the Source and the Destination ..... 17
99 3.8.2 Client To Client .................................... 18
100 3.8.3 Client To Channel ................................... 19
101 3.8.4 Server To Server .................................... 20
102 3.9 Key Exchange And Authentication ........................... 20
103 3.9.1 Authentication Payload .............................. 20
104 3.10 Algorithms ............................................... 22
105 3.10.1 Ciphers ............................................ 22
106 3.10.2 Public Key Algorithms .............................. 23
107 3.10.3 Hash Functions ..................................... 24
108 3.10.4 MAC Algorithms ..................................... 24
109 3.10.5 Compression Algorithms ............................. 25
110 3.11 SILC Public Key .......................................... 25
111 3.12 SILC Version Detection ................................... 27
112 3.13 Backup Routers ........................................... 28
113 3.13.1 Switching to Backup Router ......................... 29
114 3.13.2 Resuming Primary Router ............................ 30
115 3.13.3 Discussion on Backup Router Scheme ................. 32
116 4 SILC Procedures ............................................... 33
117 4.1 Creating Client Connection ................................ 33
118 4.2 Creating Server Connection ................................ 34
119 4.2.1 Announcing Clients, Channels and Servers ............ 35
120 4.3 Joining to a Channel ...................................... 36
121 4.4 Channel Key Generation .................................... 37
122 4.5 Private Message Sending and Reception ..................... 38
123 4.6 Private Message Key Generation ............................ 38
124 4.7 Channel Message Sending and Reception ..................... 39
125 4.8 Session Key Regeneration .................................. 39
126 4.9 Command Sending and Reception ............................. 40
127 4.10 Closing Connection ....................................... 41
128 4.11 Detaching and Resuming a Session ......................... XXXXX
129 5 Security Considerations ....................................... 41
130 6 References .................................................... 42
131 7 Author's Address .............................................. 44
139 Figure 1: SILC Network Topology
140 Figure 2: Communication Inside cell
141 Figure 3: Communication Between Cells
142 Figure 4: Router Connections
143 Figure 5: SILC Public Key
149 This document describes a Secure Internet Live Conferencing (SILC)
150 protocol which provides secure conferencing services over insecure
151 network channel. SILC is IRC [IRC] like protocol, however, it is
152 not equivalent to IRC and does not support IRC. Some of the SILC's
153 features are not found in IRC but in traditional Instant Message (IM)
154 protocols. SILC combines features from both of these chat protocol
155 styles, and SILC can be implemeneted as either IRC-like system or
158 Strong cryptographic methods are used to protect SILC packets inside
159 the SILC network. Three other Internet Drafts relates very closely
160 to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
161 Authentication Protocols [SILC3] and SILC Commands [SILC4].
163 The protocol uses extensively packets as conferencing protocol
164 requires message and command sending. The SILC Packet Protocol is
165 described in [SILC2] and should be read to fully comprehend this
166 document and protocol. [SILC2] also describes the packet encryption
167 and decryption in detail. The SILC Packet Protocol provides secured
168 and authenticated packets, and the protocol is designed to be compact.
169 This makes SILC also suitable in environment of low bandwith
170 requirements such as mobile networks. All packet payloads in SILC
171 can be also compressed.
173 The security of SILC protocol, and for any security protocol for that
174 matter, is based on strong and secure key exchange protocol. The SILC
175 Key Exchange protocol is described in [SILC3] along with connection
176 authentication protocol and should be read to fully comprehend this
177 document and protocol.
179 The SILC protocol has been developed to work on TCP/IP network
180 protocol, although it could be made to work on other network protocols
181 with only minor changes. However, it is recommended that TCP/IP
182 protocol is used under SILC protocol. Typical implementation would
183 be made in client-server model.
187 1.1 Requirements Terminology
189 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
190 MAY, and OPTIONAL, when they appear in this document, are to be
191 interpreted as described in [RFC2119].
197 This section describes various SILC protocol concepts that forms the
198 actual protocol, and in the end, the actual SILC network. The mission
199 of the protocol is to deliver messages from clients to other clients
200 through routers and servers in secure manner. The messages may also
201 be delivered from one client to many clients forming a group, also
204 This section does not focus to security issues. Instead, basic network
205 concepts are introduced to make the topology of the SILC network
210 2.1 SILC Network Topology
212 SILC network is a cellular network as opposed to tree style network
213 topology. The rationale for this is to have servers that can perform
214 specific kind of tasks what other servers cannot perform. This leads
215 to two kinds of servers; normal SILC servers and SILC routers.
217 A difference between normal server and router server is that routers
218 knows everything about everything in the network. They also do the
219 actual routing of the messages to the correct receiver. Normal servers
220 knows only about local information and nothing about global information.
221 This makes the network faster as there are less servers that needs to
222 keep global information up to date at all time.
224 This, on the other hand, leads to cellular like network, where routers
225 are in the center of the cell and servers are connected to the router.
233 The following diagram represents SILC network topology.
237 ---- ---- ---- ---- ---- ----
238 | S8 | S5 | S4 | | S7 | S5 | S6 |
239 ----- ---- ----- ----- ---- -----
240 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
241 ---- ------ ---- ---- ------ ----
242 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
243 ---- ---- ---- ---- ---- ---- | S3 | S1 |
244 Cell 1. \\ Cell 2. | \\____ ----- -----
246 ---- ---- ---- ---- ---- ---- ---- ------
247 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
248 ----- ---- ----- ----- ---- ----- ---- ----
249 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
250 ---- ------ ---- ---- ------ ----
251 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
252 ---- ---- ---- ---- ---- ----
257 Figure 1: SILC Network Topology
260 A cell is formed when a server or servers connect to one router. In
261 SILC network normal server cannot directly connect to other normal
262 server. Normal server may only connect to SILC router which then
263 routes the messages to the other servers in the cell. Router servers
264 on the other hand may connect to other routers to form the actual SILC
265 network, as seen in above figure. However, router is also normal SILC
266 server; clients may connect to it the same way as to normal SILC
267 server. Normal server also cannot have active connections to more
268 than one router. Normal server cannot be connected to two different
269 cells. Router servers, on the other hand, may have as many router to
270 router connections as needed.
272 There are many issues in this network topology that needs to be careful
273 about. Issues like the size of the cells, the number of the routers in
274 the SILC network and the capacity requirements of the routers. These
275 issues should be discussed in the Internet Community and additional
276 documents on the issue may be written.
280 2.2 Communication Inside a Cell
282 It is always guaranteed that inside a cell message is delivered to the
283 recipient with at most two server hops. A client which is connected to
284 server in the cell and is talking on channel to other client connected
285 to other server in the same cell, will have its messages delivered from
286 its local server first to the router of the cell, and from the router
287 to the other server in the cell.
289 The following diagram represents this scenario:
303 Figure 2: Communication Inside cell
306 Example: Client 1. connected to Server 1. send message to
307 Client 4. connected to Server 2. travels from Server 1.
308 first to Router which routes the message to Server 2.
309 which then sends it to the Client 4. All the other
310 servers in the cell will not see the routed message.
313 If the client is connected directly to the router, as router is also normal
314 SILC server, the messages inside the cell are always delivered only with
315 one server hop. If clients communicating with each other are connected
316 to the same server, no router interaction is needed. This is the optimal
317 situation of message delivery in the SILC network.
321 2.3 Communication in the Network
323 If the message is destined to server that does not belong to local cell
324 the message is routed to the router server to which the destination
325 server belongs, if the local router is connected to destination router.
326 If there is no direct connection to the destination router, the local
327 router routes the message to its primary route. The following diagram
328 represents message sending between cells.
333 1 --- S1 S4 --- 5 S2 --- 1
334 S/R - - - - - - - - S/R
344 Figure 3: Communication Between Cells
347 Example: Client 5. connected to Server 4. in Cell 1. sends message
348 to Client 2. connected to Server 1. in Cell 2. travels
349 from Server 4. to Router which routes the message to
350 Router in Cell 2, which then routes the message to
351 Server 1. All the other servers and routers in the
352 network will not see the routed message.
355 The optimal case of message delivery from the client point of view is
356 when clients are connected directly to the routers and the messages
357 are delivered from one router to the other.
361 2.4 Channel Communication
363 Messages may be sent to group of clients as well. Sending messages to
364 many clients works the same way as sending messages point to point, from
365 message delivery point of view. Security issues are another matter
366 which are not discussed in this section.
368 Router server handles the message routing to multiple recipients. If
369 any recipient is not in the same cell as the sender the messages are
372 Server distributes the channel message to its local clients which are
373 joined to the channel. Router also distributes the message to its
374 local clients on the channel.
378 2.5 Router Connections
380 Router connections play very important role in making the SILC like
381 network topology to work. For example, sending broadcast packets in
382 SILC network require special connections between routers; routers must
383 be connected in a specific way.
385 Every router has their primary route which is a connection to another
386 router in the network. Unless there is only two routers in the network
387 must not routers use each other as their primary routes. The router
388 connections in the network must form a ring.
396 Example with three routers in the network:
401 S/R1 - < - < - < - < - < - < - S/R2
404 \\ - > - > - S/R3 - > - > - /
409 Figure 4: Router Connections
412 Example: Network with three routers. Router 1. uses Router 2. as its
413 primary router. Router 2. uses Router 3. as its primary router,
414 and Router 3. uses Router 1. as its primary router. There may
415 be other direct connections between the routers but they must
416 not be used as primary routes.
418 The above example is applicable to any amount of routers in the network
419 except for two routers. If there are only two routers in the network both
420 routers must be able to handle situation where they use each other as their
423 The issue of router connections are very important especially with SILC
424 broadcast packets. Usually all router wide information in the network is
425 distributed by SILC broadcast packets. This sort of ring network, with
426 ability to have other direct routes in the network cause interesting
427 routing problems. The [SILC2] discusses the routing of packets in this
428 sort of network in more detail.
432 3. SILC Specification
434 This section describes the SILC protocol. However, [SILC2] and
435 [SILC3] describes other important protocols that are part of this SILC
436 specification and must be read.
442 A client is a piece of software connecting to SILC server. SILC client
443 cannot be SILC server. Purpose of clients is to provide the user
444 interface of the SILC services for end user. Clients are distinguished
445 from other clients by unique Client ID. Client ID is a 128 bit ID that
446 is used in the communication in the SILC network. The client ID is
447 based on the nickname selected by the user. User uses logical nicknames
448 in communication which are then mapped to the corresponding Client ID.
449 Client ID's are low level identifications and must not be seen by the
452 Clients provide other information about the end user as well. Information
453 such as the nickname of the user, username and the host name of the end
454 user and user's real name. See section 3.2 Server for information of
455 the requirements of keeping this information.
457 The nickname selected by the user is not unique in the SILC network.
458 There can be 2^8 same nicknames for one IP address. As for comparison
459 to IRC [IRC] where nicknames are unique this is a fundamental difference
460 between SILC and IRC. This causes the server names or client's host names
461 to be used along with the nicknames to identify specific users when sending
462 messages. This feature of SILC makes IRC style nickname-wars obsolete as
463 no one owns their nickname; there can always be someone else with the same
464 nickname. The maximum length of nickname is 128 bytes.
470 Client ID is used to identify users in the SILC network. The Client ID
471 is unique to the extent that there can be 2^128 different Client ID's,
472 and ID's based on IPv6 addresses extends this to 2^224 different Client
473 ID's. Collisions are not expected to happen. The Client ID is defined
479 128 bit Client ID based on IPv4 addresses:
481 32 bit Server ID IP address (bits 1-32)
482 8 bit Random number or counter
483 88 bit Truncated MD5 hash value of the nickname
485 224 bit Client ID based on IPv6 addresses:
487 128 bit Server ID IP address (bits 1-128)
488 8 bit Random number or counter
489 88 bit Truncated MD5 hash value of the nickname
491 o Server ID IP address - Indicates the server where this
492 client is coming from. The IP address hence equals the
493 server IP address where to the client has connected.
495 o Random number or counter - Random number to further
496 randomize the Client ID. Another choice is to use
497 a counter starting from the zero (0). This makes it
498 possible to have 2^8 same nicknames from the same
501 o MD5 hash - MD5 hash value of the lowercase nickname is
502 truncated taking 88 bits from the start of the hash value.
503 This hash value is used to search the user's Client ID
504 from the ID lists. Note that the nickname MUST be in
508 Collisions could occur when more than 2^8 clients using same nickname
509 from the same server IP address is connected to the SILC network.
510 Server MUST be able to handle this situation by refusing to accept
511 anymore of that nickname.
513 Another possible collision may happen with the truncated hash value of
514 the nickname. It could be possible to have same truncated hash value for
515 two different nicknames. However, this is not expected to happen nor
516 cause any problems if it would occur. Nicknames are usually logical and
517 it is unlikely to have two distinct logical nicknames produce same
518 truncated hash value.
524 Servers are the most important parts of the SILC network. They form the
525 basis of the SILC, providing a point to which clients may connect to.
526 There are two kinds of servers in SILC; normal servers and router servers.
527 This section focus on the normal server and router server is described
528 in the section 3.3 Router.
530 Normal servers MUST NOT directly connect to other normal server. Normal
531 servers may only directly connect to router server. If the message sent
532 by the client is destined outside the local server it is always sent to
533 the router server for further routing. Server may only have one active
534 connection to router on same port. Normal server MUST NOT connect to other
535 cell's router except in situations where its cell's router is unavailable.
539 3.2.1 Server's Local ID List
541 Normal server keeps various information about the clients and their end
542 users connected to it. Every normal server MUST keep list of all locally
543 connected clients, Client ID's, nicknames, usernames and host names and
544 user's real name. Normal servers only keeps local information and it
545 does not keep any global information. Hence, normal servers knows only
546 about their locally connected clients. This makes servers efficient as
547 they don't have to worry about global clients. Server is also responsible
548 of creating the Client ID's for their clients.
550 Normal server also keeps information about locally created channels and
554 Hence, local list for normal server includes:
557 server list - Router connection
565 client list - All clients in server
575 channel list - All channels in server
578 o Client ID's on channel
579 o Client ID modes on channel
587 Servers are distinguished from other servers by unique 64 bit Server ID
588 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
589 the SILC to route messages to correct servers. Server ID's also provide
590 information for Client ID's, see section 3.1.1 Client ID. Server ID is
594 64 bit Server ID based on IPv4 addresses:
596 32 bit IP address of the server
600 160 bit Server ID based on IPv6 addresses:
602 128 bit IP address of the server
606 o IP address of the server - This is the real IP address of
609 o Port - This is the port the server is bound to.
611 o Random number - This is used to further randomize the Server ID.
614 Collisions are not expected to happen in any conditions. The Server ID
615 is always created by the server itself and server is responsible of
616 distributing it to the router.
620 3.2.3 SILC Server Ports
622 The following ports has been assigned by IANA for the SILC protocol:
630 If there are needs to create new SILC networks in the future the port
631 numbers must be officially assigned by the IANA.
633 Server on network above privileged ports (>1023) SHOULD NOT be trusted
634 as they could have been set up by untrusted party.
640 Router server in SILC network is responsible for keeping the cell together
641 and routing messages to other servers and to other routers. Router server
642 is also a normal server thus clients may connect to it as it would be
643 just normal SILC server.
645 However, router servers has a lot of important tasks that normal servers
646 do not have. Router server knows everything about everything in the SILC.
647 They know all clients currently on SILC, all servers and routers and all
648 channels in SILC. Routers are the only servers in SILC that care about
649 global information and keeping them up to date at all time. And, this
650 is what they must do.
654 3.3.1 Router's Local ID List
656 Router server as well MUST keep local list of connected clients and
657 locally created channels. However, this list is extended to include all
658 the informations of the entire cell, not just the server itself as for
661 However, on router this list is a lot smaller since routers do not need
662 to keep information about user's nickname, username and host name and real
663 name since these are not needed by the router. The router keeps only
664 information that it needs.
667 Hence, local list for router includes:
670 server list - All servers in the cell
677 client list - All clients in the cell
681 channel list - All channels in the cell
683 o Client ID's on channel
684 o Client ID modes on channel
689 Note that locally connected clients and other information include all the
690 same information as defined in section section 3.2.1 Server's Local ID
695 3.3.2 Router's Global ID List
697 Router server MUST also keep global list. Normal servers do not have
698 global list as they know only about local information. Global list
699 includes all the clients on SILC, their Client ID's, all created channels
700 and their Channel ID's and all servers and routers on SILC and their
701 Server ID's. That is said, global list is for global information and the
702 list must not include the local information already on the router's local
705 Note that the global list does not include information like nicknames,
706 usernames and host names or user's real names. Router does not need to
707 keep these informations as they are not needed by the router. This
708 information is available from the client's server which maybe queried
711 Hence, global list includes:
714 server list - All servers in SILC
719 client list - All clients in SILC
722 channel list - All channels in SILC
724 o Client ID's on channel
725 o Client ID modes on channel
731 3.3.3 Router's Server ID
733 Router's Server ID's are equivalent to normal Server ID's. As routers
734 are normal servers as well same types of ID's applies for routers as well.
735 Thus, see section 3.2.2 Server ID.
741 A channel is a named group of one or more clients which will all receive
742 messages addressed to that channel. The channel is created when first
743 client requests JOIN command to the channel, and the channel ceases to
744 exist when the last client has left it. When channel exists, any client
745 can reference it using the name of the channel. If the channel has
746 a founder mode set and last client leaves the channel the channel does
747 not cease to exist. The founder mode can be used to make permanent
748 channels in the network. The founder of the channel can regain the
749 channel founder privileges on the channel later when he joins the
752 Channel names are unique although the real uniqueness comes from 64 bit
753 Channel ID. However, channel names are still unique and no two global
754 channels with same name may exist. The channel name is a string of
755 maximum length of 256 bytes. Channel names MUST NOT contain any
756 whitespaces (` '), any non-printable ASCII characters, commas (`,')
757 and wildcard characters.
759 Channels can have operators that can administrate the channel and
760 operate all of its modes. The following operators on channel exist on
764 o Channel founder - When channel is created the joining client becomes
765 channel founder. Channel founder is channel operator with some more
766 privileges. Basically, channel founder can fully operate the channel
767 and all of its modes. The privileges are limited only to the
768 particular channel. There can be only one channel founder per
769 channel. Channel founder supersedes channel operator's privileges.
771 Channel founder privileges cannot be removed by any other operator on
772 channel. When channel founder leaves the channel there is no channel
773 founder on the channel. However, it is possible to set a mode for
774 the channel which allows the original channel founder to regain the
775 founder privileges even after leaving the channel. Channel founder
776 also cannot be removed by force from the channel.
778 o Channel operator - When client joins to channel that has not existed
779 previously it will become automatically channel operator (and channel
780 founder discussed above). Channel operator is able administrate the
781 channel, set some modes on channel, remove a badly behaving client
782 from the channel and promote other clients to become channel
783 operator. The privileges are limited only to the particular channel.
785 Normal channel user may be promoted (opped) to channel operator
786 gaining channel operator privileges. Channel founder or other
787 channel operator may also demote (deop) channel operator to normal
795 Channels are distinguished from other channels by unique Channel ID.
796 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
797 collisions are not expected to happen in any conditions. Channel names
798 are just for logical use of channels. The Channel ID is created by the
799 server where the channel is created. The Channel ID is defined as
803 64 bit Channel ID based on IPv4 addresses:
805 32 bit Router's Server ID IP address (bits 1-32)
806 16 bit Router's Server ID port (bits 33-48)
809 160 bit Channel ID based on IPv6 addresses:
811 128 bit Router's Server ID IP address (bits 1-128)
812 16 bit Router's Server ID port (bits 129-144)
815 o Router's Server ID IP address - Indicates the IP address of
816 the router of the cell where this channel is created. This is
817 taken from the router's Server ID. This way SILC router knows
818 where this channel resides in the SILC network.
820 o Router's Server ID port - Indicates the port of the channel on
821 the server. This is taken from the router's Server ID.
823 o Random number - To further randomize the Channel ID. This makes
824 sure that there are no collisions. This also means that
825 in a cell there can be 2^16 channels.
832 Operators are normal users with extra privileges to their server or
833 router. Usually these people are SILC server and router administrators
834 that take care of their own server and clients on them. The purpose of
835 operators is to administrate the SILC server or router. However, even
836 an operator with highest privileges is not able to enter invite-only
837 channel, to gain access to the contents of a encrypted and authenticated
838 packets traveling in the SILC network or to gain channel operator
839 privileges on public channels without being promoted. They have the
840 same privileges as everyone else except they are able to administrate
841 their server or router.
847 Commands are very important part on SILC network especially for client
848 which uses commands to operate on the SILC network. Commands are used
849 to set nickname, join to channel, change modes and many other things.
851 Client usually sends the commands and server replies by sending a reply
852 packet to the command. Server MAY also send commands usually to serve
853 the original client's request. Usually server cannot send commands to
854 clients, however there MAY be commands that allow the server to send
855 commands to client. By default servers MAY send commands only to other
858 Note that the command reply is usually sent only after client has sent
859 the command request but server is allowed to send command reply packet
860 to client even if client has not requested the command. Client MAY
861 choose to ignore the command reply.
863 It is expected that some of the commands may be miss-used by clients
864 resulting various problems on the server side. Every implementation
865 SHOULD assure that commands may not be executed more than once, say,
866 in two (2) seconds. However, to keep response rate up, allowing for
867 example five (5) commands before limiting is allowed. It is RECOMMENDED
868 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
869 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
870 as they require heavy operations. This should be sufficient to prevent
871 the miss-use of commands.
873 SILC commands are described in [SILC4].
879 Packets are naturally the most important part of the protocol and the
880 packets are what actually makes the protocol. Packets in SILC network
881 are always encrypted using, usually the shared secret session key
882 or some other key, for example, channel key, when encrypting channel
883 messages. It is not possible to send packet in SILC network without
884 encryption. The SILC Packet Protocol is a wide protocol and is described
885 in [SILC2]. This document does not define or describe details of
890 3.8 Packet Encryption
892 All packets passed in SILC network MUST be encrypted. This section
893 defines how packets must be encrypted in the SILC network. The detailed
894 description of the actual encryption process of the packets are
895 described in [SILC2].
897 Client and its server shares secret symmetric session key which is
898 established by the SILC Key Exchange Protocol, described in [SILC3].
899 Every packet sent from client to server, with exception of packets for
900 channels, are encrypted with this session key.
902 Channels has a channel key that are shared by every client on the channel.
903 However, the channel keys are cell specific thus one cell does not know
904 the channel key of the other cell, even if that key is for same channel.
905 Channel key is also known by the routers and all servers that has clients
906 on the channel. However, channels MAY have channel private keys that
907 are entirely local setting for the client. All clients on the channel
908 MUST know the channel private key before hand to be able to talk on the
909 channel. In this case, no server or router know the key for channel.
911 Server shares secret symmetric session key with router which is
912 established by the SILC Key Exchange Protocol. Every packet passed from
913 server to router, with exception of packets for channels, are encrypted
914 with the shared session key. Same way, router server shares secret
915 symmetric key with its primary route. However, every packet passed
916 from router to other router, including packets for channels, are
917 encrypted with the shared session key. Every router connection has
918 their own session keys.
922 3.8.1 Determination of the Source and the Destination
924 The source and the destination of the packet needs to be determined
925 to be able to route the packets to correct receiver. This information
926 is available in the SILC Packet Header which is included in all packets
927 sent in SILC network. The SILC Packet Header is described in [SILC2].
929 The header MUST be encrypted with the session key who is next receiver
930 of the packet along the route. The receiver of the packet, for example
931 a router along the route, is able to determine the sender and the
932 destination of the packet by decrypting the SILC Packet Header and
933 checking the ID's attached to the header. The ID's in the header will
934 tell to where the packet needs to be sent and where it is coming from.
936 The header in the packet MUST NOT change during the routing of the
937 packet. The original sender, for example client, assembles the packet
938 and the packet header and server or router between the sender and the
939 receiver MUST NOT change the packet header. Note however, that some
940 packets such as commands may resent by a server to serve the client's
941 original command. In this case the command packet send by the server
942 includes the server's IDs.
944 Note that the packet and the packet header may be encrypted with
945 different keys. For example, packets to channels are encrypted with
946 the channel key, however, the header is encrypted with the session key
947 as described above. However, the header and the packet may be encrypted
948 with same key. This is the case, for example, with command packets.
952 3.8.2 Client To Client
954 The process of message delivery and encryption from client to another
955 client is as follows.
957 Example: Private message from client to another client on different
958 servers. Clients do not share private message delivery
959 keys; normal session keys are used.
961 o Client 1. sends encrypted packet to its server. The packet is
962 encrypted with the session key shared between client and its
965 o Server determines the destination of the packet and decrypts
966 the packet. Server encrypts the packet with session key shared
967 between the server and its router, and sends the packet to the
970 o Router determines the destination of the packet and decrypts
971 the packet. Router encrypts the packet with session key
972 shared between the router and the destination server, and sends
973 the packet to the server.
975 o Server determines the client to which the packet is destined
976 to and decrypts the packet. Server encrypts the packet with
977 session key shared between the server and the destination client,
978 and sends the packet to the client.
980 o Client 2. decrypts the packet.
983 Example: Private message from client to another client on different
984 servers. Clients has established secret shared private
985 message delivery key with each other and that is used in
986 the message encryption.
988 o Client 1. sends encrypted packet to its server. The packet header
989 is encrypted with the session key shared between the client and
990 server, and the private message is encrypted with the private
991 message delivery key shared between clients.
993 o Server determines the destination of the packet and sends the
994 packet to the router.
996 o Router determines the destination of the packet and sends the
997 packet to the server.
999 o Server determines the client to which the packet is destined
1000 to and sends the packet to the client.
1002 o Client 2. decrypts the packet with the secret shared key.
1005 If clients share secret key with each other the private message
1006 delivery is much simpler since servers and routers between the
1007 clients do not need to decrypt and re-encrypt the packet.
1009 The process for clients on same server is much simpler as there are
1010 no need to send the packet to the router. The process for clients
1011 on different cells is same as above except that the packet is routed
1012 outside the cell. The router of the destination cell routes the
1013 packet to the destination same way as described above.
1017 3.8.3 Client To Channel
1019 Process of message delivery from client on channel to all the clients
1022 Example: Channel of four users; two on same server, other two on
1023 different cells. Client sends message to the channel.
1025 o Client 1. encrypts the packet with channel key and sends the
1026 packet to its server.
1028 o Server determines local clients on the channel and sends the
1029 packet to the Client on the same server. Server then sends
1030 the packet to its router for further routing.
1032 o Router determines local clients on the channel, if found
1033 sends packet to the local clients. Router determines global
1034 clients on the channel and sends the packet to its primary
1035 router or fastest route.
1037 o (Other router(s) do the same thing and sends the packet to
1040 o Server determines local clients on the channel and sends the
1041 packet to the client.
1043 o All clients receiving the packet decrypts the packet.
1047 3.8.4 Server To Server
1049 Server to server packet delivery and encryption is described in above
1050 examples. Router to router packet delivery is analogous to server to
1051 server. However, some packets, such as channel packets, are processed
1052 differently. These cases are described later in this document and
1053 more in detail in [SILC2].
1057 3.9 Key Exchange And Authentication
1059 Key exchange is done always when for example client connects to server
1060 but also when server and router, and router and router connects to each
1061 other. The purpose of key exchange protocol is to provide secure key
1062 material to be used in the communication. The key material is used to
1063 derive various security parameters used to secure SILC packets. The
1064 SILC Key Exchange protocol is described in detail in [SILC3].
1066 Authentication is done after key exchange protocol has been successfully
1067 completed. The purpose of authentication is to authenticate for example
1068 client connecting to the server. However, usually clients are accepted
1069 to connect to server without explicit authentication. Servers are
1070 required use authentication protocol when connecting. The authentication
1071 may be based on passphrase (pre-shared-secret) or public key. All
1072 passphrases sent in SILC protocol MUST be UTF-8 [RFC2279] encoded.
1073 The connection authentication protocol is described in detail in [SILC3].
1077 3.9.1 Authentication Payload
1079 Authentication payload is used separately from the SKE and the Connection
1080 Authentication protocol. It can be used during the session to authenticate
1081 with the remote. For example, the client can authenticate itself to the
1082 server to become server operator. In this case, Authentication Payload is
1085 The format of the Authentication Payload is as follows:
1091 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
1092 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1093 | Payload Length | Authentication Method |
1094 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1095 | Public Data Length | |
1096 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1100 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1101 | Authentication Data Length | |
1102 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1104 ~ Authentication Data ~
1106 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1110 Figure 5: Authentication Payload
1114 o Payload Length (2 bytes) - Length of the entire payload.
1116 o Authentication Method (2 bytes) - The method of the
1117 authentication. The authentication methods are defined
1118 in [SILC2] in the Connection Auth Request Payload. The NONE
1119 authentication method SHOULD NOT be used.
1121 o Public Data Length (2 bytes) - Indicates the length of
1122 the Public Data field.
1124 o Public Data (variable length) - This is defined only if
1125 the authentication method is public key. If it is any other
1126 this field MAY include a random data for padding purposes.
1127 However, in this case the field MUST be ignored by the
1130 When the authentication method is public key this includes
1131 128 to 4096 bytes of non-zero random data that is used in
1132 the signature process, described subsequently.
1134 o Authentication Data Length (2 bytes) - Indicates the
1135 length of the Authentication Data field. If zero (0)
1136 value is found in this field the payload MUST be
1139 o Authentication Data (variable length) - Authentication
1140 method dependent authentication data.
1144 If the authentication method is password based, the Authentication
1145 Data field includes the plaintext UTF-8 encoded password. It is safe
1146 to send plaintext password since the entire payload is encrypted. In
1147 this case the Public Data Length is set to zero (0), but MAY also include
1148 random data for padding purposes. It is also RECOMMENDED that maximum
1149 amount of padding is applied to SILC packet when using password based
1150 authentication. This way it is not possible to approximate the length
1151 of the password from the encrypted packet.
1153 If the authentication method is public key based (or certificate)
1154 the Authentication Data is computed as follows:
1156 HASH = hash(random bytes | ID | public key (or certificate));
1157 Authentication Data = sign(HASH);
1159 The hash() and the sign() are the hash function and the public key
1160 cryptography function selected in the SKE protocol. The public key
1161 is SILC style public key unless certificates are used. The ID is the
1162 entity's ID (Client or Server ID) which is authenticating itself. The
1163 ID encoding is described in [SILC2]. The random bytes are non-zero
1164 random bytes of length between 128 and 4096 bytes, and will be included
1165 into the Public Data field as is.
1167 The receiver will compute the signature using the random data received
1168 in the payload, the ID associated to the connection and the public key
1169 (or certificate) received in the SKE protocol. After computing the
1170 receiver MUST verify the signature. In case of public key authentication
1171 this payload is also encrypted.
1177 This section defines all the allowed algorithms that can be used in
1178 the SILC protocol. This includes mandatory cipher, mandatory public
1179 key algorithm and MAC algorithms.
1185 Cipher is the encryption algorithm that is used to protect the data
1186 in the SILC packets. See [SILC2] of the actual encryption process and
1187 definition of how it must be done. SILC has a mandatory algorithm that
1188 must be supported in order to be compliant with this protocol.
1190 The following ciphers are defined in SILC protocol:
1193 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1194 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1195 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1196 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1197 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1198 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1199 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1200 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1201 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1202 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1203 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1204 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1205 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1206 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1207 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1208 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1209 none No encryption (OPTIONAL)
1213 Algorithm none does not perform any encryption process at all and
1214 thus is not recommended to be used. It is recommended that no client
1215 or server implementation would accept none algorithms except in special
1218 Additional ciphers MAY be defined to be used in SILC by using the
1219 same name format as above.
1223 3.10.2 Public Key Algorithms
1225 Public keys are used in SILC to authenticate entities in SILC network
1226 and to perform other tasks related to public key cryptography. The
1227 public keys are also used in the SILC Key Exchange protocol [SILC3].
1229 The following public key algorithms are defined in SILC protocol:
1236 DSS is described in [Menezes]. The RSA MUST be implemented according
1237 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1238 compliant to either PKCS #1 version 1.5 or newer with the following
1239 notes: The signature encoding is always in same format as the encryption
1240 encoding regardless of the PKCS #1 version. The signature with appendix
1241 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1242 rationale for this is that there is no binding between the PKCS #1 OIDs
1243 and the hash algorithms used in the SILC protocol. Hence, the encoding
1244 is always in PKCS #1 version 1.5 format.
1246 Additional public key algorithms MAY be defined to be used in SILC.
1248 When signatures are computed in SILC the computing of the signature is
1249 represented as sign(). The signature computing procedure is dependent
1250 of the public key algorithm, and the public key or certificate encoding.
1251 When using SILC public key the signature is computed as described in
1252 previous section for RSA and DSS keys. When using SSH2 public keys
1253 the signature is computed as described in [SSH-TRANS]. When using
1254 X.509 version 3 certificates the signature is computed as described
1255 in [PKCS7]. When using OpenPGP certificates the signature is computed
1256 as described in [PGP].
1260 3.10.3 Hash Functions
1262 Hash functions are used as part of MAC algorithms defined in the next
1263 section. They are also used in the SILC Key Exchange protocol defined
1266 The following Hash algorithm are defined in SILC protocol:
1269 sha1 SHA-1, length = 20 (REQUIRED)
1270 md5 MD5, length = 16 (OPTIONAL)
1275 3.10.4 MAC Algorithms
1277 Data integrity is protected by computing a message authentication code
1278 (MAC) of the packet data. See [SILC2] for details how to compute the
1281 The following MAC algorithms are defined in SILC protocol:
1284 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1285 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1286 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1287 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1288 none No MAC (OPTIONAL)
1291 The none MAC is not recommended to be used as the packet is not
1292 authenticated when MAC is not computed. It is recommended that no
1293 client or server would accept none MAC except in special debugging
1296 The HMAC algorithm is described in [HMAC] and hash algorithms that
1297 are used as part of the HMACs are described in [Scheneir] and in
1300 Additional MAC algorithms MAY be defined to be used in SILC.
1306 3.10.5 Compression Algorithms
1308 SILC protocol supports compression that may be applied to unencrypted
1309 data. It is recommended to use compression on slow links as it may
1310 significantly speed up the data transmission. By default, SILC does not
1311 use compression which is the mode that must be supported by all SILC
1314 The following compression algorithms are defined:
1317 none No compression (REQUIRED)
1318 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1321 Additional compression algorithms MAY be defined to be used in SILC.
1325 3.11 SILC Public Key
1327 This section defines the type and format of the SILC public key. All
1328 implementations MUST support this public key type. See [SILC3] for
1329 other optional public key and certificate types allowed in the SILC
1330 protocol. Public keys in SILC may be used to authenticate entities
1331 and to perform other tasks related to public key cryptography.
1333 The format of the SILC Public Key is as follows:
1339 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
1340 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1341 | Public Key Length |
1342 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1343 | Algorithm Name Length | |
1344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1348 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1349 | Identifier Length | |
1350 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1354 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1358 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1362 Figure 5: SILC Public Key
1366 o Public Key Length (4 bytes) - Indicates the full length
1367 of the public key, not including this field.
1369 o Algorithm Name Length (2 bytes) - Indicates the length
1370 of the Algorithm Length field, not including this field.
1372 o Algorithm name (variable length) - Indicates the name
1373 of the public key algorithm that the key is. See the
1374 section 3.10.2 Public Key Algorithms for defined names.
1376 o Identifier Length (2 bytes) - Indicates the length of
1377 the Identifier field, not including this field.
1379 o Identifier (variable length) - Indicates the identifier
1380 of the public key. This data can be used to identify
1381 the owner of the key. The identifier is of the following
1385 HN Host name or IP address
1392 Examples of an identifier:
1394 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1396 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1398 At least user name (UN) and host name (HN) MUST be provided as
1399 identifier. The fields are separated by commas (`,'). If
1400 comma is in the identifier string it must be written as `\\,',
1401 for example, `O=Company XYZ\\, Inc.'.
1403 o Public Data (variable length) - Includes the actual
1404 public data of the public key.
1406 The format of this field for RSA algorithm is
1415 The format of this field for DSS algorithm is
1427 The variable length fields are multiple precession
1428 integers encoded as strings in both examples.
1430 Other algorithms must define their own type of this
1431 field if they are used.
1434 All fields in the public key are in MSB (most significant byte first)
1435 order. All strings in the public key are UTF-8 encoded.
1439 3.12 SILC Version Detection
1441 The version detection of both client and server is performed at the
1442 connection phase while executing the SILC Key Exchange protocol. The
1443 version identifier is exchanged between initiator and responder. The
1444 version identifier is of the following format:
1447 SILC-<protocol version>-<software version>
1450 The version strings are of the following format:
1453 protocol version = <major>.<minor>
1454 software version = <major>[.<minor>[.<build or vendor string>]]
1457 Protocol version MAY provide both major and minor version. Currently
1458 implementations MUST set the protocol version and accept at least the
1459 protocol version as SILC-1.1-<software version>. If new protocol version
1460 causes incompatibilities with older version the <minor> version number
1461 MUST be incremented. The <major> is incremented if new protocol version
1462 is fully incompatible.
1464 Software version MAY provide major, minor and build (vendor) version.
1465 The software version MAY be freely set and accepted. The version string
1466 MUST consist of printable US-ASCII characters.
1469 Thus, the version strings could be, for example:
1474 SILC-1.1-1.0.VendorXYZ
1475 SILC-1.1-2.4.5 Vendor Limited
1482 Backup routers may exist in the cell in addition of the primary router.
1483 However, they must not be active routers and act as routers in the cell.
1484 Only one router may be acting as primary router in the cell. In the case
1485 of failure of the primary router may one of the backup routers become
1486 active. The purpose of backup routers are in case of failure of the
1487 primary router to maintain working connections inside the cell and outside
1488 the cell and to avoid netsplits.
1490 Backup routers are normal servers in the cell that are prepared to take
1491 over the tasks of the primary router if needed. They need to have at
1492 least one direct and active connection to the primary router of the cell.
1493 This communication channel is used to send the router information to
1494 the backup router. When the backup router connects to the primary router
1495 of the cell it MUST present itself as router server in the Connection
1496 Authentication protocol, even though it is normal server as long as the
1497 primary router is available. Reason for this is that the configuration
1498 needed in the responder end requires usually router connection level
1499 configuration. The responder, however must understand and treat the
1500 connection as normal server (except when feeding router level data to
1503 Backup router must know everything that the primary router knows to be
1504 able to take over the tasks of the primary router. It is the primary
1505 router's responsibility to feed the data to the backup router. If the
1506 backup router does not know all the data in the case of failure some
1507 connections may be lost. The primary router of the cell must consider
1508 the backup router being actual router server when it feeds the data to
1511 In addition of having direct connection to the primary router of the
1512 cell, the backup router must also have connection to the same router
1513 the primary router of the cell is connected. However, it must not be
1514 active router connection meaning that the backup router must not use
1515 that channel as its primary route and it must not notify the router
1516 about having connected servers, channels and clients behind it. It
1517 merely connects to the router. This sort of connection is later
1518 referred as being passive connection. Some keepalive actions may be
1519 needed by the router to keep the connection alive.
1521 It is required that other normal servers have passive connections to
1522 the backup router(s) in the cell. Some keepalive actions may be needed
1523 by the server to keep the connection alive. After they notice the
1524 failure of the primary router they must start using the connection to
1525 the first backup router as their primary route.
1527 Also, if any other router in the network is using the cell's primary
1528 router as its own primary router, it must also have passive connection
1529 to the cell's backup router. It too is prepared to switch to use the
1530 backup router as its new primary router as soon as the orignal primary
1531 router becomes unresponsive.
1533 All of the parties of this protocol knows which one is the backup router
1534 of the cell from their local configuration. Each of the entity must
1535 be configured accordingly and care must be taken when configuring the
1536 backup routers, servers and other routers in the network.
1538 It must be noted that some of the channel messages and private messages
1539 may be lost during the switch to the backup router. The announcements
1540 assures that the state of the network is not lost during the switch.
1542 It is RECOMMENDED that there would be at least one backup router in
1543 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
1544 as backup routers as it requires establishing several connections to
1545 several servers in the cell. Large cells can easily have several
1546 backup routers in the cell.
1548 The order of the backup routers are decided at the configuration phase.
1549 All the parties of this protocol must be configured accordingly to
1550 understand the order of the backup routers. It is not required that
1551 the backup server is actually active server in the cell. Backup router
1552 may be a spare server in the cell that does not accept normal client
1553 connections at all. It may be reserved purely for the backup purposes.
1554 These, however, are cell management issues.
1556 If also the first backup router is down as well and there is another
1557 backup router in the cell then it will start acting as the primary
1558 router as described above.
1562 3.13.1 Switching to Backup Router
1564 When the primary router of the cell becomes unresponsive, for example
1565 by sending EOF to the connection, all the parties of this protocol MUST
1566 replace the old connection to the primary router with first configured
1567 backup router. The backup router usually needs to do local modifications
1568 to its database in order to update all the information needed to maintain
1569 working routes. The backup router must understand that clients that
1570 were orignated from the primary router are now originated from some of
1571 the existing server connections and must update them accordingly. It
1572 must also remove those clients that were owned by the primary router
1573 since those connections were lost when the primary router became
1576 All the other parties of the protocol must also update their local
1577 database to understand that the route to the primary router will now go
1578 to the backup router.
1580 The servers connected to the backup router must announce their clients,
1581 channels, channel users, channel user modes and channel modes to the
1582 backup router. This is to assure that none of the important notify
1583 packets were lost during the switch to the backup router. The backup
1584 router must check which of these announced entities it already have
1585 and distribute the new ones to the primary route.
1587 The backup router too must announce its servers, clients, channels
1588 and other information to the new primary router. The primary router
1589 of the backup router too must announce its informations to the backup
1590 router. Both must process only the ones they do not know about. If
1591 any of the announced modes does not match then they are enforced in
1592 normal manner defined later in this specification.
1596 3.13.2 Resuming Primary Router
1598 Usually the primary router is unresponsive only a short period of time
1599 and it is intended that the original router of the cell will reassume
1600 its position as primary router when it comes back online. The backup
1601 router that is now acting as primary router of the cell must constantly
1602 try to connect to the original primary router of the cell. It is
1603 RECOMMENDED that it would try to reconnect in 30 second intervals to
1606 When the connection is established to the primary router the backup
1607 resuming protocol is executed. The protocol is advanced as follows:
1609 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1610 value 1 the primary router that came back online. The packet
1611 will indicate the primary router has been replaced by the backup
1612 router. After sending the packet the backup router will announce
1613 all of its channels, channel users, modes etc. to the primary
1616 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1617 value 2 to its current primary router to indicate that it will
1618 resign as being primary router. Then, backup router sends the
1619 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
1620 connected servers to also indicate that it will resign as being
1623 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
1624 type value 2 to the router that is using the backup router
1625 currently as its primary router.
1627 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1628 with type value 1 or 2 must reconnect immediately to the
1629 primary router of the cell that came back online. After they
1630 have created the connection they MUST NOT use that connection
1631 as active primary route but still route all packets to the
1632 backup router. After the connection is created they MUST send
1633 SILC_PACKET_RESUME_ROUTER with type value 3 back to the
1634 backup router. The session ID value found in the first packet
1635 MUST be set in this packet.
1637 5. Backup router MUST wait for all packets with type value 3 before
1638 it continues with the protocol. It knows from the session ID values
1639 set in the packet when it have received all packets. The session
1640 value should be different in all packets it have send earlier.
1641 After the packets is received the backup router sends the
1642 SILC_PACKET_RESUME_ROUTER packet with type value 4 to the
1643 primary router that came back online. This packet will indicate
1644 that the backup router is now ready to resign as being primary
1645 router. The session ID value in this packet MUST be the same as
1646 in first packet sent to the primary router. During this time
1647 the backup router should still route all packets it is receiving
1648 from server connections.
1650 6. The primary router receives the packet and send the
1651 SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
1652 including the backup router. It also sends the packet with type
1653 value 6 to its primary router, and to the router that is using
1654 it as its primary router. The Session ID value in this packet
1657 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1658 with type value 5 or 6 must switch their primary route to the
1659 new primary router and remove the route for the backup router, since
1660 it is not anymore the primary router of the cell. They must also
1661 update their local database to understand that the clients are
1662 not originated from the backup router but from the locally connected
1663 servers. After that they MUST announce their channels, channel
1664 users, modes etc. to the primary router. They must not use the
1665 backup router connection after this and the connection is considered
1666 to be passive connection. The implementations SHOULD be able
1667 to disable the connection without closing the actual link.
1669 After this protocol is executed the backup router is now again normal
1670 server in the cell that has the backup link to the primary router. The
1671 primary router feeds the router specific data again to the backup router.
1672 All server connections in the backup router are considered passive
1675 When the primary router of the cell comes back online and connects
1676 to its primary router, the remote primary router must send the
1677 SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
1678 connection is not allowed since the router has been replaced by an
1679 backup router. The session ID value in this packet SHOULD be zero (0).
1680 When the router receives this packet it must not use the connection
1681 as active connection but to understand that it cannot act as primary
1682 router in the cell. It must wait that the backup router connects to
1683 it, and the backup resuming protocol is executed.
1685 The following type values has been defined for SILC_PACKET_RESUME_ROUTER
1688 1 SILC_SERVER_BACKUP_START
1689 2 SILC_SERVER_BACKUP_START_GLOBAL
1690 3 SILC_SERVER_BACKUP_START_CONNECTED
1691 4 SILC_SERVER_BACKUP_START_ENDING
1692 5 SILC_SERVER_BACKUP_START_RESUMED
1693 6 SILC_SERVER_BACKUP_START_GLOBAL
1694 20 SILC_SERVER_BACKUP_START_REPLACED
1696 If any other value is found in the type field the packet must be
1697 discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
1698 is defined in [SILC2].
1702 3.13.3 Discussion on Backup Router Scheme
1704 It is clear that this backup router support is not able to handle all
1705 possible situations arrising in unreliable network environment. This
1706 scheme for example does not handle situation when the router actually
1707 does not go offline but the network link goes down temporarily. It would
1708 require some intelligence to figure out when it is best time to switch
1709 to the backup router. To make it even more complicated it is possible
1710 that the backup router may have not lost the network link to the primary
1713 Other possible situation is when the network link is lost temporarily
1714 between two primary routers in the SILC network. Unless the routers
1715 notice the link going down they cannot perhaps find alternative routes.
1716 Worst situation is when the link goes down only for a short period of
1717 time, thus causing lag. Should the routers or servers find alternative
1718 routes if they cannot get response from the router during the lag?
1719 When alternative routes are being found it must be careful not to
1720 mess up existing primary routes between routers in the network.
1722 It is suggested that the current backup router scheme is only temporary
1723 solution and existing backup router protocols are studied further. It
1724 is also suggested that the backup router specification will be separated
1725 from this SILC specification Internet-Draft and additional specification
1726 is written on the subject.
1732 This section describes various SILC procedures such as how the
1733 connections are created and registered, how channels are created and
1734 so on. The section describes the procedures only generally as details
1735 are described in [SILC2] and [SILC3].
1739 4.1 Creating Client Connection
1741 This section describes the procedure when client connects to SILC server.
1742 When client connects to server the server MUST perform IP address lookup
1743 and reverse IP address lookup to assure that the origin host really is
1744 who it claims to be. Client, host, connecting to server SHOULD have
1745 both valid IP address and fully qualified domain name (FQDN).
1747 After that the client and server performs SILC Key Exchange protocol
1748 which will provide the key material used later in the communication.
1749 The key exchange protocol MUST be completed successfully before the
1750 connection registration may continue. The SILC Key Exchange protocol
1751 is described in [SILC3].
1753 Typical server implementation would keep a list of connections that it
1754 allows to connect to the server. The implementation would check, for
1755 example, the connecting client's IP address from the connection list
1756 before the SILC Key Exchange protocol has been started. Reason for
1757 this is that if the host is not allowed to connect to the server there
1758 is no reason to perform the key exchange protocol.
1760 After successful key exchange protocol the client and server performs
1761 connection authentication protocol. The purpose of the protocol is to
1762 authenticate the client connecting to the server. Flexible
1763 implementation could also accept the client to connect to the server
1764 without explicit authentication. However, if authentication is
1765 desired for a specific client it may be based on passphrase or
1766 public key authentication. If authentication fails the connection
1767 MUST be terminated. The connection authentication protocol is described
1770 After successful key exchange and authentication protocol the client
1771 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1772 server. This packet includes various information about the client
1773 that the server uses to create the client. Server creates the client
1774 and sends SILC_PACKET_NEW_ID to the client which includes the created
1775 Client ID that the client MUST start using after that. After that
1776 all SILC packets from the client MUST have the Client ID as the
1777 Source ID in the SILC Packet Header, described in [SILC2].
1779 Client MUST also get the server's Server ID that is to be used as
1780 Destination ID in the SILC Packet Header when communicating with
1781 the server (for example when sending commands to the server). The
1782 ID may be resolved in two ways. Client can take the ID from an
1783 previously received packet from server that MUST include the ID,
1784 or to send SILC_COMMAND_INFO command and receive the Server ID as
1787 Server MAY choose not to use the information received in the
1788 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1789 certificate were used in the authentication, server MAY use those
1790 informations rather than what it received from client. This is suitable
1791 way to get the true information about client if it is available.
1793 The nickname of client is initially set to the username sent in the
1794 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1795 suitable by sending SILC_COMMAND_NICK command. However, this is not
1796 required as part of registration process.
1798 Server MUST also distribute the information about newly registered
1799 client to its router (or if the server is router, to all routers in
1800 the SILC network). More information about this in [SILC2].
1802 Router server MUST also check whether some client in the local cell
1803 is watching for the nickname this new client has, and send the
1804 SILC_NOTIFY_TYPE_WATCH to the watcher.
1808 4.2 Creating Server Connection
1810 This section describes the procedure when server connects to its
1811 router (or when router connects to other router, the cases are
1812 equivalent). The procedure is very much alike when client connects
1813 to the server thus it is not repeated here.
1815 One difference is that server MUST perform connection authentication
1816 protocol with proper authentication. A proper authentication is based
1817 on passphrase or public key authentication.
1819 After server and router has successfully performed the key exchange
1820 and connection authentication protocol, the server register itself
1821 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1822 includes the server's Server ID that it has created by itself and
1823 other relevant information about the server.
1825 After router has received the SILC_PACKET_NEW_SERVER packet it
1826 distributes the information about newly registered server to all routers
1827 in the SILC network. More information about this in [SILC2].
1829 As client needed to resolve the destination ID this MUST be done by the
1830 server that connected to the router, as well. The way to resolve it is
1831 to get the ID from previously received packet. The server MAY also
1832 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1833 using its own Server ID as Source ID in SILC Packet Header and the
1834 router's Server ID as Destination when communicating with the router.
1838 4.2.1 Announcing Clients, Channels and Servers
1840 After server or router has connected to the remote router, and it already
1841 has connected clients and channels it MUST announce them to the router.
1842 If the server is router server, also all the local servers in the cell
1845 All clients are announced by compiling a list of ID Payloads into the
1846 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1847 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also,
1848 the channel users on the channels must be announced by compiling a
1849 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1850 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1851 also be announced by compiling list of Notify Payloads with the
1852 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1855 The router MUST also announce the local servers by compiling list of
1856 ID Payloads into the SILC_PACKET_NEW_ID packet.
1858 Also, clients' modes (user modes in SILC) MUST be announced. This is
1859 done by compiling a list of Notify Payloads with the
1860 SILC_NOTIFY_UMODE_CHANGE nofity type into the SILC_PACKET_NOTIFY packet.
1862 Also, channel's topics MUST be announced by compiling a list of Notify
1863 Payloads with the SILC_NOTIFY_TOPIC_SET notify type into the
1864 SILC_PACKET_NOTIFY packet.
1866 The router which receives these lists MUST process them and broadcast
1867 the packets to its primary route.
1869 When processing the announced channels and channel users the router MUST
1870 check whether a channel exists already with the same name. If channel
1871 exists with the same name it MUST check whether the Channel ID is
1872 different. If the Channel ID is different the router MUST send the notify
1873 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1874 change to the ID the router has. If the mode of the channel is different
1875 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1876 server to force the mode change to the mode that the router has.
1878 The router MUST also generate new channel key and distribute it to the
1879 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1882 If the channel has channel founder on the router the router MUST send
1883 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1884 the mode change for the channel founder on the server. The channel
1885 founder privileges MUST be removed.
1887 The router processing the channels MUST also compile a list of
1888 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1889 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1890 server (or router) will receive the clients on the channel that
1895 4.3 Joining to a Channel
1897 This section describes the procedure when client joins to a channel.
1898 Client joins to channel by sending command SILC_COMMAND_JOIN to the
1899 server. If the receiver receiving join command is normal server the
1900 server MUST check its local list whether this channel already exists
1901 locally. This would indicate that some client connected to the server
1902 has already joined to the channel. If this is case the client is
1903 joined to the channel, new channel key is created and information about
1904 newly joined channel is sent to the router. The router is informed
1905 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1906 also be sent to the local clients on the channel. The new channel key
1907 is also sent to the router and to local clients on the channel.
1909 If the channel does not exist in the local list the client's command
1910 MUST be sent to the router which will then perform the actual joining
1911 procedure. When server receives the reply to the command from the
1912 router it MUST be sent to the client which sent the command originally.
1913 Server will also receive the channel key from the server that it MUST
1914 send to the client which originally requested the join command. The
1915 server MUST also save the channel key.
1917 If the receiver of the join command is router it MUST first check its
1918 local list whether anyone in the cell has already joined to the channel.
1919 If this is the case the client is joined to the channel and reply is
1920 sent to the client. If the command was sent by server the command reply
1921 is sent to the server which sent it. Then the router MUST also create
1922 new channel key and distribute it to all clients on the channel and
1923 all servers that has clients on the channel. Router MUST also send
1924 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1925 and to local servers that has clients on the channel.
1927 If the channel does not exist on the router's local list it MUST
1928 check the global list whether the channel exists at all. If it does
1929 the client is joined to the channel as described previously. If
1930 the channel does not exist the channel is created and the client
1931 is joined to the channel. The channel key is also created and
1932 distributed as previously described. The client joining to the created
1933 channel is made automatically channel founder and both channel founder
1934 and channel operator privileges is set for the client.
1936 If the router created the channel in the process, information about the
1937 new channel MUST be broadcasted to all routers. This is done by
1938 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1939 route. When the router joins the client to the channel it MUST also
1940 send information about newly joined client to all routers in the SILC
1941 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1942 type to the router's primary route.
1944 It is important to note that new channel key is created always when
1945 new client joins to channel, whether the channel has existed previously
1946 or not. This way the new client on the channel is not able to decrypt
1947 any of the old traffic on the channel. Client which receives the reply to
1948 the join command MUST start using the received Channel ID in the channel
1949 message communication thereafter. Client also receives the key for the
1950 channel in the command reply. Note that the channel key is never
1951 generated if the SILC_CMODE_PRIVKEY mode is set.
1955 4.4 Channel Key Generation
1957 Channel keys are created by router which creates the channel by taking
1958 enough randomness from cryptographically strong random number generator.
1959 The key is generated always when channel is created, when new client
1960 joins a channel and after the key has expired. Key could expire for
1963 The key MUST also be re-generated whenever some client leaves a channel.
1964 In this case the key is created from scratch by taking enough randomness
1965 from the random number generator. After that the key is distributed to
1966 all clients on the channel. However, channel keys are cell specific thus
1967 the key is created only on the cell where the client, which left the
1968 channel, exists. While the server or router is creating the new channel
1969 key, no other client may join to the channel. Messages that are sent
1970 while creating the new key are still processed with the old key. After
1971 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1972 using the new key. If server creates the new key the server MUST also
1973 send the new key to its router. See [SILC2] on more information about
1974 how channel messages must be encrypted and decrypted when router is
1977 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1978 Channel Key Payload it MUST process the key data to create encryption
1979 and decryption key, and to create the HMAC key that is used to compute
1980 the MACs of the channel messages. The processing is as follows:
1982 channel_key = raw key data
1983 HMAC key = hash(raw key data)
1985 The raw key data is the key data received in the Channel Key Payload.
1986 The hash() function is the hash function used in the HMAC of the channel.
1987 Note that the server MUST also save the channel key.
1991 4.5 Private Message Sending and Reception
1993 Private messages are sent point to point. Client explicitly destines
1994 a private message to specific client that is delivered to only to that
1995 client. No other client may receive the private message. The receiver
1996 of the private message is destined in the SILC Packet Header as any
1997 other packet as well.
1999 If the sender of a private message does not know the receiver's Client
2000 ID, it MUST resolve it from server. There are two ways to resolve the
2001 client ID from server; it is RECOMMENDED that client implementations
2002 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
2003 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
2004 If the sender has received earlier a private message from the receiver
2005 it should have cached the Client ID from the SILC Packet Header.
2007 If server receives a private message packet which includes invalid
2008 destionation Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2009 notify to the client with error status indicating that such Client ID
2012 See [SILC2] for description of private message encryption and decryption
2017 4.6 Private Message Key Generation
2019 Private message MAY be protected by the key generated by the client.
2020 The key may be generated and sent to the other client by sending packet
2021 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
2022 and is secured by session keys. After that the private message key
2023 is used in the private message communication between those clients.
2025 Other choice is to entirely use keys that are not sent through
2026 the SILC network at all. This significantly adds security. This key
2027 would be pre-shared-key that is known by both of the clients. Both
2028 agree about using the key and starts sending packets that indicate
2029 that the private message is secured using private message key.
2031 The key material used as private message key is implementation issue.
2032 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
2033 the key material. If the key is normal pre-shared-key or randomly
2034 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
2035 the key material SHOULD be processed as defined in the [SILC3]. In
2036 the processing, however, the HASH, as defined in [SILC3] MUST be
2037 ignored. After processing the key material it is employed as defined
2038 in [SILC3], however, the HMAC key material MUST be discarded.
2040 If the key is pre-shared-key or randomly generated the implementations
2041 SHOULD use the SILC protocol's mandatory cipher as the cipher. If the
2042 SKE was used to negotiate key material the cipher was negotiated as well,
2043 and may be different from default cipher.
2047 4.7 Channel Message Sending and Reception
2049 Channel messages are delivered to group of users. The group forms a
2050 channel and all clients on the channel receives messages sent to the
2053 Channel messages are destined to channel by specifying the Channel ID
2054 as Destination ID in the SILC Packet Header. The server MUST then
2055 distribute the message to all clients on the channel by sending the
2056 channel message destined explicitly to a client on the channel.
2058 If server receives a channel message packet which includes invalid
2059 destionation Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2060 notify to the sender with error status indicating that such Channel ID
2063 See the [SILC2] for description of channel messege routing for router
2064 servers, and channel message encryption and decryption process.
2068 4.8 Session Key Regeneration
2070 Session keys MUST be regenerated periodically, say, once in an hour.
2071 The re-key process is started by sending SILC_PACKET_REKEY packet to
2072 other end, to indicate that re-key must be performed. The initiator
2073 of the connection SHOULD initiate the re-key.
2075 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2076 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2077 SKE protocol. In this case the protocol is secured with the old key
2078 and the protocol results to new key material. See [SILC3] for more
2079 information. After the SILC_PACKET_REKEY packet is sent the sender
2080 will perform the SKE protocol.
2082 If PFS flag was set the resulted key material is processed as described
2083 in the section Processing the Key Material in [SILC3]. The difference
2084 with re-key in the processing is that the initial data for the hash
2085 function is just the resulted key material and not the HASH as it
2086 is not computed at all with re-key. Other than that, the key processing
2087 it equivalent to normal SKE negotiation.
2089 If PFS flag was not set, which is the default case, then re-key is done
2090 without executing SKE protocol. In this case, the new key is created by
2091 providing the current sending encryption key to the SKE protocol's key
2092 processing function. The process is described in the section Processing
2093 the Key Material in [SILC3]. The difference in the processing is that
2094 the initial data for the hash function is the current sending encryption
2095 key and not the SKE's KEY and HASH values. Other than that, the key
2096 processing is equivalent to normal SKE negotiation.
2098 After both parties has regenerated the session key, both MUST send
2099 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2100 secured with the old key. After these packets, the subsequent packets
2101 MUST be protected with the new key.
2105 4.9 Command Sending and Reception
2107 Client usually sends the commands in the SILC network. In this case
2108 the client simply sends the command packet to server and the server
2109 processes it and replies with command reply packet. See the [SILC3]
2110 for detailed description of all commands.
2112 However, if the server is not able to process the command, it is sent
2113 to the server's router. This is case for example with commands such
2114 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2115 are other commands as well. For example, if client sends the WHOIS
2116 command requesting specific information about some client the server must
2117 send the WHOIS command to router so that all clients in SILC network
2118 are searched. The router, on the other hand, sends the WHOIS command
2119 further to receive the exact information about the requested client.
2120 The WHOIS command travels all the way to the server which owns the client
2121 and it replies with command reply packet. Finally, the server which
2122 sent the command receives the command reply and it must be able to
2123 determine which client sent the original command. The server then
2124 sends command reply to the client. Implementations should have some
2125 kind of cache to handle, for example, WHOIS information. Servers
2126 and routers along the route could all cache the information for faster
2127 referencing in the future.
2129 The commands sent by server may be sent hop by hop until someone is able
2130 to process the command. However, it is preferred to destine the command
2131 as precisely as it is possible. In this case, other routers en route
2132 MUST route the command packet by checking the true sender and true
2133 destination of the packet. However, servers and routers MUST NOT route
2134 command reply packets to clients coming from other server. Client
2135 MUST NOT accept command reply packet originated from anyone else but
2136 from its own server.
2140 4.10 Closing Connection
2142 When remote client connection is closed the server MUST send the notify
2143 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2144 the client was joined. The server MUST also save the client's information
2145 for a period of time for history purposes.
2147 When remote server or router connection is closed the server or router
2148 MUST also remove all the clients that was behind the server or router
2149 from the SILC Network. The server or router MUST also send the notify
2150 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2151 local clients that are joined on the same channels with the remote
2152 server's or router's clients.
2154 Router server MUST also check whether some client in the local cell
2155 is watching for the nickname this client has, and send the
2156 SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
2157 the network has the SILC_UMODE_REJECT_WATCHING user mode set.
2161 4.11 Detaching and Resuming a Session
2163 SILC protocol provides a possibility for a client to detach itself from
2164 the network without actually signing off from the network. The client
2165 connection to the server is closed but the client remains as valid client
2166 in the network. The client may then later resume its session back from
2167 any server in the network.
2169 When client wishes to detach from the network it MUST send the
2170 SILC_COMMAND_DETACH command to its server. The server then MUST set
2171 SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
2172 notify to its primary router, which will then MUST broadcast it further
2173 to other routers in the network. This user mode indicates that the
2174 client is detached from the network. Implementations MUST NOT use
2175 the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
2176 to the client. All packets MUST still be sent to the client even if
2177 client is detached from the network. Only the server that originally
2178 had the active client connection is able to make the decision after it
2179 notices that the network connection is not active. In this case the
2180 default case is to discard the packet.
2182 The SILC_UMODE_DETACHED flag cannot be set by client itself directly
2183 with SILC_COMMAND_UMODE command, but only implicitly by sending the
2184 SILC_COMMAND_DETACH command. The flag also cannot be unset by the
2185 client, server or router with SILC_COMMAND_UMODE command, but only
2186 implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
2189 When the client wishes to resume its session in the SILC Network it
2190 connects to a server in the network, which MAY also be a different
2191 from the original server, and performs normal procedures regarding
2192 creating a connection as described in section 4.1. After the SKE
2193 and the Connection Authentication protocols has been successfully
2194 completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
2195 MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
2196 perform the resuming procedure. The packet MUST include the detached
2197 client's Client ID, which the client must know. It also includes
2198 Authentication Payload which includes signature made with the client's
2199 private key. The signature is computed as defined in the section
2200 3.9.1. Thus, the authentication method MUST be based in public key
2203 When server receives the SILC_PACKET_RESUME_CLIENT packet it MUST
2204 do the following: Server checks that the Client ID is valid client
2205 and that it has the SILC_UMODE_DETACHED mode set. Then it verifies
2206 the Authentication Payload with the detached client's public key.
2207 If it does not have the public key it retrieves it by sending
2208 SILC_COMMAND_GETKEY command to the server that has the public key from
2209 the original client connection. The server MUST NOT use the public
2210 key received in the SKE protocol for this connection. If the
2211 signature is valid the server unsets the SILC_UMODE_DETACHED flag,
2212 and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
2213 The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
2214 flag when the packet is received. If the server is router server it
2215 also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
2216 server whom owned the detached client.
2218 The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
2219 packet MUST know whether the packet already has been received for
2220 the client. It is protocol error to attempt to resume the client
2221 session from more than one server. The implementations could set
2222 internal flag that indicates that the client is resumed. If router
2223 receive SILC_PACKET_RESUME_CLIENT packet for client that is already
2224 resumed the client MUST be killed from the network. This would
2225 indicate that the client is attempting to resume the session more
2226 than once which is protocol error. In this case the router sends
2227 SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
2228 the same situation MUST also send the notify for the client.
2230 The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
2231 must also understand that the client may not be found behind the
2232 same server that it originally came from. They must update their
2233 caches according this. The server that now owns the client session
2234 MUST check whether the Client ID of the resumed client is based
2235 on the server's Server ID. If it is not it creates a new Client
2236 ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
2237 also send the channel keys of all channels that the client is
2238 joined to the client since it does not have them. Whether the
2239 Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
2240 packet to the client. Only after this the client is resumed back
2241 to the network and may start sending packets and messages.
2243 It is also possible that the server does not know about the channels
2244 that the client has joined. In this case it join the client internally
2245 to the channels, generate new channel keys and distribute the keys
2246 to the channels as described in section 4.4.
2248 It is implementation issue for how long servers keep detached client
2249 sessions. It is RECOMMENDED that the detached sessions would be
2250 persistent as long as the server is running.
2254 5 Security Considerations
2256 Security is central to the design of this protocol, and these security
2257 considerations permeate the specification. Common security considerations
2258 such as keeping private keys truly private and using adequate lengths for
2259 symmetric and asymmetric keys must be followed in order to maintain the
2260 security of this protocol.
2262 Special attention must also be paid on the servers and routers that are
2263 running the SILC service. The SILC protocol's security depends greatly
2264 on the security and the integrity of the servers and administrators that
2265 are running the service. It is recommended that some form of registration
2266 is required by the server and router administrator prior acceptance to
2267 the SILC Network. Even though, the SILC protocol is secure in a network
2268 of mutual distrust between clients, servers, routers and adminstrators
2269 of the servers, the client should be able to trust the servers they are
2270 using if they whish to do so.
2272 It however must be noted that if the client requires absolute security
2273 by not trusting any of the servers or routers in the SILC Network, it can
2274 be accomplished by negotiating private keys outside the SILC Network,
2275 either using SKE or some other key exchange protocol, or to use some
2276 other external means for distributing the keys. This applies for all
2277 messages, private messages and channel messages.
2279 It is important to note that SILC, like any other security protocol is
2280 not full proof system and cannot secure from insecure environment; the
2281 SILC servers and routers could very well be compromised. However, to
2282 provide acceptable level of security and usability for end user the
2283 protocol use many times session keys or other keys generated by the
2284 servers to secure the messages. This is intentional design feature to
2285 allow ease of use for end user. This way the network is still usable,
2286 and remains encrypted even if the external means of distributing the
2287 keys is not working. The implementation, however, may like to not
2288 follow this design feature, and always negotiate the keys outside SILC
2289 network. This is acceptable solution and many times recommended. The
2290 implementation still must be able to work with the server generated keys.
2292 If this is unacceptable for the client or end user, the private keys
2293 negotiatied outside the SILC Network should always be used. In the end
2294 it is always implementor's choice whether to negotiate private keys by
2295 default or whether to use the keys generated by the servers.
2297 It is also recommended that router operators in the SILC Network would
2298 form a joint forum to discuss the router and SILC Network management
2299 issues. Also, router operators along with the cell's server operators
2300 should have a forum to discuss the cell management issues.
2306 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2309 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2310 Protocols", Internet Draft, April 2001.
2312 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001.
2314 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2317 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2320 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2323 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2326 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2329 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2332 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2335 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2338 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2339 Infrastructure, Certificate and CRL Profile", RFC 2459,
2342 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2343 John Wiley & Sons, New York, NY, 1996.
2345 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2348 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2349 RFC 2412, November 1998.
2351 [ISAKMP] Maughan D., et al, "Internet Security Association and
2352 Key Management Protocol (ISAKMP)", RFC 2408, November
2355 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2356 (IKE)", RFC 2409, November 1998.
2358 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2359 Authentication", RFC 2104, February 1997.
2361 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2362 Specifications, Version 2.0", RFC 2437, October 1998.
2364 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2365 Requirement Levels", BCP 14, RFC 2119, March 1997.
2367 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
2368 10646", RFC 2279, January 1998.
2370 [PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
2371 Version 1.5", RFC 2315, March 1998.
2379 Snellmanninkatu 34 A 15
2383 EMail: priikone@iki.fi
2385 This Internet-Draft expires XXX