8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-06.txt XXX
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-06.txt>
32 This document is an Internet-Draft and is in full conformance with
33 all provisions of Section 10 of RFC 2026. Internet-Drafts are
34 working documents of the Internet Engineering Task Force (IETF), its
35 areas, and its working groups. Note that other groups may also
36 distribute working documents as Internet-Drafts.
38 Internet-Drafts are draft documents valid for a maximum of six months
39 and may be updated, replaced, or obsoleted by other documents at any
40 time. It is inappropriate to use Internet-Drafts as reference
41 material or to cite them other than as "work in progress."
43 The list of current Internet-Drafts can be accessed at
44 http://www.ietf.org/ietf/1id-abstracts.txt
46 The list of Internet-Draft Shadow Directories can be accessed at
47 http://www.ietf.org/shadow.html
49 The distribution of this memo is unlimited.
55 This memo describes a Secure Internet Live Conferencing (SILC)
56 protocol which provides secure conferencing services over insecure
57 network channel. SILC is IRC [IRC] like protocol, however, it is
58 not equivalent to IRC and does not support IRC. Strong cryptographic
59 methods are used to protect SILC packets inside the SILC network.
60 Three other Internet Drafts relates very closely to this memo;
61 SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
62 Protocols [SILC3] and SILC Commands [SILC4].
73 1 Introduction .................................................. 3
74 1.1 Requirements Terminology .................................. 4
75 2 SILC Concepts ................................................. 4
76 2.1 SILC Network Topology ..................................... 4
77 2.2 Communication Inside a Cell ............................... 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 .............................. 13
90 3.3.2 Router's Global ID List ............................. 13
91 3.3.3 Router's Server ID .................................. 14
92 3.4 Channels .................................................. 14
93 3.4.1 Channel ID .......................................... 15
94 3.5 Operators ................................................. 16
95 3.6 SILC Commands ............................................. 16
96 3.7 SILC Packets .............................................. 17
97 3.8 Packet Encryption ......................................... 17
98 3.8.1 Determination of the Source and the Destination ..... 18
99 3.8.2 Client To Client .................................... 18
100 3.8.3 Client To Channel ................................... 20
101 3.8.4 Server To Server .................................... 20
102 3.9 Key Exchange And Authentication ........................... 20
103 3.9.1 Authentication Payload .............................. 21
104 3.10 Algorithms ............................................... 23
105 3.10.1 Ciphers ............................................ 23
106 3.10.2 Public Key Algorithms .............................. 24
107 3.10.3 Hash Functions ..................................... 24
108 3.10.4 MAC Algorithms ..................................... 25
109 3.10.5 Compression Algorithms ............................. 25
110 3.11 SILC Public Key .......................................... 26
111 3.12 SILC Version Detection ................................... 28
112 3.13 Backup Routers ........................................... 28
113 3.13.1 Switching to Backup Router ......................... 30
114 3.13.2 Resuming Primary Router ............................ 31
115 3.13.3 Discussion on Backup Router Scheme ................. 33
116 4 SILC Procedures ............................................... 34
117 4.1 Creating Client Connection ................................ 34
118 4.2 Creating Server Connection ................................ 35
119 4.2.1 Announcing Clients, Channels and Servers ............ 36
120 4.3 Joining to a Channel ...................................... 37
121 4.4 Channel Key Generation .................................... 38
122 4.5 Private Message Sending and Reception ..................... 39
123 4.6 Private Message Key Generation ............................ 39
124 4.7 Channel Message Sending and Reception ..................... 40
125 4.8 Session Key Regeneration .................................. 40
126 4.9 Command Sending and Reception ............................. 41
127 4.10 Closing Connection ....................................... 42
128 4.11 Detaching and Resuming a Session ......................... 42
129 5 Security Considerations ....................................... 44
130 6 References .................................................... 45
131 7 Author's Address .............................................. 47
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 implemented 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 bandwidth
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.
228 The following diagram represents SILC network topology.
232 ---- ---- ---- ---- ---- ----
233 | S8 | S5 | S4 | | S7 | S5 | S6 |
234 ----- ---- ----- ----- ---- -----
235 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
236 ---- ------ ---- ---- ------ ----
237 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
238 ---- ---- ---- ---- ---- ---- | S3 | S1 |
239 Cell 1. \\ Cell 2. | \\____ ----- -----
241 ---- ---- ---- ---- ---- ---- ---- ------
242 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
243 ----- ---- ----- ----- ---- ----- ---- ----
244 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
245 ---- ------ ---- ---- ------ ----
246 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
247 ---- ---- ---- ---- ---- ----
252 Figure 1: SILC Network Topology
255 A cell is formed when a server or servers connect to one router. In
256 SILC network normal server cannot directly connect to other normal
257 server. Normal server may only connect to SILC router which then
258 routes the messages to the other servers in the cell. Router servers
259 on the other hand may connect to other routers to form the actual SILC
260 network, as seen in above figure. However, router is also normal SILC
261 server; clients may connect to it the same way as to normal SILC
262 server. Normal server also cannot have active connections to more
263 than one router. Normal server cannot be connected to two different
264 cells. Router servers, on the other hand, may have as many router to
265 router connections as needed.
267 There are many issues in this network topology that needs to be careful
268 about. Issues like the size of the cells, the number of the routers in
269 the SILC network and the capacity requirements of the routers. These
270 issues should be discussed in the Internet Community and additional
271 documents on the issue may be written.
275 2.2 Communication Inside a Cell
277 It is always guaranteed that inside a cell message is delivered to the
278 recipient with at most two server hops. A client which is connected to
279 server in the cell and is talking on channel to other client connected
280 to other server in the same cell, will have its messages delivered from
281 its local server first to the router of the cell, and from the router
282 to the other server in the cell.
284 The following diagram represents this scenario:
298 Figure 2: Communication Inside cell
301 Example: Client 1. connected to Server 1. send message to
302 Client 4. connected to Server 2. travels from Server 1.
303 first to Router which routes the message to Server 2.
304 which then sends it to the Client 4. All the other
305 servers in the cell will not see the routed message.
308 If the client is connected directly to the router, as router is also normal
309 SILC server, the messages inside the cell are always delivered only with
310 one server hop. If clients communicating with each other are connected
311 to the same server, no router interaction is needed. This is the optimal
312 situation of message delivery in the SILC network.
316 2.3 Communication in the Network
318 If the message is destined to server that does not belong to local cell
319 the message is routed to the router server to which the destination
320 server belongs, if the local router is connected to destination router.
321 If there is no direct connection to the destination router, the local
322 router routes the message to its primary route. The following diagram
323 represents message sending between cells.
331 1 --- S1 S4 --- 5 S2 --- 1
332 S/R - - - - - - - - S/R
342 Figure 3: Communication Between Cells
345 Example: Client 5. connected to Server 4. in Cell 1. sends message
346 to Client 2. connected to Server 1. in Cell 2. travels
347 from Server 4. to Router which routes the message to
348 Router in Cell 2, which then routes the message to
349 Server 1. All the other servers and routers in the
350 network will not see the routed message.
353 The optimal case of message delivery from the client point of view is
354 when clients are connected directly to the routers and the messages
355 are delivered from one router to the other.
359 2.4 Channel Communication
361 Messages may be sent to group of clients as well. Sending messages to
362 many clients works the same way as sending messages point to point, from
363 message delivery point of view. Security issues are another matter
364 which are not discussed in this section.
366 Router server handles the message routing to multiple recipients. If
367 any recipient is not in the same cell as the sender the messages are
370 Server distributes the channel message to its local clients which are
371 joined to the channel. Router also distributes the message to its
372 local clients on the channel.
376 2.5 Router Connections
378 Router connections play very important role in making the SILC like
379 network topology to work. For example, sending broadcast packets in
380 SILC network require special connections between routers; routers must
381 be connected in a specific way.
383 Every router has their primary route which is a connection to another
384 router in the network. Unless there is only two routers in the network
385 must not routers use each other as their primary routes. The router
386 connections in the network must form a ring.
388 Example with three routers in the network:
393 S/R1 - < - < - < - < - < - < - S/R2
396 \\ - > - > - S/R3 - > - > - /
401 Figure 4: Router Connections
404 Example: Network with three routers. Router 1. uses Router 2. as its
405 primary router. Router 2. uses Router 3. as its primary router,
406 and Router 3. uses Router 1. as its primary router. There may
407 be other direct connections between the routers but they must
408 not be used as primary routes.
410 The above example is applicable to any amount of routers in the network
411 except for two routers. If there are only two routers in the network both
412 routers must be able to handle situation where they use each other as their
415 The issue of router connections are very important especially with SILC
416 broadcast packets. Usually all router wide information in the network is
417 distributed by SILC broadcast packets. This sort of ring network, with
418 ability to have other direct routes in the network cause interesting
419 routing problems. The [SILC2] discusses the routing of packets in this
420 sort of network in more detail.
424 3. SILC Specification
426 This section describes the SILC protocol. However, [SILC2] and
427 [SILC3] describes other important protocols that are part of this SILC
428 specification and must be read.
434 A client is a piece of software connecting to SILC server. SILC client
435 cannot be SILC server. Purpose of clients is to provide the user
436 interface of the SILC services for end user. Clients are distinguished
437 from other clients by unique Client ID. Client ID is a 128 bit ID that
438 is used in the communication in the SILC network. The client ID is
439 based on the nickname selected by the user. User uses logical nicknames
440 in communication which are then mapped to the corresponding Client ID.
441 Client ID's are low level identifications and must not be seen by the
444 Clients provide other information about the end user as well. Information
445 such as the nickname of the user, username and the host name of the end
446 user and user's real name. See section 3.2 Server for information of
447 the requirements of keeping this information.
449 The nickname selected by the user is not unique in the SILC network.
450 There can be 2^8 same nicknames for one IP address. As for comparison
451 to IRC [IRC] where nicknames are unique this is a fundamental difference
452 between SILC and IRC. This causes the server names or client's host names
453 to be used along with the nicknames to identify specific users when sending
454 messages. This feature of SILC makes IRC style nickname-wars obsolete as
455 no one owns their nickname; there can always be someone else with the same
456 nickname. The maximum length of nickname is 128 bytes.
462 Client ID is used to identify users in the SILC network. The Client ID
463 is unique to the extent that there can be 2^128 different Client ID's,
464 and ID's based on IPv6 addresses extends this to 2^224 different Client
465 ID's. Collisions are not expected to happen. The Client ID is defined
471 128 bit Client ID based on IPv4 addresses:
473 32 bit Server ID IP address (bits 1-32)
474 8 bit Random number or counter
475 88 bit Truncated MD5 hash value of the nickname
477 224 bit Client ID based on IPv6 addresses:
479 128 bit Server ID IP address (bits 1-128)
480 8 bit Random number or counter
481 88 bit Truncated MD5 hash value of the nickname
483 o Server ID IP address - Indicates the server where this
484 client is coming from. The IP address hence equals the
485 server IP address where to the client has connected.
487 o Random number or counter - Random number to further
488 randomize the Client ID. Another choice is to use
489 a counter starting from the zero (0). This makes it
490 possible to have 2^8 same nicknames from the same
493 o MD5 hash - MD5 hash value of the lowercase nickname is
494 truncated taking 88 bits from the start of the hash value.
495 This hash value is used to search the user's Client ID
496 from the ID lists. Note that the nickname MUST be in
500 Collisions could occur when more than 2^8 clients using same nickname
501 from the same server IP address is connected to the SILC network.
502 Server MUST be able to handle this situation by refusing to accept
503 anymore of that nickname.
505 Another possible collision may happen with the truncated hash value of
506 the nickname. It could be possible to have same truncated hash value for
507 two different nicknames. However, this is not expected to happen nor
508 cause any problems if it would occur. Nicknames are usually logical and
509 it is unlikely to have two distinct logical nicknames produce same
510 truncated hash value.
516 Servers are the most important parts of the SILC network. They form the
517 basis of the SILC, providing a point to which clients may connect to.
518 There are two kinds of servers in SILC; normal servers and router servers.
519 This section focus on the normal server and router server is described
520 in the section 3.3 Router.
522 Normal servers MUST NOT directly connect to other normal server. Normal
523 servers may only directly connect to router server. If the message sent
524 by the client is destined outside the local server it is always sent to
525 the router server for further routing. Server may only have one active
526 connection to router on same port. Normal server MUST NOT connect to other
527 cell's router except in situations where its cell's router is unavailable.
531 3.2.1 Server's Local ID List
533 Normal server keeps various information about the clients and their end
534 users connected to it. Every normal server MUST keep list of all locally
535 connected clients, Client ID's, nicknames, usernames and host names and
536 user's real name. Normal servers only keeps local information and it
537 does not keep any global information. Hence, normal servers knows only
538 about their locally connected clients. This makes servers efficient as
539 they don't have to worry about global clients. Server is also responsible
540 of creating the Client ID's for their clients.
542 Normal server also keeps information about locally created channels and
546 Hence, local list for normal server includes:
549 server list - Router connection
557 client list - All clients in server
567 channel list - All channels in server
570 o Client ID's on channel
571 o Client ID modes on channel
579 Servers are distinguished from other servers by unique 64 bit Server ID
580 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
581 the SILC to route messages to correct servers. Server ID's also provide
582 information for Client ID's, see section 3.1.1 Client ID. Server ID is
586 64 bit Server ID based on IPv4 addresses:
588 32 bit IP address of the server
592 160 bit Server ID based on IPv6 addresses:
594 128 bit IP address of the server
598 o IP address of the server - This is the real IP address of
601 o Port - This is the port the server is bound to.
603 o Random number - This is used to further randomize the Server ID.
606 Collisions are not expected to happen in any conditions. The Server ID
607 is always created by the server itself and server is responsible of
608 distributing it to the router.
612 3.2.3 SILC Server Ports
614 The following ports has been assigned by IANA for the SILC protocol:
622 If there are needs to create new SILC networks in the future the port
623 numbers must be officially assigned by the IANA.
625 Server on network above privileged ports (>1023) SHOULD NOT be trusted
626 as they could have been set up by untrusted party.
632 Router server in SILC network is responsible for keeping the cell together
633 and routing messages to other servers and to other routers. Router server
634 is also a normal server thus clients may connect to it as it would be
635 just normal SILC server.
637 However, router servers has a lot of important tasks that normal servers
638 do not have. Router server knows everything about everything in the SILC.
639 They know all clients currently on SILC, all servers and routers and all
640 channels in SILC. Routers are the only servers in SILC that care about
641 global information and keeping them up to date at all time. And, this
642 is what they must do.
646 3.3.1 Router's Local ID List
648 Router server as well MUST keep local list of connected clients and
649 locally created channels. However, this list is extended to include all
650 the informations of the entire cell, not just the server itself as for
653 However, on router this list is a lot smaller since routers do not need
654 to keep information about user's nickname, username and host name and real
655 name since these are not needed by the router. The router keeps only
656 information that it needs.
659 Hence, local list for router includes:
662 server list - All servers in the cell
669 client list - All clients in the cell
673 channel list - All channels in the cell
675 o Client ID's on channel
676 o Client ID modes on channel
681 Note that locally connected clients and other information include all the
682 same information as defined in section section 3.2.1 Server's Local ID
687 3.3.2 Router's Global ID List
689 Router server MUST also keep global list. Normal servers do not have
690 global list as they know only about local information. Global list
691 includes all the clients on SILC, their Client ID's, all created channels
692 and their Channel ID's and all servers and routers on SILC and their
693 Server ID's. That is said, global list is for global information and the
694 list must not include the local information already on the router's local
697 Note that the global list does not include information like nicknames,
698 usernames and host names or user's real names. Router does not need to
699 keep these informations as they are not needed by the router. This
700 information is available from the client's server which maybe queried
703 Hence, global list includes:
706 server list - All servers in SILC
711 client list - All clients in SILC
714 channel list - All channels in SILC
716 o Client ID's on channel
717 o Client ID modes on channel
723 3.3.3 Router's Server ID
725 Router's Server ID's are equivalent to normal Server ID's. As routers
726 are normal servers as well same types of ID's applies for routers as well.
727 Thus, see section 3.2.2 Server ID.
733 A channel is a named group of one or more clients which will all receive
734 messages addressed to that channel. The channel is created when first
735 client requests JOIN command to the channel, and the channel ceases to
736 exist when the last client has left it. When channel exists, any client
737 can reference it using the name of the channel. If the channel has
738 a founder mode set and last client leaves the channel the channel does
739 not cease to exist. The founder mode can be used to make permanent
740 channels in the network. The founder of the channel can regain the
741 channel founder privileges on the channel later when he joins the
744 Channel names are unique although the real uniqueness comes from 64 bit
745 Channel ID. However, channel names are still unique and no two global
746 channels with same name may exist. The channel name is a string of
747 maximum length of 256 bytes. Channel names MUST NOT contain any
748 whitespaces (` '), any non-printable ASCII characters, commas (`,')
749 and wildcard characters.
751 Channels can have operators that can administrate the channel and
752 operate all of its modes. The following operators on channel exist on
756 o Channel founder - When channel is created the joining client becomes
757 channel founder. Channel founder is channel operator with some more
758 privileges. Basically, channel founder can fully operate the channel
759 and all of its modes. The privileges are limited only to the
760 particular channel. There can be only one channel founder per
761 channel. Channel founder supersedes channel operator's privileges.
763 Channel founder privileges cannot be removed by any other operator on
764 channel. When channel founder leaves the channel there is no channel
765 founder on the channel. However, it is possible to set a mode for
766 the channel which allows the original channel founder to regain the
767 founder privileges even after leaving the channel. Channel founder
768 also cannot be removed by force from the channel.
770 o Channel operator - When client joins to channel that has not existed
771 previously it will become automatically channel operator (and channel
772 founder discussed above). Channel operator is able administrate the
773 channel, set some modes on channel, remove a badly behaving client
774 from the channel and promote other clients to become channel
775 operator. The privileges are limited only to the particular channel.
777 Normal channel user may be promoted (opped) to channel operator
778 gaining channel operator privileges. Channel founder or other
779 channel operator may also demote (deop) channel operator to normal
787 Channels are distinguished from other channels by unique Channel ID.
788 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
789 collisions are not expected to happen in any conditions. Channel names
790 are just for logical use of channels. The Channel ID is created by the
791 server where the channel is created. The Channel ID is defined as
795 64 bit Channel ID based on IPv4 addresses:
797 32 bit Router's Server ID IP address (bits 1-32)
798 16 bit Router's Server ID port (bits 33-48)
801 160 bit Channel ID based on IPv6 addresses:
803 128 bit Router's Server ID IP address (bits 1-128)
804 16 bit Router's Server ID port (bits 129-144)
807 o Router's Server ID IP address - Indicates the IP address of
808 the router of the cell where this channel is created. This is
809 taken from the router's Server ID. This way SILC router knows
810 where this channel resides in the SILC network.
812 o Router's Server ID port - Indicates the port of the channel on
813 the server. This is taken from the router's Server ID.
815 o Random number - To further randomize the Channel ID. This makes
816 sure that there are no collisions. This also means that
817 in a cell there can be 2^16 channels.
824 Operators are normal users with extra privileges to their server or
825 router. Usually these people are SILC server and router administrators
826 that take care of their own server and clients on them. The purpose of
827 operators is to administrate the SILC server or router. However, even
828 an operator with highest privileges is not able to enter invite-only
829 channel, to gain access to the contents of a encrypted and authenticated
830 packets traveling in the SILC network or to gain channel operator
831 privileges on public channels without being promoted. They have the
832 same privileges as everyone else except they are able to administrate
833 their server or router.
839 Commands are very important part on SILC network especially for client
840 which uses commands to operate on the SILC network. Commands are used
841 to set nickname, join to channel, change modes and many other things.
843 Client usually sends the commands and server replies by sending a reply
844 packet to the command. Server MAY also send commands usually to serve
845 the original client's request. Usually server cannot send commands to
846 clients, however there MAY be commands that allow the server to send
847 commands to client. By default servers MAY send commands only to other
850 Note that the command reply is usually sent only after client has sent
851 the command request but server is allowed to send command reply packet
852 to client even if client has not requested the command. Client MAY
853 choose to ignore the command reply.
855 It is expected that some of the commands may be miss-used by clients
856 resulting various problems on the server side. Every implementation
857 SHOULD assure that commands may not be executed more than once, say,
858 in two (2) seconds. However, to keep response rate up, allowing for
859 example five (5) commands before limiting is allowed. It is RECOMMENDED
860 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
861 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
862 as they require heavy operations. This should be sufficient to prevent
863 the miss-use of commands.
865 SILC commands are described in [SILC4].
871 Packets are naturally the most important part of the protocol and the
872 packets are what actually makes the protocol. Packets in SILC network
873 are always encrypted using, usually the shared secret session key
874 or some other key, for example, channel key, when encrypting channel
875 messages. It is not possible to send packet in SILC network without
876 encryption. The SILC Packet Protocol is a wide protocol and is described
877 in [SILC2]. This document does not define or describe details of
882 3.8 Packet Encryption
884 All packets passed in SILC network MUST be encrypted. This section
885 defines how packets must be encrypted in the SILC network. The detailed
886 description of the actual encryption process of the packets are
887 described in [SILC2].
889 Client and its server shares secret symmetric session key which is
890 established by the SILC Key Exchange Protocol, described in [SILC3].
891 Every packet sent from client to server, with exception of packets for
892 channels, are encrypted with this session key.
894 Channels has a channel key that are shared by every client on the channel.
895 However, the channel keys are cell specific thus one cell does not know
896 the channel key of the other cell, even if that key is for same channel.
897 Channel key is also known by the routers and all servers that has clients
898 on the channel. However, channels MAY have channel private keys that
899 are entirely local setting for the client. All clients on the channel
900 MUST know the channel private key before hand to be able to talk on the
901 channel. In this case, no server or router know the key for channel.
903 Server shares secret symmetric session key with router which is
904 established by the SILC Key Exchange Protocol. Every packet passed from
905 server to router, with exception of packets for channels, are encrypted
906 with the shared session key. Same way, router server shares secret
907 symmetric key with its primary route. However, every packet passed
908 from router to other router, including packets for channels, are
909 encrypted with the shared session key. Every router connection has
910 their own session keys.
914 3.8.1 Determination of the Source and the Destination
916 The source and the destination of the packet needs to be determined
917 to be able to route the packets to correct receiver. This information
918 is available in the SILC Packet Header which is included in all packets
919 sent in SILC network. The SILC Packet Header is described in [SILC2].
921 The header MUST be encrypted with the session key who is next receiver
922 of the packet along the route. The receiver of the packet, for example
923 a router along the route, is able to determine the sender and the
924 destination of the packet by decrypting the SILC Packet Header and
925 checking the ID's attached to the header. The ID's in the header will
926 tell to where the packet needs to be sent and where it is coming from.
928 The header in the packet MUST NOT change during the routing of the
929 packet. The original sender, for example client, assembles the packet
930 and the packet header and server or router between the sender and the
931 receiver MUST NOT change the packet header. Note however, that some
932 packets such as commands may resent by a server to serve the client's
933 original command. In this case the command packet send by the server
934 includes the server's IDs.
936 Note that the packet and the packet header may be encrypted with
937 different keys. For example, packets to channels are encrypted with
938 the channel key, however, the header is encrypted with the session key
939 as described above. However, the header and the packet may be encrypted
940 with same key. This is the case, for example, with command packets.
944 3.8.2 Client To Client
946 The process of message delivery and encryption from client to another
947 client is as follows.
949 Example: Private message from client to another client on different
950 servers. Clients do not share private message delivery
951 keys; normal session keys are used.
953 o Client 1. sends encrypted packet to its server. The packet is
954 encrypted with the session key shared between client and its
957 o Server determines the destination of the packet and decrypts
958 the packet. Server encrypts the packet with session key shared
959 between the server and its router, and sends the packet to the
962 o Router determines the destination of the packet and decrypts
963 the packet. Router encrypts the packet with session key
964 shared between the router and the destination server, and sends
965 the packet to the server.
967 o Server determines the client to which the packet is destined
968 to and decrypts the packet. Server encrypts the packet with
969 session key shared between the server and the destination client,
970 and sends the packet to the client.
972 o Client 2. decrypts the packet.
975 Example: Private message from client to another client on different
976 servers. Clients has established secret shared private
977 message delivery key with each other and that is used in
978 the message encryption.
980 o Client 1. sends encrypted packet to its server. The packet header
981 is encrypted with the session key shared between the client and
982 server, and the private message is encrypted with the private
983 message delivery key shared between clients.
985 o Server determines the destination of the packet and sends the
986 packet to the router.
988 o Router determines the destination of the packet and sends the
989 packet to the server.
991 o Server determines the client to which the packet is destined
992 to and sends the packet to the client.
994 o Client 2. decrypts the packet with the secret shared key.
997 If clients share secret key with each other the private message
998 delivery is much simpler since servers and routers between the
999 clients do not need to decrypt and re-encrypt the packet.
1001 The process for clients on same server is much simpler as there are
1002 no need to send the packet to the router. The process for clients
1003 on different cells is same as above except that the packet is routed
1004 outside the cell. The router of the destination cell routes the
1005 packet to the destination same way as described above.
1009 3.8.3 Client To Channel
1011 Process of message delivery from client on channel to all the clients
1014 Example: Channel of four users; two on same server, other two on
1015 different cells. Client sends message to the channel.
1017 o Client 1. encrypts the packet with channel key and sends the
1018 packet to its server.
1020 o Server determines local clients on the channel and sends the
1021 packet to the Client on the same server. Server then sends
1022 the packet to its router for further routing.
1024 o Router determines local clients on the channel, if found
1025 sends packet to the local clients. Router determines global
1026 clients on the channel and sends the packet to its primary
1027 router or fastest route.
1029 o (Other router(s) do the same thing and sends the packet to
1032 o Server determines local clients on the channel and sends the
1033 packet to the client.
1035 o All clients receiving the packet decrypts the packet.
1039 3.8.4 Server To Server
1041 Server to server packet delivery and encryption is described in above
1042 examples. Router to router packet delivery is analogous to server to
1043 server. However, some packets, such as channel packets, are processed
1044 differently. These cases are described later in this document and
1045 more in detail in [SILC2].
1049 3.9 Key Exchange And Authentication
1051 Key exchange is done always when for example client connects to server
1052 but also when server and router, and router and router connects to each
1053 other. The purpose of key exchange protocol is to provide secure key
1054 material to be used in the communication. The key material is used to
1055 derive various security parameters used to secure SILC packets. The
1056 SILC Key Exchange protocol is described in detail in [SILC3].
1058 Authentication is done after key exchange protocol has been successfully
1059 completed. The purpose of authentication is to authenticate for example
1060 client connecting to the server. However, usually clients are accepted
1061 to connect to server without explicit authentication. Servers are
1062 required use authentication protocol when connecting. The authentication
1063 may be based on passphrase (pre-shared-secret) or public key. All
1064 passphrases sent in SILC protocol MUST be UTF-8 [RFC2279] encoded.
1065 The connection authentication protocol is described in detail in [SILC3].
1069 3.9.1 Authentication Payload
1071 Authentication payload is used separately from the SKE and the Connection
1072 Authentication protocol. It can be used during the session to authenticate
1073 with the remote. For example, the client can authenticate itself to the
1074 server to become server operator. In this case, Authentication Payload is
1077 The format of the Authentication Payload is as follows:
1083 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
1084 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1085 | Payload Length | Authentication Method |
1086 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1087 | Public Data Length | |
1088 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1092 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1093 | Authentication Data Length | |
1094 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1096 ~ Authentication Data ~
1098 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1102 Figure 5: Authentication Payload
1106 o Payload Length (2 bytes) - Length of the entire payload.
1108 o Authentication Method (2 bytes) - The method of the
1109 authentication. The authentication methods are defined
1110 in [SILC2] in the Connection Auth Request Payload. The NONE
1111 authentication method SHOULD NOT be used.
1113 o Public Data Length (2 bytes) - Indicates the length of
1114 the Public Data field.
1116 o Public Data (variable length) - This is defined only if
1117 the authentication method is public key. If it is any other
1118 this field MAY include a random data for padding purposes.
1119 However, in this case the field MUST be ignored by the
1122 When the authentication method is public key this includes
1123 128 to 4096 bytes of non-zero random data that is used in
1124 the signature process, described subsequently.
1126 o Authentication Data Length (2 bytes) - Indicates the
1127 length of the Authentication Data field. If zero (0)
1128 value is found in this field the payload MUST be
1131 o Authentication Data (variable length) - Authentication
1132 method dependent authentication data.
1136 If the authentication method is password based, the Authentication
1137 Data field includes the plaintext UTF-8 encoded password. It is safe
1138 to send plaintext password since the entire payload is encrypted. In
1139 this case the Public Data Length is set to zero (0), but MAY also include
1140 random data for padding purposes. It is also RECOMMENDED that maximum
1141 amount of padding is applied to SILC packet when using password based
1142 authentication. This way it is not possible to approximate the length
1143 of the password from the encrypted packet.
1145 If the authentication method is public key based (or certificate)
1146 the Authentication Data is computed as follows:
1148 HASH = hash(random bytes | ID | public key (or certificate));
1149 Authentication Data = sign(HASH);
1151 The hash() and the sign() are the hash function and the public key
1152 cryptography function selected in the SKE protocol, unless otherwise
1153 stated in the context where this payload is used. The public key
1154 is SILC style public key unless certificates are used. The ID is the
1155 entity's ID (Client or Server ID) which is authenticating itself. The
1156 ID encoding is described in [SILC2]. The random bytes are non-zero
1157 random bytes of length between 128 and 4096 bytes, and will be included
1158 into the Public Data field as is.
1160 The receiver will compute the signature using the random data received
1161 in the payload, the ID associated to the connection and the public key
1162 (or certificate) received in the SKE protocol. After computing the
1163 receiver MUST verify the signature. In case of public key authentication
1164 this payload is also encrypted.
1170 This section defines all the allowed algorithms that can be used in
1171 the SILC protocol. This includes mandatory cipher, mandatory public
1172 key algorithm and MAC algorithms.
1178 Cipher is the encryption algorithm that is used to protect the data
1179 in the SILC packets. See [SILC2] of the actual encryption process and
1180 definition of how it must be done. SILC has a mandatory algorithm that
1181 must be supported in order to be compliant with this protocol.
1183 The following ciphers are defined in SILC protocol:
1186 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1187 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1188 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1189 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1190 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1191 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1192 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1193 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1194 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1195 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1196 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1197 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1198 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1199 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1200 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1201 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1202 none No encryption (OPTIONAL)
1206 Algorithm none does not perform any encryption process at all and
1207 thus is not recommended to be used. It is recommended that no client
1208 or server implementation would accept none algorithms except in special
1211 Additional ciphers MAY be defined to be used in SILC by using the
1212 same name format as above.
1216 3.10.2 Public Key Algorithms
1218 Public keys are used in SILC to authenticate entities in SILC network
1219 and to perform other tasks related to public key cryptography. The
1220 public keys are also used in the SILC Key Exchange protocol [SILC3].
1222 The following public key algorithms are defined in SILC protocol:
1229 DSS is described in [Menezes]. The RSA MUST be implemented according
1230 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1231 compliant to either PKCS #1 version 1.5 or newer with the following
1232 notes: The signature encoding is always in same format as the encryption
1233 encoding regardless of the PKCS #1 version. The signature with appendix
1234 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1235 rationale for this is that there is no binding between the PKCS #1 OIDs
1236 and the hash algorithms used in the SILC protocol. Hence, the encoding
1237 is always in PKCS #1 version 1.5 format.
1239 Additional public key algorithms MAY be defined to be used in SILC.
1241 When signatures are computed in SILC the computing of the signature is
1242 represented as sign(). The signature computing procedure is dependent
1243 of the public key algorithm, and the public key or certificate encoding.
1244 When using SILC public key the signature is computed as described in
1245 previous section for RSA and DSS keys. When using SSH2 public keys
1246 the signature is computed as described in [SSH-TRANS]. When using
1247 X.509 version 3 certificates the signature is computed as described
1248 in [PKCS7]. When using OpenPGP certificates the signature is computed
1249 as described in [PGP].
1253 3.10.3 Hash Functions
1255 Hash functions are used as part of MAC algorithms defined in the next
1256 section. They are also used in the SILC Key Exchange protocol defined
1259 The following Hash algorithm are defined in SILC protocol:
1262 sha1 SHA-1, length = 20 (REQUIRED)
1263 md5 MD5, length = 16 (OPTIONAL)
1268 3.10.4 MAC Algorithms
1270 Data integrity is protected by computing a message authentication code
1271 (MAC) of the packet data. See [SILC2] for details how to compute the
1274 The following MAC algorithms are defined in SILC protocol:
1277 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1278 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1279 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1280 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1281 none No MAC (OPTIONAL)
1284 The none MAC is not recommended to be used as the packet is not
1285 authenticated when MAC is not computed. It is recommended that no
1286 client or server would accept none MAC except in special debugging
1289 The HMAC algorithm is described in [HMAC] and hash algorithms that
1290 are used as part of the HMACs are described in [Scheneir] and in
1293 Additional MAC algorithms MAY be defined to be used in SILC.
1299 3.10.5 Compression Algorithms
1301 SILC protocol supports compression that may be applied to unencrypted
1302 data. It is recommended to use compression on slow links as it may
1303 significantly speed up the data transmission. By default, SILC does not
1304 use compression which is the mode that must be supported by all SILC
1307 The following compression algorithms are defined:
1310 none No compression (REQUIRED)
1311 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1314 Additional compression algorithms MAY be defined to be used in SILC.
1318 3.11 SILC Public Key
1320 This section defines the type and format of the SILC public key. All
1321 implementations MUST support this public key type. See [SILC3] for
1322 other optional public key and certificate types allowed in the SILC
1323 protocol. Public keys in SILC may be used to authenticate entities
1324 and to perform other tasks related to public key cryptography.
1326 The format of the SILC Public Key is as follows:
1332 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
1333 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1334 | Public Key Length |
1335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1336 | Algorithm Name Length | |
1337 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1341 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1342 | Identifier Length | |
1343 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1347 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1351 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1355 Figure 5: SILC Public Key
1359 o Public Key Length (4 bytes) - Indicates the full length
1360 of the public key, not including this field.
1362 o Algorithm Name Length (2 bytes) - Indicates the length
1363 of the Algorithm Length field, not including this field.
1365 o Algorithm name (variable length) - Indicates the name
1366 of the public key algorithm that the key is. See the
1367 section 3.10.2 Public Key Algorithms for defined names.
1369 o Identifier Length (2 bytes) - Indicates the length of
1370 the Identifier field, not including this field.
1372 o Identifier (variable length) - Indicates the identifier
1373 of the public key. This data can be used to identify
1374 the owner of the key. The identifier is of the following
1378 HN Host name or IP address
1385 Examples of an identifier:
1387 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1389 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1391 At least user name (UN) and host name (HN) MUST be provided as
1392 identifier. The fields are separated by commas (`,'). If
1393 comma is in the identifier string it must be written as `\\,',
1394 for example, `O=Company XYZ\\, Inc.'.
1396 o Public Data (variable length) - Includes the actual
1397 public data of the public key.
1399 The format of this field for RSA algorithm is
1408 The format of this field for DSS algorithm is
1420 The variable length fields are multiple precession
1421 integers encoded as strings in both examples.
1423 Other algorithms must define their own type of this
1424 field if they are used.
1427 All fields in the public key are in MSB (most significant byte first)
1428 order. All strings in the public key are UTF-8 encoded.
1430 If an external protocol need to refer to SILC Public Key by name, the
1431 name "silc-rsa" and "silc-dss" for SILC Public Key based on RSA algorithm
1432 and SILC Public Key based on DSS algorithm, respectively, are to be used.
1433 However, this SILC specification does not use these names directly, and
1434 they are defined here for external protocols (protocols that may like
1435 to use SILC Public Key).
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 original 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 originated 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 resume
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 sent 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 must 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_RESUMED_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 arising 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.
1848 Channels' mode and founder public key and other channel mode specific
1849 data is announced by sending SILC_NOTIFY_TYPE_CMODE_CHANGE notify list.
1850 Also, the channel users on the channels must be announced by compiling a
1851 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1852 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1853 also be announced by compiling list of Notify Payloads with the
1854 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1857 The router MUST also announce the local servers by compiling list of
1858 ID Payloads into the SILC_PACKET_NEW_ID packet.
1860 Also, clients' modes (user modes in SILC) MUST be announced. This is
1861 done by compiling a list of Notify Payloads with the
1862 SILC_NOTIFY_UMODE_CHANGE nofity type into the SILC_PACKET_NOTIFY packet.
1864 Also, channel's topics MUST be announced by compiling a list of Notify
1865 Payloads with the SILC_NOTIFY_TOPIC_SET notify type into the
1866 SILC_PACKET_NOTIFY packet.
1868 The router which receives these lists MUST process them and broadcast
1869 the packets to its primary route.
1871 When processing the announced channels and channel users the router MUST
1872 check whether a channel exists already with the same name. If channel
1873 exists with the same name it MUST check whether the Channel ID is
1874 different. If the Channel ID is different the router MUST send the notify
1875 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1876 change to the ID the router has. If the mode of the channel is different
1877 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1878 server to force the mode change to the mode that the router has.
1880 The router MUST also generate new channel key and distribute it to the
1881 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1884 If the channel has channel founder on the router the router MUST send
1885 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1886 the mode change for the channel founder on the server. The channel
1887 founder privileges MUST be removed.
1889 The router processing the channels MUST also compile a list of
1890 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1891 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1892 server (or router) will receive the clients on the channel that
1897 4.3 Joining to a Channel
1899 This section describes the procedure when client joins to a channel.
1900 Client joins to channel by sending command SILC_COMMAND_JOIN to the
1901 server. If the receiver receiving join command is normal server the
1902 server MUST check its local list whether this channel already exists
1903 locally. This would indicate that some client connected to the server
1904 has already joined to the channel. If this is case the client is
1905 joined to the channel, new channel key is created and information about
1906 newly joined channel is sent to the router. The router is informed
1907 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1908 also be sent to the local clients on the channel. The new channel key
1909 is also sent to the router and to local clients on the channel.
1911 If the channel does not exist in the local list the client's command
1912 MUST be sent to the router which will then perform the actual joining
1913 procedure. When server receives the reply to the command from the
1914 router it MUST be sent to the client which sent the command originally.
1915 Server will also receive the channel key from the server that it MUST
1916 send to the client which originally requested the join command. The
1917 server MUST also save the channel key.
1919 If the receiver of the join command is router it MUST first check its
1920 local list whether anyone in the cell has already joined to the channel.
1921 If this is the case the client is joined to the channel and reply is
1922 sent to the client. If the command was sent by server the command reply
1923 is sent to the server which sent it. Then the router MUST also create
1924 new channel key and distribute it to all clients on the channel and
1925 all servers that has clients on the channel. Router MUST also send
1926 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1927 and to local servers that has clients on the channel.
1929 If the channel does not exist on the router's local list it MUST
1930 check the global list whether the channel exists at all. If it does
1931 the client is joined to the channel as described previously. If
1932 the channel does not exist the channel is created and the client
1933 is joined to the channel. The channel key is also created and
1934 distributed as previously described. The client joining to the created
1935 channel is made automatically channel founder and both channel founder
1936 and channel operator privileges is set for the client.
1938 If the router created the channel in the process, information about the
1939 new channel MUST be broadcasted to all routers. This is done by
1940 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1941 route. When the router joins the client to the channel it MUST also
1942 send information about newly joined client to all routers in the SILC
1943 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1944 type to the router's primary route.
1946 It is important to note that new channel key is created always when
1947 new client joins to channel, whether the channel has existed previously
1948 or not. This way the new client on the channel is not able to decrypt
1949 any of the old traffic on the channel. Client which receives the reply to
1950 the join command MUST start using the received Channel ID in the channel
1951 message communication thereafter. Client also receives the key for the
1952 channel in the command reply. Note that the channel key is never
1953 generated if the SILC_CMODE_PRIVKEY mode is set.
1957 4.4 Channel Key Generation
1959 Channel keys are created by router which creates the channel by taking
1960 enough randomness from cryptographically strong random number generator.
1961 The key is generated always when channel is created, when new client
1962 joins a channel and after the key has expired. Key could expire for
1965 The key MUST also be re-generated whenever some client leaves a channel.
1966 In this case the key is created from scratch by taking enough randomness
1967 from the random number generator. After that the key is distributed to
1968 all clients on the channel. However, channel keys are cell specific thus
1969 the key is created only on the cell where the client, which left the
1970 channel, exists. While the server or router is creating the new channel
1971 key, no other client may join to the channel. Messages that are sent
1972 while creating the new key are still processed with the old key. After
1973 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1974 using the new key. If server creates the new key the server MUST also
1975 send the new key to its router. See [SILC2] on more information about
1976 how channel messages must be encrypted and decrypted when router is
1979 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1980 Channel Key Payload it MUST process the key data to create encryption
1981 and decryption key, and to create the HMAC key that is used to compute
1982 the MACs of the channel messages. The processing is as follows:
1984 channel_key = raw key data
1985 HMAC key = hash(raw key data)
1987 The raw key data is the key data received in the Channel Key Payload.
1988 The hash() function is the hash function used in the HMAC of the channel.
1989 Note that the server MUST also save the channel key.
1993 4.5 Private Message Sending and Reception
1995 Private messages are sent point to point. Client explicitly destines
1996 a private message to specific client that is delivered to only to that
1997 client. No other client may receive the private message. The receiver
1998 of the private message is destined in the SILC Packet Header as any
1999 other packet as well.
2001 If the sender of a private message does not know the receiver's Client
2002 ID, it MUST resolve it from server. There are two ways to resolve the
2003 client ID from server; it is RECOMMENDED that client implementations
2004 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
2005 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
2006 If the sender has received earlier a private message from the receiver
2007 it should have cached the Client ID from the SILC Packet Header.
2009 If server receives a private message packet which includes invalid
2010 destination Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2011 notify to the client with error status indicating that such Client ID
2014 See [SILC2] for description of private message encryption and decryption
2019 4.6 Private Message Key Generation
2021 Private message MAY be protected by the key generated by the client.
2022 The key may be generated and sent to the other client by sending packet
2023 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
2024 and is secured by session keys. After that the private message key
2025 is used in the private message communication between those clients.
2027 Other choice is to entirely use keys that are not sent through
2028 the SILC network at all. This significantly adds security. This key
2029 would be pre-shared-key that is known by both of the clients. Both
2030 agree about using the key and starts sending packets that indicate
2031 that the private message is secured using private message key.
2033 The key material used as private message key is implementation issue.
2034 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
2035 the key material. If the key is normal pre-shared-key or randomly
2036 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
2037 the key material SHOULD be processed as defined in the [SILC3]. In
2038 the processing, however, the HASH, as defined in [SILC3] MUST be
2039 ignored. After processing the key material it is employed as defined
2040 in [SILC3], however, the HMAC key material MUST be discarded.
2042 If the key is pre-shared-key or randomly generated the implementations
2043 SHOULD use the SILC protocol's mandatory cipher as the cipher. If the
2044 SKE was used to negotiate key material the cipher was negotiated as well,
2045 and may be different from default cipher.
2049 4.7 Channel Message Sending and Reception
2051 Channel messages are delivered to group of users. The group forms a
2052 channel and all clients on the channel receives messages sent to the
2055 Channel messages are destined to channel by specifying the Channel ID
2056 as Destination ID in the SILC Packet Header. The server MUST then
2057 distribute the message to all clients on the channel by sending the
2058 channel message destined explicitly to a client on the channel.
2060 If server receives a channel message packet which includes invalid
2061 destination Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2062 notify to the sender with error status indicating that such Channel ID
2065 See the [SILC2] for description of channel message routing for router
2066 servers, and channel message encryption and decryption process.
2070 4.8 Session Key Regeneration
2072 Session keys MUST be regenerated periodically, say, once in an hour.
2073 The re-key process is started by sending SILC_PACKET_REKEY packet to
2074 other end, to indicate that re-key must be performed. The initiator
2075 of the connection SHOULD initiate the re-key.
2077 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2078 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2079 SKE protocol. In this case the protocol is secured with the old key
2080 and the protocol results to new key material. See [SILC3] for more
2081 information. After the SILC_PACKET_REKEY packet is sent the sender
2082 will perform the SKE protocol.
2084 If PFS flag was set the resulted key material is processed as described
2085 in the section Processing the Key Material in [SILC3]. The difference
2086 with re-key in the processing is that the initial data for the hash
2087 function is just the resulted key material and not the HASH as it
2088 is not computed at all with re-key. Other than that, the key processing
2089 it equivalent to normal SKE negotiation.
2091 If PFS flag was not set, which is the default case, then re-key is done
2092 without executing SKE protocol. In this case, the new key is created by
2093 providing the current sending encryption key to the SKE protocol's key
2094 processing function. The process is described in the section Processing
2095 the Key Material in [SILC3]. The difference in the processing is that
2096 the initial data for the hash function is the current sending encryption
2097 key and not the SKE's KEY and HASH values. Other than that, the key
2098 processing is equivalent to normal SKE negotiation.
2100 After both parties has regenerated the session key, both MUST send
2101 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2102 secured with the old key. After these packets, the subsequent packets
2103 MUST be protected with the new key.
2107 4.9 Command Sending and Reception
2109 Client usually sends the commands in the SILC network. In this case
2110 the client simply sends the command packet to server and the server
2111 processes it and replies with command reply packet. See the [SILC3]
2112 for detailed description of all commands.
2114 However, if the server is not able to process the command, it is sent
2115 to the server's router. This is case for example with commands such
2116 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2117 are other commands as well. For example, if client sends the WHOIS
2118 command requesting specific information about some client the server must
2119 send the WHOIS command to router so that all clients in SILC network
2120 are searched. The router, on the other hand, sends the WHOIS command
2121 further to receive the exact information about the requested client.
2122 The WHOIS command travels all the way to the server which owns the client
2123 and it replies with command reply packet. Finally, the server which
2124 sent the command receives the command reply and it must be able to
2125 determine which client sent the original command. The server then
2126 sends command reply to the client. Implementations should have some
2127 kind of cache to handle, for example, WHOIS information. Servers
2128 and routers along the route could all cache the information for faster
2129 referencing in the future.
2131 The commands sent by server may be sent hop by hop until someone is able
2132 to process the command. However, it is preferred to destine the command
2133 as precisely as it is possible. In this case, other routers en route
2134 MUST route the command packet by checking the true sender and true
2135 destination of the packet. However, servers and routers MUST NOT route
2136 command reply packets to clients coming from other server. Client
2137 MUST NOT accept command reply packet originated from anyone else but
2138 from its own server.
2142 4.10 Closing Connection
2144 When remote client connection is closed the server MUST send the notify
2145 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2146 the client was joined. The server MUST also save the client's information
2147 for a period of time for history purposes.
2149 When remote server or router connection is closed the server or router
2150 MUST also remove all the clients that was behind the server or router
2151 from the SILC Network. The server or router MUST also send the notify
2152 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2153 local clients that are joined on the same channels with the remote
2154 server's or router's clients.
2156 Router server MUST also check whether some client in the local cell
2157 is watching for the nickname this client has, and send the
2158 SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
2159 the network has the SILC_UMODE_REJECT_WATCHING user mode set.
2163 4.11 Detaching and Resuming a Session
2165 SILC protocol provides a possibility for a client to detach itself from
2166 the network without actually signing off from the network. The client
2167 connection to the server is closed but the client remains as valid client
2168 in the network. The client may then later resume its session back from
2169 any server in the network.
2171 When client wishes to detach from the network it MUST send the
2172 SILC_COMMAND_DETACH command to its server. The server then MUST set
2173 SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
2174 notify to its primary router, which will then MUST broadcast it further
2175 to other routers in the network. This user mode indicates that the
2176 client is detached from the network. Implementations MUST NOT use
2177 the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
2178 to the client. All packets MUST still be sent to the client even if
2179 client is detached from the network. Only the server that originally
2180 had the active client connection is able to make the decision after it
2181 notices that the network connection is not active. In this case the
2182 default case is to discard the packet.
2184 The SILC_UMODE_DETACHED flag cannot be set by client itself directly
2185 with SILC_COMMAND_UMODE command, but only implicitly by sending the
2186 SILC_COMMAND_DETACH command. The flag also cannot be unset by the
2187 client, server or router with SILC_COMMAND_UMODE command, but only
2188 implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
2191 When the client wishes to resume its session in the SILC Network it
2192 connects to a server in the network, which MAY also be a different
2193 from the original server, and performs normal procedures regarding
2194 creating a connection as described in section 4.1. After the SKE
2195 and the Connection Authentication protocols has been successfully
2196 completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
2197 MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
2198 perform the resuming procedure. The packet MUST include the detached
2199 client's Client ID, which the client must know. It also includes
2200 Authentication Payload which includes signature made with the client's
2201 private key. The signature is computed as defined in the section
2202 3.9.1. Thus, the authentication method MUST be based in public key
2205 When server receives the SILC_PACKET_RESUME_CLIENT packet it MUST
2206 do the following: Server checks that the Client ID is valid client
2207 and that it has the SILC_UMODE_DETACHED mode set. Then it verifies
2208 the Authentication Payload with the detached client's public key.
2209 If it does not have the public key it retrieves it by sending
2210 SILC_COMMAND_GETKEY command to the server that has the public key from
2211 the original client connection. The server MUST NOT use the public
2212 key received in the SKE protocol for this connection. If the
2213 signature is valid the server unsets the SILC_UMODE_DETACHED flag,
2214 and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
2215 The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
2216 flag when the packet is received. If the server is router server it
2217 also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
2218 server whom owned the detached client.
2220 The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
2221 packet MUST know whether the packet already has been received for
2222 the client. It is protocol error to attempt to resume the client
2223 session from more than one server. The implementations could set
2224 internal flag that indicates that the client is resumed. If router
2225 receive SILC_PACKET_RESUME_CLIENT packet for client that is already
2226 resumed the client MUST be killed from the network. This would
2227 indicate that the client is attempting to resume the session more
2228 than once which is protocol error. In this case the router sends
2229 SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
2230 the same situation MUST also send the notify for the client.
2232 The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
2233 must also understand that the client may not be found behind the
2234 same server that it originally came from. They must update their
2235 caches according this. The server that now owns the client session
2236 MUST check whether the Client ID of the resumed client is based
2237 on the server's Server ID. If it is not it creates a new Client
2238 ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
2239 also send the channel keys of all channels that the client is
2240 joined to the client since it does not have them. Whether the
2241 Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
2242 packet to the client. Only after this the client is resumed back
2243 to the network and may start sending packets and messages.
2245 It is also possible that the server does not know about the channels
2246 that the client has joined. In this case it join the client internally
2247 to the channels, generate new channel keys and distribute the keys
2248 to the channels as described in section 4.4.
2250 It is implementation issue for how long servers keep detached client
2251 sessions. It is RECOMMENDED that the detached sessions would be
2252 persistent as long as the server is running.
2256 5 Security Considerations
2258 Security is central to the design of this protocol, and these security
2259 considerations permeate the specification. Common security considerations
2260 such as keeping private keys truly private and using adequate lengths for
2261 symmetric and asymmetric keys must be followed in order to maintain the
2262 security of this protocol.
2264 Special attention must also be paid on the servers and routers that are
2265 running the SILC service. The SILC protocol's security depends greatly
2266 on the security and the integrity of the servers and administrators that
2267 are running the service. It is recommended that some form of registration
2268 is required by the server and router administrator prior acceptance to
2269 the SILC Network. Even though, the SILC protocol is secure in a network
2270 of mutual distrust between clients, servers, routers and administrators
2271 of the servers, the client should be able to trust the servers they are
2272 using if they wish to do so.
2274 It however must be noted that if the client requires absolute security
2275 by not trusting any of the servers or routers in the SILC Network, it can
2276 be accomplished by negotiating private keys outside the SILC Network,
2277 either using SKE or some other key exchange protocol, or to use some
2278 other external means for distributing the keys. This applies for all
2279 messages, private messages and channel messages.
2281 It is important to note that SILC, like any other security protocol is
2282 not full proof system; the SILC servers and routers could very well be
2283 compromised. However, to provide acceptable level of security and
2284 usability for end user the protocol use many times session keys or other
2285 keys generated by the servers to secure the messages. This is
2286 intentional design feature to allow ease of use for end user. This way
2287 the network is still usable, and remains encrypted even if the external
2288 means of distributing the keys is not working. The implementation,
2289 however, may like to not follow this design feature, and always negotiate
2290 the keys outside SILC network. This is acceptable solution and many times
2291 recommended. The implementation still must be able to work with the
2292 server generated keys.
2294 If this is unacceptable for the client or end user, the private keys
2295 negotiated outside the SILC Network should always be used. In the end
2296 it is always implementor's choice whether to negotiate private keys by
2297 default or whether to use the keys generated by the servers.
2299 It is also recommended that router operators in the SILC Network would
2300 form a joint forum to discuss the router and SILC Network management
2301 issues. Also, router operators along with the cell's server operators
2302 should have a forum to discuss the cell management issues.
2308 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2311 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2312 Protocols", Internet Draft, May 2002.
2314 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, May 2002.
2316 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2319 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2322 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2325 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2328 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2331 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2334 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2337 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2340 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2341 Infrastructure, Certificate and CRL Profile", RFC 2459,
2344 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2345 John Wiley & Sons, New York, NY, 1996.
2347 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2350 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2351 RFC 2412, November 1998.
2353 [ISAKMP] Maughan D., et al, "Internet Security Association and
2354 Key Management Protocol (ISAKMP)", RFC 2408, November
2357 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2358 (IKE)", RFC 2409, November 1998.
2360 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2361 Authentication", RFC 2104, February 1997.
2363 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2364 Specifications, Version 2.0", RFC 2437, October 1998.
2366 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2367 Requirement Levels", BCP 14, RFC 2119, March 1997.
2369 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
2370 10646", RFC 2279, January 1998.
2372 [PKCS7] Kalinski, B., "PKCS #7: Cryptographic Message Syntax,
2373 Version 1.5", RFC 2315, March 1998.
2381 Snellmaninkatu 34 A 15
2385 EMail: priikone@iki.fi
2387 This Internet-Draft expires XXX