8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-spec-05.txt XXX
25 Secure Internet Live Conferencing (SILC),
26 Protocol Specification
27 <draft-riikonen-silc-spec-05.txt>
32 This document is an Internet-Draft and is in full conformance with
33 all provisions of Section 10 of RFC 2026. Internet-Drafts are
34 working documents of the Internet Engineering Task Force (IETF), its
35 areas, and its working groups. Note that other groups may also
36 distribute working documents as Internet-Drafts.
38 Internet-Drafts are draft documents valid for a maximum of six months
39 and may be updated, replaced, or obsoleted by other documents at any
40 time. It is inappropriate to use Internet-Drafts as reference
41 material or to cite them other than as "work in progress."
43 The list of current Internet-Drafts can be accessed at
44 http://www.ietf.org/ietf/1id-abstracts.txt
46 The list of Internet-Draft Shadow Directories can be accessed at
47 http://www.ietf.org/shadow.html
49 The distribution of this memo is unlimited.
55 This memo describes a Secure Internet Live Conferencing (SILC)
56 protocol which provides secure conferencing services over insecure
57 network channel. SILC is IRC [IRC] like protocol, however, it is
58 not equivalent to IRC and does not support IRC. Strong cryptographic
59 methods are used to protect SILC packets inside the SILC network.
60 Three other Internet Drafts relates very closely to this memo;
61 SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication
62 Protocols [SILC3] and SILC Commands [SILC4].
73 1 Introduction .................................................. 3
74 1.1 Requirements Terminology .................................. 4
75 2 SILC Concepts ................................................. 4
76 2.1 SILC Network Topology ..................................... 4
77 2.2 Communication Inside a Cell ............................... 5
78 2.3 Communication in the Network .............................. 6
79 2.4 Channel Communication ..................................... 7
80 2.5 Router Connections ........................................ 7
81 3 SILC Specification ............................................ 8
82 3.1 Client .................................................... 8
83 3.1.1 Client ID ........................................... 9
84 3.2 Server .................................................... 10
85 3.2.1 Server's Local ID List .............................. 10
86 3.2.2 Server ID ........................................... 11
87 3.2.3 SILC Server Ports ................................... 12
88 3.3 Router .................................................... 12
89 3.3.1 Router's Local ID List .............................. 12
90 3.3.2 Router's Global ID List ............................. 13
91 3.3.3 Router's Server ID .................................. 14
92 3.4 Channels .................................................. 14
93 3.4.1 Channel ID .......................................... 16
94 3.5 Operators ................................................. 16
95 3.6 SILC Commands ............................................. 16
96 3.7 SILC Packets .............................................. 17
97 3.8 Packet Encryption ......................................... 17
98 3.8.1 Determination of the Source and the Destination ..... 17
99 3.8.2 Client To Client .................................... 18
100 3.8.3 Client To Channel ................................... 19
101 3.8.4 Server To Server .................................... 20
102 3.9 Key Exchange And Authentication ........................... 20
103 3.9.1 Authentication Payload .............................. 20
104 3.10 Algorithms ............................................... 22
105 3.10.1 Ciphers ............................................ 22
106 3.10.2 Public Key Algorithms .............................. 23
107 3.10.3 Hash Functions ..................................... 24
108 3.10.4 MAC Algorithms ..................................... 24
109 3.10.5 Compression Algorithms ............................. 25
110 3.11 SILC Public Key .......................................... 25
111 3.12 SILC Version Detection ................................... 27
112 3.13 Backup Routers ........................................... 28
113 3.13.1 Switching to Backup Router ......................... 29
114 3.13.2 Resuming Primary Router ............................ 30
115 3.13.3 Discussion on Backup Router Scheme ................. 32
116 4 SILC Procedures ............................................... 33
117 4.1 Creating Client Connection ................................ 33
118 4.2 Creating Server Connection ................................ 34
119 4.2.1 Announcing Clients, Channels and Servers ............ 35
120 4.3 Joining to a Channel ...................................... 36
121 4.4 Channel Key Generation .................................... 37
122 4.5 Private Message Sending and Reception ..................... 38
123 4.6 Private Message Key Generation ............................ 38
124 4.7 Channel Message Sending and Reception ..................... 39
125 4.8 Session Key Regeneration .................................. 39
126 4.9 Command Sending and Reception ............................. 40
127 4.10 Closing Connection ....................................... 41
128 4.11 Detaching and Resuming a Session ......................... XXXXX
129 5 Security Considerations ....................................... 41
130 6 References .................................................... 42
131 7 Author's Address .............................................. 44
139 Figure 1: SILC Network Topology
140 Figure 2: Communication Inside cell
141 Figure 3: Communication Between Cells
142 Figure 4: Router Connections
143 Figure 5: SILC Public Key
149 This document describes a Secure Internet Live Conferencing (SILC)
150 protocol which provides secure conferencing services over insecure
151 network channel. SILC is IRC [IRC] like protocol, however, it is
152 not equivalent to IRC and does not support IRC.
154 Strong cryptographic methods are used to protect SILC packets inside
155 the SILC network. Three other Internet Drafts relates very closely
156 to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and
157 Authentication Protocols [SILC3] and SILC Commands [SILC4].
159 The protocol uses extensively packets as conferencing protocol
160 requires message and command sending. The SILC Packet Protocol is
161 described in [SILC2] and should be read to fully comprehend this
162 document and protocol. [SILC2] also describes the packet encryption
163 and decryption in detail.
165 The security of SILC protocol, and for any security protocol for that
166 matter, is based on strong and secure key exchange protocol. The SILC
167 Key Exchange protocol is described in [SILC3] along with connection
168 authentication protocol and should be read to fully comprehend this
169 document and protocol.
171 The SILC protocol has been developed to work on TCP/IP network
172 protocol, although it could be made to work on other network protocols
173 with only minor changes. However, it is recommended that TCP/IP
174 protocol is used under SILC protocol. Typical implementation would
175 be made in client-server model.
179 1.1 Requirements Terminology
181 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
182 MAY, and OPTIONAL, when they appear in this document, are to be
183 interpreted as described in [RFC2119].
189 This section describes various SILC protocol concepts that forms the
190 actual protocol, and in the end, the actual SILC network. The mission
191 of the protocol is to deliver messages from clients to other clients
192 through routers and servers in secure manner. The messages may also
193 be delivered from one client to many clients forming a group, also
196 This section does not focus to security issues. Instead, basic network
197 concepts are introduced to make the topology of the SILC network
202 2.1 SILC Network Topology
204 SILC network is a cellular network as opposed to tree style network
205 topology. The rationale for this is to have servers that can perform
206 specific kind of tasks what other servers cannot perform. This leads
207 to two kinds of servers; normal SILC servers and SILC routers.
209 A difference between normal server and router server is that routers
210 knows everything about everything in the network. They also do the
211 actual routing of the messages to the correct receiver. Normal servers
212 knows only about local information and nothing about global information.
213 This makes the network faster as there are less servers that needs to
214 keep global information up to date at all time.
216 This, on the other hand, leads to cellular like network, where routers
217 are in the center of the cell and servers are connected to the router.
225 The following diagram represents SILC network topology.
229 ---- ---- ---- ---- ---- ----
230 | S8 | S5 | S4 | | S7 | S5 | S6 |
231 ----- ---- ----- ----- ---- -----
232 | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 |
233 ---- ------ ---- ---- ------ ----
234 | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ----
235 ---- ---- ---- ---- ---- ---- | S3 | S1 |
236 Cell 1. \\ Cell 2. | \\____ ----- -----
238 ---- ---- ---- ---- ---- ---- ---- ------
239 | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 |
240 ----- ---- ----- ----- ---- ----- ---- ----
241 | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4.
242 ---- ------ ---- ---- ------ ----
243 | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ...
244 ---- ---- ---- ---- ---- ----
249 Figure 1: SILC Network Topology
252 A cell is formed when a server or servers connect to one router. In
253 SILC network normal server cannot directly connect to other normal
254 server. Normal server may only connect to SILC router which then
255 routes the messages to the other servers in the cell. Router servers
256 on the other hand may connect to other routers to form the actual SILC
257 network, as seen in above figure. However, router is also normal SILC
258 server; clients may connect to it the same way as to normal SILC
259 server. Normal server also cannot have active connections to more
260 than one router. Normal server cannot be connected to two different
261 cells. Router servers, on the other hand, may have as many router to
262 router connections as needed.
264 There are many issues in this network topology that needs to be careful
265 about. Issues like the size of the cells, the number of the routers in
266 the SILC network and the capacity requirements of the routers. These
267 issues should be discussed in the Internet Community and additional
268 documents on the issue may be written.
272 2.2 Communication Inside a Cell
274 It is always guaranteed that inside a cell message is delivered to the
275 recipient with at most two server hops. A client which is connected to
276 server in the cell and is talking on channel to other client connected
277 to other server in the same cell, will have its messages delivered from
278 its local server first to the router of the cell, and from the router
279 to the other server in the cell.
281 The following diagram represents this scenario:
295 Figure 2: Communication Inside cell
298 Example: Client 1. connected to Server 1. send message to
299 Client 4. connected to Server 2. travels from Server 1.
300 first to Router which routes the message to Server 2.
301 which then sends it to the Client 4. All the other
302 servers in the cell will not see the routed message.
305 If the client is connected directly to the router, as router is also normal
306 SILC server, the messages inside the cell are always delivered only with
307 one server hop. If clients communicating with each other are connected
308 to the same server, no router interaction is needed. This is the optimal
309 situation of message delivery in the SILC network.
313 2.3 Communication in the Network
315 If the message is destined to server that does not belong to local cell
316 the message is routed to the router server to which the destination
317 server belongs, if the local router is connected to destination router.
318 If there is no direct connection to the destination router, the local
319 router routes the message to its primary route. The following diagram
320 represents message sending between cells.
325 1 --- S1 S4 --- 5 S2 --- 1
326 S/R - - - - - - - - S/R
336 Figure 3: Communication Between Cells
339 Example: Client 5. connected to Server 4. in Cell 1. sends message
340 to Client 2. connected to Server 1. in Cell 2. travels
341 from Server 4. to Router which routes the message to
342 Router in Cell 2, which then routes the message to
343 Server 1. All the other servers and routers in the
344 network will not see the routed message.
347 The optimal case of message delivery from the client point of view is
348 when clients are connected directly to the routers and the messages
349 are delivered from one router to the other.
353 2.4 Channel Communication
355 Messages may be sent to group of clients as well. Sending messages to
356 many clients works the same way as sending messages point to point, from
357 message delivery point of view. Security issues are another matter
358 which are not discussed in this section.
360 Router server handles the message routing to multiple recipients. If
361 any recipient is not in the same cell as the sender the messages are
364 Server distributes the channel message to its local clients which are
365 joined to the channel. Router also distributes the message to its
366 local clients on the channel.
370 2.5 Router Connections
372 Router connections play very important role in making the SILC like
373 network topology to work. For example, sending broadcast packets in
374 SILC network require special connections between routers; routers must
375 be connected in a specific way.
377 Every router has their primary route which is a connection to another
378 router in the network. Unless there is only two routers in the network
379 must not routers use each other as their primary routes. The router
380 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.
739 Channel names are unique although the real uniqueness comes from 64 bit
740 Channel ID. However, channel names are still unique and no two global
741 channels with same name may exist. The Channel name is a string of
742 maximum length of 256 bytes. Channel names MUST NOT contain any
743 spaces (` '), any non-printable ASCII characters, commas (`,') and
746 Channels can have operators that can administrate the channel and
747 operate all of its modes. The following operators on channel exist on
751 o Channel founder - When channel is created the joining client becomes
752 channel founder. Channel founder is channel operator with some more
753 privileges. Basically, channel founder can fully operate the channel
754 and all of its modes. The privileges are limited only to the
755 particular channel. There can be only one channel founder per
756 channel. Channel founder supersedes channel operator's privileges.
758 Channel founder privileges cannot be removed by any other operator on
759 channel. When channel founder leaves the channel there is no channel
760 founder on the channel. However, it is possible to set a mode for
761 the channel which allows the original channel founder to regain the
762 founder privileges even after leaving the channel. Channel founder
763 also cannot be removed by force from the channel.
765 o Channel operator - When client joins to channel that has not existed
766 previously it will become automatically channel operator (and channel
767 founder discussed above). Channel operator is able administrate the
768 channel, set some modes on channel, remove a badly behaving client
769 from the channel and promote other clients to become channel
770 operator. The privileges are limited only to the particular channel.
772 Normal channel user may be promoted (opped) to channel operator
773 gaining channel operator privileges. Channel founder or other
774 channel operator may also demote (deop) channel operator to normal
782 Channels are distinguished from other channels by unique Channel ID.
783 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
784 collisions are not expected to happen in any conditions. Channel names
785 are just for logical use of channels. The Channel ID is created by the
786 server where the channel is created. The Channel ID is defined as
790 64 bit Channel ID based on IPv4 addresses:
792 32 bit Router's Server ID IP address (bits 1-32)
793 16 bit Router's Server ID port (bits 33-48)
796 160 bit Channel ID based on IPv6 addresses:
798 128 bit Router's Server ID IP address (bits 1-128)
799 16 bit Router's Server ID port (bits 129-144)
802 o Router's Server ID IP address - Indicates the IP address of
803 the router of the cell where this channel is created. This is
804 taken from the router's Server ID. This way SILC router knows
805 where this channel resides in the SILC network.
807 o Router's Server ID port - Indicates the port of the channel on
808 the server. This is taken from the router's Server ID.
810 o Random number - To further randomize the Channel ID. This makes
811 sure that there are no collisions. This also means that
812 in a cell there can be 2^16 channels.
819 Operators are normal users with extra privileges to their server or
820 router. Usually these people are SILC server and router administrators
821 that take care of their own server and clients on them. The purpose of
822 operators is to administrate the SILC server or router. However, even
823 an operator with highest privileges is not able to enter invite-only
824 channel, to gain access to the contents of a encrypted and authenticated
825 packets traveling in the SILC network or to gain channel operator
826 privileges on public channels without being promoted. They have the
827 same privileges as everyone else except they are able to administrate
828 their server or router.
834 Commands are very important part on SILC network especially for client
835 which uses commands to operate on the SILC network. Commands are used
836 to set nickname, join to channel, change modes and many other things.
838 Client usually sends the commands and server replies by sending a reply
839 packet to the command. Server MAY also send commands usually to serve
840 the original client's request. Usually server cannot send commands to
841 clients, however there MAY be commands that allow the server to send
842 commands to client. By default servers MAY send commands only to other
845 Note that the command reply is usually sent only after client has sent
846 the command request but server is allowed to send command reply packet
847 to client even if client has not requested the command. Client MAY
848 choose to ignore the command reply.
850 It is expected that some of the commands may be miss-used by clients
851 resulting various problems on the server side. Every implementation
852 SHOULD assure that commands may not be executed more than once, say,
853 in two (2) seconds. However, to keep response rate up, allowing for
854 example five (5) commands before limiting is allowed. It is RECOMMENDED
855 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
856 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
857 as they require heavy operations. This should be sufficient to prevent
858 the miss-use of commands.
860 SILC commands are described in [SILC4].
866 Packets are naturally the most important part of the protocol and the
867 packets are what actually makes the protocol. Packets in SILC network
868 are always encrypted using, usually the shared secret session key
869 or some other key, for example, channel key, when encrypting channel
870 messages. It is not possible to send packet in SILC network without
871 encryption. The SILC Packet Protocol is a wide protocol and is described
872 in [SILC2]. This document does not define or describe details of
877 3.8 Packet Encryption
879 All packets passed in SILC network MUST be encrypted. This section
880 defines how packets must be encrypted in the SILC network. The detailed
881 description of the actual encryption process of the packets are
882 described in [SILC2].
884 Client and its server shares secret symmetric session key which is
885 established by the SILC Key Exchange Protocol, described in [SILC3].
886 Every packet sent from client to server, with exception of packets for
887 channels, are encrypted with this session key.
889 Channels has a channel key that are shared by every client on the channel.
890 However, the channel keys are cell specific thus one cell does not know
891 the channel key of the other cell, even if that key is for same channel.
892 Channel key is also known by the routers and all servers that has clients
893 on the channel. However, channels MAY have channel private keys that
894 are entirely local setting for the client. All clients on the channel
895 MUST know the channel private key before hand to be able to talk on the
896 channel. In this case, no server or router know the key for channel.
898 Server shares secret symmetric session key with router which is
899 established by the SILC Key Exchange Protocol. Every packet passed from
900 server to router, with exception of packets for channels, are encrypted
901 with the shared session key. Same way, router server shares secret
902 symmetric key with its primary route. However, every packet passed
903 from router to other router, including packets for channels, are
904 encrypted with the shared session key. Every router connection has
905 their own session keys.
909 3.8.1 Determination of the Source and the Destination
911 The source and the destination of the packet needs to be determined
912 to be able to route the packets to correct receiver. This information
913 is available in the SILC Packet Header which is included in all packets
914 sent in SILC network. The SILC Packet Header is described in [SILC2].
916 The header MUST be encrypted with the session key who is next receiver
917 of the packet along the route. The receiver of the packet, for example
918 a router along the route, is able to determine the sender and the
919 destination of the packet by decrypting the SILC Packet Header and
920 checking the ID's attached to the header. The ID's in the header will
921 tell to where the packet needs to be sent and where it is coming from.
923 The header in the packet MUST NOT change during the routing of the
924 packet. The original sender, for example client, assembles the packet
925 and the packet header and server or router between the sender and the
926 receiver MUST NOT change the packet header. Note however, that some
927 packets such as commands may resent by a server to serve the client's
928 original command. In this case the command packet send by the server
929 includes the server's IDs.
931 Note that the packet and the packet header may be encrypted with
932 different keys. For example, packets to channels are encrypted with
933 the channel key, however, the header is encrypted with the session key
934 as described above. However, the header and the packet may be encrypted
935 with same key. This is the case, for example, with command packets.
939 3.8.2 Client To Client
941 The process of message delivery and encryption from client to another
942 client is as follows.
944 Example: Private message from client to another client on different
945 servers. Clients do not share private message delivery
946 keys; normal session keys are used.
948 o Client 1. sends encrypted packet to its server. The packet is
949 encrypted with the session key shared between client and its
952 o Server determines the destination of the packet and decrypts
953 the packet. Server encrypts the packet with session key shared
954 between the server and its router, and sends the packet to the
957 o Router determines the destination of the packet and decrypts
958 the packet. Router encrypts the packet with session key
959 shared between the router and the destination server, and sends
960 the packet to the server.
962 o Server determines the client to which the packet is destined
963 to and decrypts the packet. Server encrypts the packet with
964 session key shared between the server and the destination client,
965 and sends the packet to the client.
967 o Client 2. decrypts the packet.
970 Example: Private message from client to another client on different
971 servers. Clients has established secret shared private
972 message delivery key with each other and that is used in
973 the message encryption.
975 o Client 1. sends encrypted packet to its server. The packet header
976 is encrypted with the session key shared between the client and
977 server, and the private message is encrypted with the private
978 message delivery key shared between clients.
980 o Server determines the destination of the packet and sends the
981 packet to the router.
983 o Router determines the destination of the packet and sends the
984 packet to the server.
986 o Server determines the client to which the packet is destined
987 to and sends the packet to the client.
989 o Client 2. decrypts the packet with the secret shared key.
992 If clients share secret key with each other the private message
993 delivery is much simpler since servers and routers between the
994 clients do not need to decrypt and re-encrypt the packet.
996 The process for clients on same server is much simpler as there are
997 no need to send the packet to the router. The process for clients
998 on different cells is same as above except that the packet is routed
999 outside the cell. The router of the destination cell routes the
1000 packet to the destination same way as described above.
1004 3.8.3 Client To Channel
1006 Process of message delivery from client on channel to all the clients
1009 Example: Channel of four users; two on same server, other two on
1010 different cells. Client sends message to the channel.
1012 o Client 1. encrypts the packet with channel key and sends the
1013 packet to its server.
1015 o Server determines local clients on the channel and sends the
1016 packet to the Client on the same server. Server then sends
1017 the packet to its router for further routing.
1019 o Router determines local clients on the channel, if found
1020 sends packet to the local clients. Router determines global
1021 clients on the channel and sends the packet to its primary
1022 router or fastest route.
1024 o (Other router(s) do the same thing and sends the packet to
1027 o Server determines local clients on the channel and sends the
1028 packet to the client.
1030 o All clients receiving the packet decrypts the packet.
1034 3.8.4 Server To Server
1036 Server to server packet delivery and encryption is described in above
1037 examples. Router to router packet delivery is analogous to server to
1038 server. However, some packets, such as channel packets, are processed
1039 differently. These cases are described later in this document and
1040 more in detail in [SILC2].
1044 3.9 Key Exchange And Authentication
1046 Key exchange is done always when for example client connects to server
1047 but also when server and router, and router and router connects to each
1048 other. The purpose of key exchange protocol is to provide secure key
1049 material to be used in the communication. The key material is used to
1050 derive various security parameters used to secure SILC packets. The
1051 SILC Key Exchange protocol is described in detail in [SILC3].
1053 Authentication is done after key exchange protocol has been successfully
1054 completed. The purpose of authentication is to authenticate for example
1055 client connecting to the server. However, usually clients are accepted
1056 to connect to server without explicit authentication. Servers are
1057 required use authentication protocol when connecting. The authentication
1058 may be based on passphrase (pre-shared-secret) or public key. All
1059 passphrases sent in SILC protocol MUST be UTF-8 [RFC2279] encoded.
1060 The connection authentication protocol is described in detail in [SILC3].
1064 3.9.1 Authentication Payload
1066 Authentication payload is used separately from the SKE and the Connection
1067 Authentication protocol. It can be used during the session to authenticate
1068 with the remote. For example, the client can authenticate itself to the
1069 server to become server operator. In this case, Authentication Payload is
1072 The format of the Authentication Payload is as follows:
1078 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
1079 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1080 | Payload Length | Authentication Method |
1081 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1082 | Public Data Length | |
1083 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1087 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1088 | Authentication Data Length | |
1089 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1091 ~ Authentication Data ~
1093 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1097 Figure 5: Authentication Payload
1101 o Payload Length (2 bytes) - Length of the entire payload.
1103 o Authentication Method (2 bytes) - The method of the
1104 authentication. The authentication methods are defined
1105 in [SILC2] in the Connection Auth Request Payload. The NONE
1106 authentication method SHOULD NOT be used.
1108 o Public Data Length (2 bytes) - Indicates the length of
1109 the Public Data field.
1111 o Public Data (variable length) - This is defined only if
1112 the authentication method is public key. If it is any other
1113 this field MAY include a random data for padding purposes.
1114 However, in this case the field MUST be ignored by the
1117 When the authentication method is public key this includes
1118 128 to 4096 bytes of non-zero random data that is used in
1119 the signature process, described subsequently.
1121 o Authentication Data Length (2 bytes) - Indicates the
1122 length of the Authentication Data field. If zero (0)
1123 value is found in this field the payload MUST be
1126 o Authentication Data (variable length) - Authentication
1127 method dependent authentication data.
1131 If the authentication method is password based, the Authentication
1132 Data field includes the plaintext UTF-8 encoded password. It is safe
1133 to send plaintext password since the entire payload is encrypted. In
1134 this case the Public Data Length is set to zero (0), but MAY also include
1135 random data for padding purposes. It is also RECOMMENDED that maximum
1136 amount of padding is applied to SILC packet when using password based
1137 authentication. This way it is not possible to approximate the length
1138 of the password from the encrypted packet.
1140 If the authentication method is public key based (or certificate)
1141 the Authentication Data is computed as follows:
1143 HASH = hash(random bytes | ID | public key (or certificate));
1144 Authentication Data = sign(HASH);
1146 The hash() and the sign() are the hash function and the public key
1147 cryptography function selected in the SKE protocol. The public key
1148 is SILC style public key unless certificates are used. The ID is the
1149 entity's ID (Client or Server ID) which is authenticating itself. The
1150 ID encoding is described in [SILC2]. The random bytes are non-zero
1151 random bytes of length between 128 and 4096 bytes, and will be included
1152 into the Public Data field as is.
1154 The receiver will compute the signature using the random data received
1155 in the payload, the ID associated to the connection and the public key
1156 (or certificate) received in the SKE protocol. After computing the
1157 receiver MUST verify the signature. In case of public key authentication
1158 this payload is also encrypted.
1164 This section defines all the allowed algorithms that can be used in
1165 the SILC protocol. This includes mandatory cipher, mandatory public
1166 key algorithm and MAC algorithms.
1172 Cipher is the encryption algorithm that is used to protect the data
1173 in the SILC packets. See [SILC2] of the actual encryption process and
1174 definition of how it must be done. SILC has a mandatory algorithm that
1175 must be supported in order to be compliant with this protocol.
1177 The following ciphers are defined in SILC protocol:
1180 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1181 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1182 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1183 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1184 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1185 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1186 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1187 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1188 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1189 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1190 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1191 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1192 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1193 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1194 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1195 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1196 none No encryption (OPTIONAL)
1200 Algorithm none does not perform any encryption process at all and
1201 thus is not recommended to be used. It is recommended that no client
1202 or server implementation would accept none algorithms except in special
1205 Additional ciphers MAY be defined to be used in SILC by using the
1206 same name format as above.
1210 3.10.2 Public Key Algorithms
1212 Public keys are used in SILC to authenticate entities in SILC network
1213 and to perform other tasks related to public key cryptography. The
1214 public keys are also used in the SILC Key Exchange protocol [SILC3].
1216 The following public key algorithms are defined in SILC protocol:
1223 DSS is described in [Menezes]. The RSA MUST be implemented according
1224 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1225 compliant to either PKCS #1 version 1.5 or newer with the following
1226 notes: The signature encoding is always in same format as the encryption
1227 encoding regardless of the PKCS #1 version. The signature with appendix
1228 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1229 rationale for this is that there is no binding between the PKCS #1 OIDs
1230 and the hash algorithms used in the SILC protocol. Hence, the encoding
1231 is always in PKCS #1 version 1.5 format.
1233 Additional public key algorithms MAY be defined to be used in SILC.
1237 3.10.3 Hash Functions
1239 Hash functions are used as part of MAC algorithms defined in the next
1240 section. They are also used in the SILC Key Exchange protocol defined
1243 The following Hash algorithm are defined in SILC protocol:
1246 sha1 SHA-1, length = 20 (REQUIRED)
1247 md5 MD5, length = 16 (OPTIONAL)
1252 3.10.4 MAC Algorithms
1254 Data integrity is protected by computing a message authentication code
1255 (MAC) of the packet data. See [SILC2] for details how to compute the
1258 The following MAC algorithms are defined in SILC protocol:
1261 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1262 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1263 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1264 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1265 none No MAC (OPTIONAL)
1268 The none MAC is not recommended to be used as the packet is not
1269 authenticated when MAC is not computed. It is recommended that no
1270 client or server would accept none MAC except in special debugging
1273 The HMAC algorithm is described in [HMAC] and hash algorithms that
1274 are used as part of the HMACs are described in [Scheneir] and in
1277 Additional MAC algorithms MAY be defined to be used in SILC.
1283 3.10.5 Compression Algorithms
1285 SILC protocol supports compression that may be applied to unencrypted
1286 data. It is recommended to use compression on slow links as it may
1287 significantly speed up the data transmission. By default, SILC does not
1288 use compression which is the mode that must be supported by all SILC
1291 The following compression algorithms are defined:
1294 none No compression (REQUIRED)
1295 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1298 Additional compression algorithms MAY be defined to be used in SILC.
1302 3.11 SILC Public Key
1304 This section defines the type and format of the SILC public key. All
1305 implementations MUST support this public key type. See [SILC3] for
1306 other optional public key and certificate types allowed in the SILC
1307 protocol. Public keys in SILC may be used to authenticate entities
1308 and to perform other tasks related to public key cryptography.
1310 The format of the SILC Public Key is as follows:
1316 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
1317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1318 | Public Key Length |
1319 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1320 | Algorithm Name Length | |
1321 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1325 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1326 | Identifier Length | |
1327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1331 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1339 Figure 5: SILC Public Key
1343 o Public Key Length (4 bytes) - Indicates the full length
1344 of the public key, not including this field.
1346 o Algorithm Name Length (2 bytes) - Indicates the length
1347 of the Algorithm Length field, not including this field.
1349 o Algorithm name (variable length) - Indicates the name
1350 of the public key algorithm that the key is. See the
1351 section 3.10.2 Public Key Algorithms for defined names.
1353 o Identifier Length (2 bytes) - Indicates the length of
1354 the Identifier field, not including this field.
1356 o Identifier (variable length) - Indicates the identifier
1357 of the public key. This data can be used to identify
1358 the owner of the key. The identifier is of the following
1362 HN Host name or IP address
1369 Examples of an identifier:
1371 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1373 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1375 At least user name (UN) and host name (HN) MUST be provided as
1376 identifier. The fields are separated by commas (`,'). If
1377 comma is in the identifier string it must be written as `\\,',
1378 for example, `O=Company XYZ\\, Inc.'.
1380 o Public Data (variable length) - Includes the actual
1381 public data of the public key.
1383 The format of this field for RSA algorithm is
1392 The format of this field for DSS algorithm is
1404 The variable length fields are multiple precession
1405 integers encoded as strings in both examples.
1407 Other algorithms must define their own type of this
1408 field if they are used.
1411 All fields in the public key are in MSB (most significant byte first)
1416 3.12 SILC Version Detection
1418 The version detection of both client and server is performed at the
1419 connection phase while executing the SILC Key Exchange protocol. The
1420 version identifier is exchanged between initiator and responder. The
1421 version identifier is of the following format:
1424 SILC-<protocol version>-<software version>
1427 The version strings are of the following format:
1430 protocol version = <major>.<minor>
1431 software version = <major>[.<minor>[.<build or vendor string>]]
1434 Protocol version MAY provide both major and minor version. Currently
1435 implementations MUST set the protocol version and accept at least the
1436 protocol version as SILC-1.1-<software version>. If new protocol version
1437 causes incompatibilities with older version the <minor> version number
1438 MUST be incremented. The <major> is incremented if new protocol version
1439 is fully incompatible.
1441 Software version MAY provide major, minor and build (vendor) version.
1442 The software version MAY be freely set and accepted. The version string
1443 MUST consist of printable US-ASCII characters.
1446 Thus, the version strings could be, for example:
1451 SILC-1.1-1.0.VendorXYZ
1452 SILC-1.1-2.4.5 Vendor Limited
1459 Backup routers may exist in the cell in addition of the primary router.
1460 However, they must not be active routers and act as routers in the cell.
1461 Only one router may be acting as primary router in the cell. In the case
1462 of failure of the primary router may one of the backup routers become
1463 active. The purpose of backup routers are in case of failure of the
1464 primary router to maintain working connections inside the cell and outside
1465 the cell and to avoid netsplits.
1467 Backup routers are normal servers in the cell that are prepared to take
1468 over the tasks of the primary router if needed. They need to have at
1469 least one direct and active connection to the primary router of the cell.
1470 This communication channel is used to send the router information to
1471 the backup router. When the backup router connects to the primary router
1472 of the cell it MUST present itself as router server in the Connection
1473 Authentication protocol, even though it is normal server as long as the
1474 primary router is available. Reason for this is that the configuration
1475 needed in the responder end requires usually router connection level
1476 configuration. The responder, however must understand and treat the
1477 connection as normal server (except when feeding router level data to
1480 Backup router must know everything that the primary router knows to be
1481 able to take over the tasks of the primary router. It is the primary
1482 router's responsibility to feed the data to the backup router. If the
1483 backup router does not know all the data in the case of failure some
1484 connections may be lost. The primary router of the cell must consider
1485 the backup router being actual router server when it feeds the data to
1488 In addition of having direct connection to the primary router of the
1489 cell, the backup router must also have connection to the same router
1490 the primary router of the cell is connected. However, it must not be
1491 active router connection meaning that the backup router must not use
1492 that channel as its primary route and it must not notify the router
1493 about having connected servers, channels and clients behind it. It
1494 merely connects to the router. This sort of connection is later
1495 referred as being passive connection. Some keepalive actions may be
1496 needed by the router to keep the connection alive.
1498 It is required that other normal servers have passive connections to
1499 the backup router(s) in the cell. Some keepalive actions may be needed
1500 by the server to keep the connection alive. After they notice the
1501 failure of the primary router they must start using the connection to
1502 the first backup router as their primary route.
1504 Also, if any other router in the network is using the cell's primary
1505 router as its own primary router, it must also have passive connection
1506 to the cell's backup router. It too is prepared to switch to use the
1507 backup router as its new primary router as soon as the orignal primary
1508 router becomes unresponsive.
1510 All of the parties of this protocol knows which one is the backup router
1511 of the cell from their local configuration. Each of the entity must
1512 be configured accordingly and care must be taken when configuring the
1513 backup routers, servers and other routers in the network.
1515 It must be noted that some of the channel messages and private messages
1516 may be lost during the switch to the backup router. The announcements
1517 assures that the state of the network is not lost during the switch.
1519 It is RECOMMENDED that there would be at least one backup router in
1520 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
1521 as backup routers as it requires establishing several connections to
1522 several servers in the cell. Large cells can easily have several
1523 backup routers in the cell.
1525 The order of the backup routers are decided at the configuration phase.
1526 All the parties of this protocol must be configured accordingly to
1527 understand the order of the backup routers. It is not required that
1528 the backup server is actually active server in the cell. Backup router
1529 may be a spare server in the cell that does not accept normal client
1530 connections at all. It may be reserved purely for the backup purposes.
1531 These, however, are cell management issues.
1533 If also the first backup router is down as well and there is another
1534 backup router in the cell then it will start acting as the primary
1535 router as described above.
1539 3.13.1 Switching to Backup Router
1541 When the primary router of the cell becomes unresponsive, for example
1542 by sending EOF to the connection, all the parties of this protocol MUST
1543 replace the old connection to the primary router with first configured
1544 backup router. The backup router usually needs to do local modifications
1545 to its database in order to update all the information needed to maintain
1546 working routes. The backup router must understand that clients that
1547 were orignated from the primary router are now originated from some of
1548 the existing server connections and must update them accordingly. It
1549 must also remove those clients that were owned by the primary router
1550 since those connections were lost when the primary router became
1553 All the other parties of the protocol must also update their local
1554 database to understand that the route to the primary router will now go
1555 to the backup router.
1557 The servers connected to the backup router must announce their clients,
1558 channels, channel users, channel user modes and channel modes to the
1559 backup router. This is to assure that none of the important notify
1560 packets were lost during the switch to the backup router. The backup
1561 router must check which of these announced entities it already have
1562 and distribute the new ones to the primary route.
1564 The backup router too must announce its servers, clients, channels
1565 and other information to the new primary router. The primary router
1566 of the backup router too must announce its informations to the backup
1567 router. Both must process only the ones they do not know about. If
1568 any of the announced modes does not match then they are enforced in
1569 normal manner defined later in this specification.
1573 3.13.2 Resuming Primary Router
1575 Usually the primary router is unresponsive only a short period of time
1576 and it is intended that the original router of the cell will reassume
1577 its position as primary router when it comes back online. The backup
1578 router that is now acting as primary router of the cell must constantly
1579 try to connect to the original primary router of the cell. It is
1580 RECOMMENDED that it would try to reconnect in 30 second intervals to
1583 When the connection is established to the primary router the backup
1584 resuming protocol is executed. The protocol is advanced as follows:
1586 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1587 value 1 the primary router that came back online. The packet
1588 will indicate the primary router has been replaced by the backup
1589 router. After sending the packet the backup router will announce
1590 all of its channels, channel users, modes etc. to the primary
1593 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1594 value 2 to its current primary router to indicate that it will
1595 resign as being primary router. Then, backup router sends the
1596 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
1597 connected servers to also indicate that it will resign as being
1600 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
1601 type value 2 to the router that is using the backup router
1602 currently as its primary router.
1604 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1605 with type value 1 or 2 must reconnect immediately to the
1606 primary router of the cell that came back online. After they
1607 have created the connection they MUST NOT use that connection
1608 as active primary route but still route all packets to the
1609 backup router. After the connection is created they MUST send
1610 SILC_PACKET_RESUME_ROUTER with type value 3 back to the
1611 backup router. The session ID value found in the first packet
1612 MUST be set in this packet.
1614 5. Backup router MUST wait for all packets with type value 3 before
1615 it continues with the protocol. It knows from the session ID values
1616 set in the packet when it have received all packets. The session
1617 value should be different in all packets it have send earlier.
1618 After the packets is received the backup router sends the
1619 SILC_PACKET_RESUME_ROUTER packet with type value 4 to the
1620 primary router that came back online. This packet will indicate
1621 that the backup router is now ready to resign as being primary
1622 router. The session ID value in this packet MUST be the same as
1623 in first packet sent to the primary router. During this time
1624 the backup router should still route all packets it is receiving
1625 from server connections.
1627 6. The primary router receives the packet and send the
1628 SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
1629 including the backup router. It also sends the packet with type
1630 value 6 to its primary router, and to the router that is using
1631 it as its primary router. The Session ID value in this packet
1634 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1635 with type value 5 or 6 must switch their primary route to the
1636 new primary router and remove the route for the backup router, since
1637 it is not anymore the primary router of the cell. They must also
1638 update their local database to understand that the clients are
1639 not originated from the backup router but from the locally connected
1640 servers. After that they MUST announce their channels, channel
1641 users, modes etc. to the primary router. They must not use the
1642 backup router connection after this and the connection is considered
1643 to be passive connection. The implementations SHOULD be able
1644 to disable the connection without closing the actual link.
1646 After this protocol is executed the backup router is now again normal
1647 server in the cell that has the backup link to the primary router. The
1648 primary router feeds the router specific data again to the backup router.
1649 All server connections in the backup router are considered passive
1652 When the primary router of the cell comes back online and connects
1653 to its primary router, the remote primary router must send the
1654 SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
1655 connection is not allowed since the router has been replaced by an
1656 backup router. The session ID value in this packet SHOULD be zero (0).
1657 When the router receives this packet it must not use the connection
1658 as active connection but to understand that it cannot act as primary
1659 router in the cell. It must wait that the backup router connects to
1660 it, and the backup resuming protocol is executed.
1662 The following type values has been defined for SILC_PACKET_RESUME_ROUTER
1665 1 SILC_SERVER_BACKUP_START
1666 2 SILC_SERVER_BACKUP_START_GLOBAL
1667 3 SILC_SERVER_BACKUP_START_CONNECTED
1668 4 SILC_SERVER_BACKUP_START_ENDING
1669 5 SILC_SERVER_BACKUP_START_RESUMED
1670 6 SILC_SERVER_BACKUP_START_GLOBAL
1671 20 SILC_SERVER_BACKUP_START_REPLACED
1673 If any other value is found in the type field the packet must be
1674 discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
1675 is defined in [SILC2].
1679 3.13.3 Discussion on Backup Router Scheme
1681 It is clear that this backup router support is not able to handle all
1682 possible situations arrising in unreliable network environment. This
1683 scheme for example does not handle situation when the router actually
1684 does not go offline but the network link goes down temporarily. It would
1685 require some intelligence to figure out when it is best time to switch
1686 to the backup router. To make it even more complicated it is possible
1687 that the backup router may have not lost the network link to the primary
1690 Other possible situation is when the network link is lost temporarily
1691 between two primary routers in the SILC network. Unless the routers
1692 notice the link going down they cannot perhaps find alternative routes.
1693 Worst situation is when the link goes down only for a short period of
1694 time, thus causing lag. Should the routers or servers find alternative
1695 routes if they cannot get response from the router during the lag?
1696 When alternative routes are being found it must be careful not to
1697 mess up existing primary routes between routers in the network.
1699 It is suggested that the current backup router scheme is only temporary
1700 solution and existing backup router protocols are studied further. It
1701 is also suggested that the backup router specification will be separated
1702 from this SILC specification Internet-Draft and additional specification
1703 is written on the subject.
1709 This section describes various SILC procedures such as how the
1710 connections are created and registered, how channels are created and
1711 so on. The section describes the procedures only generally as details
1712 are described in [SILC2] and [SILC3].
1716 4.1 Creating Client Connection
1718 This section describes the procedure when client connects to SILC server.
1719 When client connects to server the server MUST perform IP address lookup
1720 and reverse IP address lookup to assure that the origin host really is
1721 who it claims to be. Client, host, connecting to server SHOULD have
1722 both valid IP address and fully qualified domain name (FQDN).
1724 After that the client and server performs SILC Key Exchange protocol
1725 which will provide the key material used later in the communication.
1726 The key exchange protocol MUST be completed successfully before the
1727 connection registration may continue. The SILC Key Exchange protocol
1728 is described in [SILC3].
1730 Typical server implementation would keep a list of connections that it
1731 allows to connect to the server. The implementation would check, for
1732 example, the connecting client's IP address from the connection list
1733 before the SILC Key Exchange protocol has been started. Reason for
1734 this is that if the host is not allowed to connect to the server there
1735 is no reason to perform the key exchange protocol.
1737 After successful key exchange protocol the client and server performs
1738 connection authentication protocol. The purpose of the protocol is to
1739 authenticate the client connecting to the server. Flexible
1740 implementation could also accept the client to connect to the server
1741 without explicit authentication. However, if authentication is
1742 desired for a specific client it may be based on passphrase or
1743 public key authentication. If authentication fails the connection
1744 MUST be terminated. The connection authentication protocol is described
1747 After successful key exchange and authentication protocol the client
1748 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1749 server. This packet includes various information about the client
1750 that the server uses to create the client. Server creates the client
1751 and sends SILC_PACKET_NEW_ID to the client which includes the created
1752 Client ID that the client MUST start using after that. After that
1753 all SILC packets from the client MUST have the Client ID as the
1754 Source ID in the SILC Packet Header, described in [SILC2].
1756 Client MUST also get the server's Server ID that is to be used as
1757 Destination ID in the SILC Packet Header when communicating with
1758 the server (for example when sending commands to the server). The
1759 ID may be resolved in two ways. Client can take the ID from an
1760 previously received packet from server that MUST include the ID,
1761 or to send SILC_COMMAND_INFO command and receive the Server ID as
1764 Server MAY choose not to use the information received in the
1765 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1766 certificate were used in the authentication, server MAY use those
1767 informations rather than what it received from client. This is suitable
1768 way to get the true information about client if it is available.
1770 The nickname of client is initially set to the username sent in the
1771 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1772 suitable by sending SILC_COMMAND_NICK command. However, this is not
1773 required as part of registration process.
1775 Server MUST also distribute the information about newly registered
1776 client to its router (or if the server is router, to all routers in
1777 the SILC network). More information about this in [SILC2].
1779 Router server MUST also check whether some client in the local cell
1780 is watching for the nickname this new client has, and send the
1781 SILC_NOTIFY_TYPE_WATCH to the watcher.
1785 4.2 Creating Server Connection
1787 This section describes the procedure when server connects to its
1788 router (or when router connects to other router, the cases are
1789 equivalent). The procedure is very much alike when client connects
1790 to the server thus it is not repeated here.
1792 One difference is that server MUST perform connection authentication
1793 protocol with proper authentication. A proper authentication is based
1794 on passphrase or public key authentication.
1796 After server and router has successfully performed the key exchange
1797 and connection authentication protocol, the server register itself
1798 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1799 includes the server's Server ID that it has created by itself and
1800 other relevant information about the server.
1802 After router has received the SILC_PACKET_NEW_SERVER packet it
1803 distributes the information about newly registered server to all routers
1804 in the SILC network. More information about this in [SILC2].
1806 As client needed to resolve the destination ID this MUST be done by the
1807 server that connected to the router, as well. The way to resolve it is
1808 to get the ID from previously received packet. The server MAY also
1809 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1810 using its own Server ID as Source ID in SILC Packet Header and the
1811 router's Server ID as Destination when communicating with the router.
1815 4.2.1 Announcing Clients, Channels and Servers
1817 After server or router has connected to the remote router, and it already
1818 has connected clients and channels it MUST announce them to the router.
1819 If the server is router server, also all the local servers in the cell
1822 All clients are announced by compiling a list of ID Payloads into the
1823 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1824 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also,
1825 the channel users on the channels must be announced by compiling a
1826 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1827 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1828 also be announced by compiling list of Notify Payloads with the
1829 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1832 The router MUST also announce the local servers by compiling list of
1833 ID Payloads into the SILC_PACKET_NEW_ID packet.
1835 Also, clients' modes (user modes in SILC) MUST be announced. This is
1836 done by compiling a list of Notify Payloads with the
1837 SILC_NOTIFY_UMODE_CHANGE nofity type into the SILC_PACKET_NOTIFY packet.
1839 Also, channel's topics MUST be announced by compiling a list of Notify
1840 Payloads with the SILC_NOTIFY_TOPIC_SET notify type into the
1841 SILC_PACKET_NOTIFY packet.
1843 The router which receives these lists MUST process them and broadcast
1844 the packets to its primary route.
1846 When processing the announced channels and channel users the router MUST
1847 check whether a channel exists already with the same name. If channel
1848 exists with the same name it MUST check whether the Channel ID is
1849 different. If the Channel ID is different the router MUST send the notify
1850 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1851 change to the ID the router has. If the mode of the channel is different
1852 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1853 server to force the mode change to the mode that the router has.
1855 The router MUST also generate new channel key and distribute it to the
1856 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1859 If the channel has channel founder on the router the router MUST send
1860 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1861 the mode change for the channel founder on the server. The channel
1862 founder privileges MUST be removed.
1864 The router processing the channels MUST also compile a list of
1865 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1866 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1867 server (or router) will receive the clients on the channel that
1872 4.3 Joining to a Channel
1874 This section describes the procedure when client joins to a channel.
1875 Client joins to channel by sending command SILC_COMMAND_JOIN to the
1876 server. If the receiver receiving join command is normal server the
1877 server MUST check its local list whether this channel already exists
1878 locally. This would indicate that some client connected to the server
1879 has already joined to the channel. If this is case the client is
1880 joined to the channel, new channel key is created and information about
1881 newly joined channel is sent to the router. The router is informed
1882 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1883 also be sent to the local clients on the channel. The new channel key
1884 is also sent to the router and to local clients on the channel.
1886 If the channel does not exist in the local list the client's command
1887 MUST be sent to the router which will then perform the actual joining
1888 procedure. When server receives the reply to the command from the
1889 router it MUST be sent to the client which sent the command originally.
1890 Server will also receive the channel key from the server that it MUST
1891 send to the client which originally requested the join command. The
1892 server MUST also save the channel key.
1894 If the receiver of the join command is router it MUST first check its
1895 local list whether anyone in the cell has already joined to the channel.
1896 If this is the case the client is joined to the channel and reply is
1897 sent to the client. If the command was sent by server the command reply
1898 is sent to the server which sent it. Then the router MUST also create
1899 new channel key and distribute it to all clients on the channel and
1900 all servers that has clients on the channel. Router MUST also send
1901 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1902 and to local servers that has clients on the channel.
1904 If the channel does not exist on the router's local list it MUST
1905 check the global list whether the channel exists at all. If it does
1906 the client is joined to the channel as described previously. If
1907 the channel does not exist the channel is created and the client
1908 is joined to the channel. The channel key is also created and
1909 distributed as previously described. The client joining to the created
1910 channel is made automatically channel founder and both channel founder
1911 and channel operator privileges is set for the client.
1913 If the router created the channel in the process, information about the
1914 new channel MUST be broadcasted to all routers. This is done by
1915 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1916 route. When the router joins the client to the channel it MUST also
1917 send information about newly joined client to all routers in the SILC
1918 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1919 type to the router's primary route.
1921 It is important to note that new channel key is created always when
1922 new client joins to channel, whether the channel has existed previously
1923 or not. This way the new client on the channel is not able to decrypt
1924 any of the old traffic on the channel. Client which receives the reply to
1925 the join command MUST start using the received Channel ID in the channel
1926 message communication thereafter. Client also receives the key for the
1927 channel in the command reply. Note that the channel key is never
1928 generated if the SILC_CMODE_PRIVKEY mode is set.
1932 4.4 Channel Key Generation
1934 Channel keys are created by router which creates the channel by taking
1935 enough randomness from cryptographically strong random number generator.
1936 The key is generated always when channel is created, when new client
1937 joins a channel and after the key has expired. Key could expire for
1940 The key MUST also be re-generated whenever some client leaves a channel.
1941 In this case the key is created from scratch by taking enough randomness
1942 from the random number generator. After that the key is distributed to
1943 all clients on the channel. However, channel keys are cell specific thus
1944 the key is created only on the cell where the client, which left the
1945 channel, exists. While the server or router is creating the new channel
1946 key, no other client may join to the channel. Messages that are sent
1947 while creating the new key are still processed with the old key. After
1948 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1949 using the new key. If server creates the new key the server MUST also
1950 send the new key to its router. See [SILC2] on more information about
1951 how channel messages must be encrypted and decrypted when router is
1954 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1955 Channel Key Payload it MUST process the key data to create encryption
1956 and decryption key, and to create the HMAC key that is used to compute
1957 the MACs of the channel messages. The processing is as follows:
1959 channel_key = raw key data
1960 HMAC key = hash(raw key data)
1962 The raw key data is the key data received in the Channel Key Payload.
1963 The hash() function is the hash function used in the HMAC of the channel.
1964 Note that the server MUST also save the channel key.
1968 4.5 Private Message Sending and Reception
1970 Private messages are sent point to point. Client explicitly destines
1971 a private message to specific client that is delivered to only to that
1972 client. No other client may receive the private message. The receiver
1973 of the private message is destined in the SILC Packet Header as any
1974 other packet as well.
1976 If the sender of a private message does not know the receiver's Client
1977 ID, it MUST resolve it from server. There are two ways to resolve the
1978 client ID from server; it is RECOMMENDED that client implementations
1979 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
1980 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
1981 If the sender has received earlier a private message from the receiver
1982 it should have cached the Client ID from the SILC Packet Header.
1984 If server receives a private message packet which includes invalid
1985 destionation Client ID the server MUST send SILC_NOTIFY_TYPE_ERROR
1986 notify to the client with error status indicating that such Client ID
1989 See [SILC2] for description of private message encryption and decryption
1994 4.6 Private Message Key Generation
1996 Private message MAY be protected by the key generated by the client.
1997 The key may be generated and sent to the other client by sending packet
1998 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
1999 and is secured by session keys. After that the private message key
2000 is used in the private message communication between those clients.
2002 Other choice is to entirely use keys that are not sent through
2003 the SILC network at all. This significantly adds security. This key
2004 would be pre-shared-key that is known by both of the clients. Both
2005 agree about using the key and starts sending packets that indicate
2006 that the private message is secured using private message key.
2008 The key material used as private message key is implementation issue.
2009 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
2010 the key material. If the key is normal pre-shared-key or randomly
2011 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
2012 the key material SHOULD be processed as defined in the [SILC3]. In
2013 the processing, however, the HASH, as defined in [SILC3] MUST be
2014 ignored. After processing the key material it is employed as defined
2015 in [SILC3], however, the HMAC key material MUST be discarded.
2017 If the key is pre-shared-key or randomly generated the implementations
2018 SHOULD use the SILC protocol's mandatory cipher as the cipher. If the
2019 SKE was used to negotiate key material the cipher was negotiated as well,
2020 and may be different from default cipher.
2024 4.7 Channel Message Sending and Reception
2026 Channel messages are delivered to group of users. The group forms a
2027 channel and all clients on the channel receives messages sent to the
2030 Channel messages are destined to channel by specifying the Channel ID
2031 as Destination ID in the SILC Packet Header. The server MUST then
2032 distribute the message to all clients on the channel by sending the
2033 channel message destined explicitly to a client on the channel.
2035 If server receives a channel message packet which includes invalid
2036 destionation Channel ID the server MUST send SILC_NOTIFY_TYPE_ERROR
2037 notify to the sender with error status indicating that such Channel ID
2040 See the [SILC2] for description of channel messege routing for router
2041 servers, and channel message encryption and decryption process.
2045 4.8 Session Key Regeneration
2047 Session keys MUST be regenerated periodically, say, once in an hour.
2048 The re-key process is started by sending SILC_PACKET_REKEY packet to
2049 other end, to indicate that re-key must be performed. The initiator
2050 of the connection SHOULD initiate the re-key.
2052 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2053 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2054 SKE protocol. In this case the protocol is secured with the old key
2055 and the protocol results to new key material. See [SILC3] for more
2056 information. After the SILC_PACKET_REKEY packet is sent the sender
2057 will perform the SKE protocol.
2059 If PFS flag was set the resulted key material is processed as described
2060 in the section Processing the Key Material in [SILC3]. The difference
2061 with re-key in the processing is that the initial data for the hash
2062 function is just the resulted key material and not the HASH as it
2063 is not computed at all with re-key. Other than that, the key processing
2064 it equivalent to normal SKE negotiation.
2066 If PFS flag was not set, which is the default case, then re-key is done
2067 without executing SKE protocol. In this case, the new key is created by
2068 providing the current sending encryption key to the SKE protocol's key
2069 processing function. The process is described in the section Processing
2070 the Key Material in [SILC3]. The difference in the processing is that
2071 the initial data for the hash function is the current sending encryption
2072 key and not the SKE's KEY and HASH values. Other than that, the key
2073 processing is equivalent to normal SKE negotiation.
2075 After both parties has regenerated the session key, both MUST send
2076 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2077 secured with the old key. After these packets, the subsequent packets
2078 MUST be protected with the new key.
2082 4.9 Command Sending and Reception
2084 Client usually sends the commands in the SILC network. In this case
2085 the client simply sends the command packet to server and the server
2086 processes it and replies with command reply packet. See the [SILC3]
2087 for detailed description of all commands.
2089 However, if the server is not able to process the command, it is sent
2090 to the server's router. This is case for example with commands such
2091 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2092 are other commands as well. For example, if client sends the WHOIS
2093 command requesting specific information about some client the server must
2094 send the WHOIS command to router so that all clients in SILC network
2095 are searched. The router, on the other hand, sends the WHOIS command
2096 further to receive the exact information about the requested client.
2097 The WHOIS command travels all the way to the server which owns the client
2098 and it replies with command reply packet. Finally, the server which
2099 sent the command receives the command reply and it must be able to
2100 determine which client sent the original command. The server then
2101 sends command reply to the client. Implementations should have some
2102 kind of cache to handle, for example, WHOIS information. Servers
2103 and routers along the route could all cache the information for faster
2104 referencing in the future.
2106 The commands sent by server may be sent hop by hop until someone is able
2107 to process the command. However, it is preferred to destine the command
2108 as precisely as it is possible. In this case, other routers en route
2109 MUST route the command packet by checking the true sender and true
2110 destination of the packet. However, servers and routers MUST NOT route
2111 command reply packets to clients coming from other server. Client
2112 MUST NOT accept command reply packet originated from anyone else but
2113 from its own server.
2117 4.10 Closing Connection
2119 When remote client connection is closed the server MUST send the notify
2120 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2121 the client was joined. The server MUST also save the client's information
2122 for a period of time for history purposes.
2124 When remote server or router connection is closed the server or router
2125 MUST also remove all the clients that was behind the server or router
2126 from the SILC Network. The server or router MUST also send the notify
2127 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2128 local clients that are joined on the same channels with the remote
2129 server's or router's clients.
2131 Router server MUST also check whether some client in the local cell
2132 is watching for the nickname this client has, and send the
2133 SILC_NOTIFY_TYPE_WATCH to the watcher, unless the client which left
2134 the network has the SILC_UMODE_REJECT_WATCHING user mode set.
2138 4.11 Detaching and Resuming a Session
2140 SILC protocol provides a possibility for a client to detach itself from
2141 the network without actually signing off from the network. The client
2142 connection to the server is closed but the client remains as valid client
2143 in the network. The client may then later resume its session back from
2144 any server in the network.
2146 When client wishes to detach from the network it MUST send the
2147 SILC_COMMAND_DETACH command to its server. The server then MUST set
2148 SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
2149 notify to its primary router, which will then MUST broadcast it further
2150 to other routers in the network. This user mode indicates that the
2151 client is detached from the network. Implementations MUST NOT use
2152 the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
2153 to the client. All packets MUST still be sent to the client even if
2154 client is detached from the network. Only the server that originally
2155 had the active client connection is able to make the decision after it
2156 notices that the network connection is not active. In this case the
2157 default case is to discard the packet.
2159 The SILC_UMODE_DETACHED flag cannot be set by client itself directly
2160 with SILC_COMMAND_UMODE command, but only implicitly by sending the
2161 SILC_COMMAND_DETACH command. The flag also cannot be unset by the
2162 client, server or router with SILC_COMMAND_UMODE command, but only
2163 implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
2166 When the client wishes to resume its session in the SILC Network it
2167 connects to a server in the network, which MAY also be a different
2168 from the original server, and performs normal procedures regarding
2169 creating a connection as described in section 4.1. After the SKE
2170 and the Connection Authentication protocols has been successfully
2171 completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
2172 MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
2173 perform the resuming procedure. The packet MUST include the detached
2174 client's Client ID, which the client must know. It also includes
2175 Authentication Payload which includes signature made with the client's
2176 private key. The signature is computed as defined in the section
2177 3.9.1. Thus, the authentication method MUST be based in public key
2180 When server receives the SILC_PACKET_RESUME_CLIENT packet it MUST
2181 do the following: Server checks that the Client ID is valid client
2182 and that it has the SILC_UMODE_DETACHED mode set. Then it verifies
2183 the Authentication Payload with the detached client's public key.
2184 If it does not have the public key it retrieves it by sending
2185 SILC_COMMAND_GETKEY command to the server that has the public key from
2186 the original client connection. The server MUST NOT use the public
2187 key received in the SKE protocol for this connection. If the
2188 signature is valid the server unsets the SILC_UMODE_DETACHED flag,
2189 and sends the SILC_PACKET_RESUME_CLIENT packet to its primary router.
2190 The routers MUST broadcast the packet and unset the SILC_UMODE_DETACHED
2191 flag when the packet is received. If the server is router server it
2192 also MUST send the SILC_PACKET_RESUME_CLIENT packet to the original
2193 server whom owned the detached client.
2195 The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
2196 packet MUST know whether the packet already has been received for
2197 the client. It is protocol error to attempt to resume the client
2198 session from more than one server. The implementations could set
2199 internal flag that indicates that the client is resumed. If router
2200 receive SILC_PACKET_RESUME_CLIENT packet for client that is already
2201 resumed the client MUST be killed from the network. This would
2202 indicate that the client is attempting to resume the session more
2203 than once which is protocol error. In this case the router sends
2204 SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
2205 the same situation MUST also send the notify for the client.
2207 The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
2208 must also understand that the client may not be found behind the
2209 same server that it originally came from. They must update their
2210 caches according this. The server that now owns the client session
2211 MUST check whether the Client ID of the resumed client is based
2212 on the server's Server ID. If it is not it creates a new Client
2213 ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
2214 also send the channel keys of all channels that the client is
2215 joined to the client since it does not have them. Whether the
2216 Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
2217 packet to the client. Only after this the client is resumed back
2218 to the network and may start sending packets and messages.
2220 It is also possible that the server does not know about the channels
2221 that the client has joined. In this case it join the client internally
2222 to the channels, generate new channel keys and distribute the keys
2223 to the channels as described in section 4.4.
2225 It is implementation issue for how long servers keep detached client
2226 sessions. It is RECOMMENDED that the detached sessions would be
2227 persistent as long as the server is running.
2231 5 Security Considerations
2233 Security is central to the design of this protocol, and these security
2234 considerations permeate the specification. Common security considerations
2235 such as keeping private keys truly private and using adequate lengths for
2236 symmetric and asymmetric keys must be followed in order to maintain the
2237 security of this protocol.
2239 Special attention must also be paid on the servers and routers that are
2240 running the SILC service. The SILC protocol's security depends greatly
2241 on the security and the integrity of the servers and administrators that
2242 are running the service. It is recommended that some form of registration
2243 is required by the server and router administrator prior acceptance to
2244 the SILC Network. Even though, the SILC protocol is secure in a network
2245 of mutual distrust between clients, servers, routers and adminstrators
2246 of the servers, the client should be able to trust the servers they are
2247 using if they whish to do so.
2249 It however must be noted that if the client requires absolute security
2250 by not trusting any of the servers or routers in the SILC Network, it can
2251 be accomplished by negotiating private keys outside the SILC Network,
2252 either using SKE or some other key exchange protocol, or to use some
2253 other external means for distributing the keys. This applies for all
2254 messages, private messages and channel messages.
2256 It is important to note that SILC, like any other security protocol is
2257 not full proof system and cannot secure from insecure environment; the
2258 SILC servers and routers could very well be compromised. However, to
2259 provide acceptable level of security and usability for end user the
2260 protocol use many times session keys or other keys generated by the
2261 servers to secure the messages. This is intentional design feature to
2262 allow ease of use for end user. This way the network is still usable,
2263 and remains encrypted even if the external means of distributing the
2264 keys is not working. The implementation, however, may like to not
2265 follow this design feature, and always negotiate the keys outside SILC
2266 network. This is acceptable solution and many times recommended. The
2267 implementation still must be able to work with the server generated keys.
2269 If this is unacceptable for the client or end user, the private keys
2270 negotiatied outside the SILC Network should always be used. In the end
2271 it is always implementor's choice whether to negotiate private keys by
2272 default or whether to use the keys generated by the servers.
2274 It is also recommended that router operators in the SILC Network would
2275 form a joint forum to discuss the router and SILC Network management
2276 issues. Also, router operators along with the cell's server operators
2277 should have a forum to discuss the cell management issues.
2283 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2286 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2287 Protocols", Internet Draft, April 2001.
2289 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001.
2291 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2294 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2297 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2300 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2303 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2306 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2309 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2312 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2315 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2316 Infrastructure, Certificate and CRL Profile", RFC 2459,
2319 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2320 John Wiley & Sons, New York, NY, 1996.
2322 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2325 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2326 RFC 2412, November 1998.
2328 [ISAKMP] Maughan D., et al, "Internet Security Association and
2329 Key Management Protocol (ISAKMP)", RFC 2408, November
2332 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2333 (IKE)", RFC 2409, November 1998.
2335 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2336 Authentication", RFC 2104, February 1997.
2338 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2339 Specifications, Version 2.0", RFC 2437, October 1998.
2341 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2342 Requirement Levels", BCP 14, RFC 2119, March 1997.
2344 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
2345 10646", RFC 2279, January 1998.
2354 Snellmanninkatu 34 A 15
2358 EMail: priikone@iki.fi
2360 This Internet-Draft expires XXX