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 nickname is truncated
494 taking 88 bits from the start of the hash value. This
495 hash value is used to search the user's Client ID from
499 Collisions could occur when more than 2^8 clients using same nickname
500 from the same server IP address is connected to the SILC network.
501 Server MUST be able to handle this situation by refusing to accept
502 anymore of that nickname.
504 Another possible collision may happen with the truncated hash value of
505 the nickname. It could be possible to have same truncated hash value for
506 two different nicknames. However, this is not expected to happen nor
507 cause any problems if it would occur. Nicknames are usually logical and
508 it is unlikely to have two distinct logical nicknames produce same
509 truncated hash value.
515 Servers are the most important parts of the SILC network. They form the
516 basis of the SILC, providing a point to which clients may connect to.
517 There are two kinds of servers in SILC; normal servers and router servers.
518 This section focus on the normal server and router server is described
519 in the section 3.3 Router.
521 Normal servers MUST NOT directly connect to other normal server. Normal
522 servers may only directly connect to router server. If the message sent
523 by the client is destined outside the local server it is always sent to
524 the router server for further routing. Server may only have one active
525 connection to router on same port. Normal server MUST NOT connect to other
526 cell's router except in situations where its cell's router is unavailable.
530 3.2.1 Server's Local ID List
532 Normal server keeps various information about the clients and their end
533 users connected to it. Every normal server MUST keep list of all locally
534 connected clients, Client ID's, nicknames, usernames and host names and
535 user's real name. Normal servers only keeps local information and it
536 does not keep any global information. Hence, normal servers knows only
537 about their locally connected clients. This makes servers efficient as
538 they don't have to worry about global clients. Server is also responsible
539 of creating the Client ID's for their clients.
541 Normal server also keeps information about locally created channels and
545 Hence, local list for normal server includes:
548 server list - Router connection
556 client list - All clients in server
566 channel list - All channels in server
569 o Client ID's on channel
570 o Client ID modes on channel
578 Servers are distinguished from other servers by unique 64 bit Server ID
579 (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in
580 the SILC to route messages to correct servers. Server ID's also provide
581 information for Client ID's, see section 3.1.1 Client ID. Server ID is
585 64 bit Server ID based on IPv4 addresses:
587 32 bit IP address of the server
591 160 bit Server ID based on IPv6 addresses:
593 128 bit IP address of the server
597 o IP address of the server - This is the real IP address of
600 o Port - This is the port the server is bound to.
602 o Random number - This is used to further randomize the Server ID.
605 Collisions are not expected to happen in any conditions. The Server ID
606 is always created by the server itself and server is responsible of
607 distributing it to the router.
611 3.2.3 SILC Server Ports
613 The following ports has been assigned by IANA for the SILC protocol:
621 If there are needs to create new SILC networks in the future the port
622 numbers must be officially assigned by the IANA.
624 Server on network above privileged ports (>1023) SHOULD NOT be trusted
625 as they could have been set up by untrusted party.
631 Router server in SILC network is responsible for keeping the cell together
632 and routing messages to other servers and to other routers. Router server
633 is also a normal server thus clients may connect to it as it would be
634 just normal SILC server.
636 However, router servers has a lot of important tasks that normal servers
637 do not have. Router server knows everything about everything in the SILC.
638 They know all clients currently on SILC, all servers and routers and all
639 channels in SILC. Routers are the only servers in SILC that care about
640 global information and keeping them up to date at all time. And, this
641 is what they must do.
645 3.3.1 Router's Local ID List
647 Router server as well MUST keep local list of connected clients and
648 locally created channels. However, this list is extended to include all
649 the informations of the entire cell, not just the server itself as for
652 However, on router this list is a lot smaller since routers do not need
653 to keep information about user's nickname, username and host name and real
654 name since these are not needed by the router. The router keeps only
655 information that it needs.
658 Hence, local list for router includes:
661 server list - All servers in the cell
668 client list - All clients in the cell
672 channel list - All channels in the cell
674 o Client ID's on channel
675 o Client ID modes on channel
680 Note that locally connected clients and other information include all the
681 same information as defined in section section 3.2.1 Server's Local ID
686 3.3.2 Router's Global ID List
688 Router server MUST also keep global list. Normal servers do not have
689 global list as they know only about local information. Global list
690 includes all the clients on SILC, their Client ID's, all created channels
691 and their Channel ID's and all servers and routers on SILC and their
692 Server ID's. That is said, global list is for global information and the
693 list must not include the local information already on the router's local
696 Note that the global list does not include information like nicknames,
697 usernames and host names or user's real names. Router does not need to
698 keep these informations as they are not needed by the router. This
699 information is available from the client's server which maybe queried
702 Hence, global list includes:
705 server list - All servers in SILC
710 client list - All clients in SILC
713 channel list - All channels in SILC
715 o Client ID's on channel
716 o Client ID modes on channel
722 3.3.3 Router's Server ID
724 Router's Server ID's are equivalent to normal Server ID's. As routers
725 are normal servers as well same types of ID's applies for routers as well.
726 Thus, see section 3.2.2 Server ID.
732 A channel is a named group of one or more clients which will all receive
733 messages addressed to that channel. The channel is created when first
734 client requests JOIN command to the channel, and the channel ceases to
735 exist when the last client has left it. When channel exists, any client
736 can reference it using the name of the channel.
738 Channel names are unique although the real uniqueness comes from 64 bit
739 Channel ID. However, channel names are still unique and no two global
740 channels with same name may exist. The Channel name is a string of
741 maximum length of 256 bytes. Channel names MUST NOT contain any
742 spaces (` '), any non-printable ASCII characters, commas (`,') and
745 Channels can have operators that can administrate the channel and
746 operate all of its modes. The following operators on channel exist on
750 o Channel founder - When channel is created the joining client becomes
751 channel founder. Channel founder is channel operator with some more
752 privileges. Basically, channel founder can fully operate the channel
753 and all of its modes. The privileges are limited only to the
754 particular channel. There can be only one channel founder per
755 channel. Channel founder supersedes channel operator's privileges.
757 Channel founder privileges cannot be removed by any other operator on
758 channel. When channel founder leaves the channel there is no channel
759 founder on the channel. However, it is possible to set a mode for
760 the channel which allows the original channel founder to regain the
761 founder privileges even after leaving the channel. Channel founder
762 also cannot be removed by force from the channel.
764 o Channel operator - When client joins to channel that has not existed
765 previously it will become automatically channel operator (and channel
766 founder discussed above). Channel operator is able administrate the
767 channel, set some modes on channel, remove a badly behaving client
768 from the channel and promote other clients to become channel
769 operator. The privileges are limited only to the particular channel.
771 Normal channel user may be promoted (opped) to channel operator
772 gaining channel operator privileges. Channel founder or other
773 channel operator may also demote (deop) channel operator to normal
781 Channels are distinguished from other channels by unique Channel ID.
782 The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and
783 collisions are not expected to happen in any conditions. Channel names
784 are just for logical use of channels. The Channel ID is created by the
785 server where the channel is created. The Channel ID is defined as
789 64 bit Channel ID based on IPv4 addresses:
791 32 bit Router's Server ID IP address (bits 1-32)
792 16 bit Router's Server ID port (bits 33-48)
795 160 bit Channel ID based on IPv6 addresses:
797 128 bit Router's Server ID IP address (bits 1-128)
798 16 bit Router's Server ID port (bits 129-144)
801 o Router's Server ID IP address - Indicates the IP address of
802 the router of the cell where this channel is created. This is
803 taken from the router's Server ID. This way SILC router knows
804 where this channel resides in the SILC network.
806 o Router's Server ID port - Indicates the port of the channel on
807 the server. This is taken from the router's Server ID.
809 o Random number - To further randomize the Channel ID. This makes
810 sure that there are no collisions. This also means that
811 in a cell there can be 2^16 channels.
818 Operators are normal users with extra privileges to their server or
819 router. Usually these people are SILC server and router administrators
820 that take care of their own server and clients on them. The purpose of
821 operators is to administrate the SILC server or router. However, even
822 an operator with highest privileges is not able to enter invite-only
823 channel, to gain access to the contents of a encrypted and authenticated
824 packets traveling in the SILC network or to gain channel operator
825 privileges on public channels without being promoted. They have the
826 same privileges as everyone else except they are able to administrate
827 their server or router.
833 Commands are very important part on SILC network especially for client
834 which uses commands to operate on the SILC network. Commands are used
835 to set nickname, join to channel, change modes and many other things.
837 Client usually sends the commands and server replies by sending a reply
838 packet to the command. Server MAY also send commands usually to serve
839 the original client's request. However, server MUST NOT send commands
840 to client and there are some commands that server must not send.
842 Note that the command reply is usually sent only after client has sent
843 the command request but server is allowed to send command reply packet
844 to client even if client has not requested the command. Client MAY,
845 choose to ignore the command reply.
847 It is expected that some of the commands may be miss-used by clients
848 resulting various problems on the server side. Every implementation
849 SHOULD assure that commands may not be executed more than once, say,
850 in two (2) seconds. However, to keep response rate up, allowing for
851 example five (5) commands before limiting is allowed. It is RECOMMENDED
852 that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN,
853 SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases
854 as they require heavy operations. This should be sufficient to prevent
855 the miss-use of commands.
857 SILC commands are described in [SILC4].
863 Packets are naturally the most important part of the protocol and the
864 packets are what actually makes the protocol. Packets in SILC network
865 are always encrypted using, usually the shared secret session key
866 or some other key, for example, channel key, when encrypting channel
867 messages. It is not possible to send packet in SILC network without
868 encryption. The SILC Packet Protocol is a wide protocol and is described
869 in [SILC2]. This document does not define or describe details of
874 3.8 Packet Encryption
876 All packets passed in SILC network MUST be encrypted. This section
877 defines how packets must be encrypted in the SILC network. The detailed
878 description of the actual encryption process of the packets are
879 described in [SILC2].
881 Client and its server shares secret symmetric session key which is
882 established by the SILC Key Exchange Protocol, described in [SILC3].
883 Every packet sent from client to server, with exception of packets for
884 channels, are encrypted with this session key.
886 Channels has a channel key that are shared by every client on the channel.
887 However, the channel keys are cell specific thus one cell does not know
888 the channel key of the other cell, even if that key is for same channel.
889 Channel key is also known by the routers and all servers that has clients
890 on the channel. However, channels MAY have channel private keys that
891 are entirely local setting for the client. All clients on the channel
892 MUST know the channel private key before hand to be able to talk on the
893 channel. In this case, no server or router know the key for channel.
895 Server shares secret symmetric session key with router which is
896 established by the SILC Key Exchange Protocol. Every packet passed from
897 server to router, with exception of packets for channels, are encrypted
898 with the shared session key. Same way, router server shares secret
899 symmetric key with its primary route. However, every packet passed
900 from router to other router, including packets for channels, are
901 encrypted with the shared session key. Every router connection has
902 their own session keys.
906 3.8.1 Determination of the Source and the Destination
908 The source and the destination of the packet needs to be determined
909 to be able to route the packets to correct receiver. This information
910 is available in the SILC Packet Header which is included in all packets
911 sent in SILC network. The SILC Packet Header is described in [SILC2].
913 The header MUST be encrypted with the session key who is next receiver
914 of the packet along the route. The receiver of the packet, for example
915 a router along the route, is able to determine the sender and the
916 destination of the packet by decrypting the SILC Packet Header and
917 checking the ID's attached to the header. The ID's in the header will
918 tell to where the packet needs to be sent and where it is coming from.
920 The header in the packet MUST NOT change during the routing of the
921 packet. The original sender, for example client, assembles the packet
922 and the packet header and server or router between the sender and the
923 receiver MUST NOT change the packet header.
925 Note that the packet and the packet header may be encrypted with
926 different keys. For example, packets to channels are encrypted with
927 the channel key, however, the header is encrypted with the session key
928 as described above. However, the header and the packet may be encrypted
929 with same key. This is the case, for example, with command packets.
933 3.8.2 Client To Client
935 The process of message delivery and encryption from client to another
936 client is as follows.
938 Example: Private message from client to another client on different
939 servers. Clients do not share private message delivery
940 keys; normal session keys are used.
942 o Client 1. sends encrypted packet to its server. The packet is
943 encrypted with the session key shared between client and its
946 o Server determines the destination of the packet and decrypts
947 the packet. Server encrypts the packet with session key shared
948 between the server and its router, and sends the packet to the
951 o Router determines the destination of the packet and decrypts
952 the packet. Router encrypts the packet with session key
953 shared between the router and the destination server, and sends
954 the packet to the server.
956 o Server determines the client to which the packet is destined
957 to and decrypts the packet. Server encrypts the packet with
958 session key shared between the server and the destination client,
959 and sends the packet to the client.
961 o Client 2. decrypts the packet.
964 Example: Private message from client to another client on different
965 servers. Clients has established secret shared private
966 message delivery key with each other and that is used in
967 the message encryption.
969 o Client 1. sends encrypted packet to its server. The packet header
970 is encrypted with the session key shared between the client and
971 server, and the private message is encrypted with the private
972 message delivery key shared between clients.
974 o Server determines the destination of the packet and sends the
975 packet to the router.
977 o Router determines the destination of the packet and sends the
978 packet to the server.
980 o Server determines the client to which the packet is destined
981 to and sends the packet to the client.
983 o Client 2. decrypts the packet with the secret shared key.
986 If clients share secret key with each other the private message
987 delivery is much simpler since servers and routers between the
988 clients do not need to decrypt and re-encrypt the packet.
990 The process for clients on same server is much simpler as there are
991 no need to send the packet to the router. The process for clients
992 on different cells is same as above except that the packet is routed
993 outside the cell. The router of the destination cell routes the
994 packet to the destination same way as described above.
998 3.8.3 Client To Channel
1000 Process of message delivery from client on channel to all the clients
1003 Example: Channel of four users; two on same server, other two on
1004 different cells. Client sends message to the channel.
1006 o Client 1. encrypts the packet with channel key and sends the
1007 packet to its server.
1009 o Server determines local clients on the channel and sends the
1010 packet to the Client on the same server. Server then sends
1011 the packet to its router for further routing.
1013 o Router determines local clients on the channel, if found
1014 sends packet to the local clients. Router determines global
1015 clients on the channel and sends the packet to its primary
1016 router or fastest route.
1018 o (Other router(s) do the same thing and sends the packet to
1021 o Server determines local clients on the channel and sends the
1022 packet to the client.
1024 o All clients receiving the packet decrypts the packet.
1028 3.8.4 Server To Server
1030 Server to server packet delivery and encryption is described in above
1031 examples. Router to router packet delivery is analogous to server to
1032 server. However, some packets, such as channel packets, are processed
1033 differently. These cases are described later in this document and
1034 more in detail in [SILC2].
1038 3.9 Key Exchange And Authentication
1040 Key exchange is done always when for example client connects to server
1041 but also when server and router, and router and router connects to each
1042 other. The purpose of key exchange protocol is to provide secure key
1043 material to be used in the communication. The key material is used to
1044 derive various security parameters used to secure SILC packets. The
1045 SILC Key Exchange protocol is described in detail in [SILC3].
1047 Authentication is done after key exchange protocol has been successfully
1048 completed. The purpose of authentication is to authenticate for example
1049 client connecting to the server. However, usually clients are accepted
1050 to connect to server without explicit authentication. Servers are
1051 required use authentication protocol when connecting. The authentication
1052 may be based on passphrase (pre-shared-secret) or public key. All
1053 passphrases sent in SILC protocol MUST be UTF-8 [RFC2279] encoded.
1054 The connection authentication protocol is described in detail in [SILC3].
1058 3.9.1 Authentication Payload
1060 Authentication payload is used separately from the SKE and the Connection
1061 Authentication protocol. It is used during the session to authenticate
1062 with the remote. For example, the client can authenticate itself to the
1063 server to become server operator. In this case, Authentication Payload is
1066 The format of the Authentication Payload is as follows:
1072 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
1073 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1074 | Payload Length | Authentication Method |
1075 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1076 | Public Data Length | |
1077 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1081 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1082 | Authentication Data Length | |
1083 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1085 ~ Authentication Data ~
1087 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1091 Figure 5: Authentication Payload
1095 o Payload Length (2 bytes) - Length of the entire payload.
1097 o Authentication Method (2 bytes) - The method of the
1098 authentication. The authentication methods are defined
1099 in [SILC2] in the Connection Auth Request Payload. The NONE
1100 authentication method SHOULD NOT be used.
1102 o Public Data Length (2 bytes) - Indicates the length of
1103 the Public Data field.
1105 o Public Data (variable length) - This is defined only if
1106 the authentication method is public key. If it is any other
1107 this field MAY include a random data for padding purposes.
1108 However, in this case the field MUST be ignored by the
1111 When the authentication method is public key this includes
1112 128 to 4096 bytes of non-zero random data that is used in
1113 the signature process, described subsequently.
1115 o Authentication Data Length (2 bytes) - Indicates the
1116 length of the Authentication Data field.
1118 o Authentication Data (variable length) - Authentication
1119 method dependent authentication data.
1123 If the authentication method is password based, the Authentication
1124 Data field includes the plaintext UTF-8 encoded password. It is safe
1125 to send plaintext password since the entire payload is encrypted. In
1126 this case the Public Data Length is set to zero (0), but MAY also include
1127 random data for padding purposes. It is also RECOMMENDED that maximum
1128 amount of padding is applied to SILC packet when using password based
1129 authentication. This way it is not possible to approximate the length
1130 of the password from the encrypted packet.
1132 If the authentication method is public key based (or certificate)
1133 the Authentication Data is computed as follows:
1135 HASH = hash(random bytes | ID | public key (or certificate));
1136 Authentication Data = sign(HASH);
1138 The hash() and the sign() are the hash function and the public key
1139 cryptography function selected in the SKE protocol. The public key
1140 is SILC style public key unless certificates are used. The ID is the
1141 entity's ID (Client or Server ID) which is authenticating itself. The
1142 ID is raw ID data. The random bytes are non-zero random bytes of
1143 length between 128 and 4096 bytes, and will be included into the
1144 Public Data field as is.
1146 The receiver will compute the signature using the random data received
1147 in the payload, the ID associated to the connection and the public key
1148 (or certificate) received in the SKE protocol. After computing the
1149 receiver MUST verify the signature. In this case also, the entire
1150 payload is encrypted.
1156 This section defines all the allowed algorithms that can be used in
1157 the SILC protocol. This includes mandatory cipher, mandatory public
1158 key algorithm and MAC algorithms.
1164 Cipher is the encryption algorithm that is used to protect the data
1165 in the SILC packets. See [SILC2] of the actual encryption process and
1166 definition of how it must be done. SILC has a mandatory algorithm that
1167 must be supported in order to be compliant with this protocol.
1169 The following ciphers are defined in SILC protocol:
1172 aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED)
1173 aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL)
1174 aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL)
1175 twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL)
1176 twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL)
1177 twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL)
1178 blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL)
1179 cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL)
1180 cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL)
1181 cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL)
1182 rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL)
1183 rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL)
1184 rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL)
1185 mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL)
1186 mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL)
1187 mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL)
1188 none No encryption (OPTIONAL)
1192 Algorithm none does not perform any encryption process at all and
1193 thus is not recommended to be used. It is recommended that no client
1194 or server implementation would accept none algorithms except in special
1197 Additional ciphers MAY be defined to be used in SILC by using the
1198 same name format as above.
1202 3.10.2 Public Key Algorithms
1204 Public keys are used in SILC to authenticate entities in SILC network
1205 and to perform other tasks related to public key cryptography. The
1206 public keys are also used in the SILC Key Exchange protocol [SILC3].
1208 The following public key algorithms are defined in SILC protocol:
1215 DSS is described in [Menezes]. The RSA MUST be implemented according
1216 PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be
1217 compliant to either PKCS #1 version 1.5 or newer with the following
1218 notes: The signature encoding is always in same format as the encryption
1219 encoding regardless of the PKCS #1 version. The signature with appendix
1220 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The
1221 rationale for this is that there is no binding between the PKCS #1 OIDs
1222 and the hash algorithms used in the SILC protocol. Hence, the encoding
1223 is always in PKCS #1 version 1.5 format.
1225 Additional public key algorithms MAY be defined to be used in SILC.
1229 3.10.3 Hash Functions
1231 Hash functions are used as part of MAC algorithms defined in the next
1232 section. They are also used in the SILC Key Exchange protocol defined
1235 The following Hash algorithm are defined in SILC protocol:
1238 sha1 SHA-1, length = 20 (REQUIRED)
1239 md5 MD5, length = 16 (OPTIONAL)
1244 3.10.4 MAC Algorithms
1246 Data integrity is protected by computing a message authentication code
1247 (MAC) of the packet data. See [SILC2] for details how to compute the
1250 The following MAC algorithms are defined in SILC protocol:
1253 hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED)
1254 hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL)
1255 hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL)
1256 hmac-md5 HMAC-MD5, length = 16 (OPTIONAL)
1257 none No MAC (OPTIONAL)
1260 The none MAC is not recommended to be used as the packet is not
1261 authenticated when MAC is not computed. It is recommended that no
1262 client or server would accept none MAC except in special debugging
1265 The HMAC algorithm is described in [HMAC] and hash algorithms that
1266 are used as part of the HMACs are described in [Scheneir] and in
1269 Additional MAC algorithms MAY be defined to be used in SILC.
1275 3.10.5 Compression Algorithms
1277 SILC protocol supports compression that may be applied to unencrypted
1278 data. It is recommended to use compression on slow links as it may
1279 significantly speed up the data transmission. By default, SILC does not
1280 use compression which is the mode that must be supported by all SILC
1283 The following compression algorithms are defined:
1286 none No compression (REQUIRED)
1287 zlib GNU ZLIB (LZ77) compression (OPTIONAL)
1290 Additional compression algorithms MAY be defined to be used in SILC.
1294 3.11 SILC Public Key
1296 This section defines the type and format of the SILC public key. All
1297 implementations MUST support this public key type. See [SILC3] for
1298 other optional public key and certificate types allowed in the SILC
1299 protocol. Public keys in SILC may be used to authenticate entities
1300 and to perform other tasks related to public key cryptography.
1302 The format of the SILC Public Key is as follows:
1308 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
1309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1310 | Public Key Length |
1311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1312 | Algorithm Name Length | |
1313 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1317 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1318 | Identifier Length | |
1319 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
1323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1327 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1331 Figure 5: SILC Public Key
1335 o Public Key Length (4 bytes) - Indicates the full length
1336 of the public key, not including this field.
1338 o Algorithm Name Length (2 bytes) - Indicates the length
1339 of the Algorithm Length field, not including this field.
1341 o Algorithm name (variable length) - Indicates the name
1342 of the public key algorithm that the key is. See the
1343 section 3.10.2 Public Key Algorithms for defined names.
1345 o Identifier Length (2 bytes) - Indicates the length of
1346 the Identifier field, not including this field.
1348 o Identifier (variable length) - Indicates the identifier
1349 of the public key. This data can be used to identify
1350 the owner of the key. The identifier is of the following
1354 HN Host name or IP address
1361 Examples of an identifier:
1363 `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi'
1365 `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland'
1367 At least user name (UN) and host name (HN) MUST be provided as
1368 identifier. The fields are separated by commas (`,'). If
1369 comma is in the identifier string it must be written as `\\,',
1370 for example, `O=Company XYZ\\, Inc.'.
1372 o Public Data (variable length) - Includes the actual
1373 public data of the public key.
1375 The format of this field for RSA algorithm is
1384 The format of this field for DSS algorithm is
1396 The variable length fields are multiple precession
1397 integers encoded as strings in both examples.
1399 Other algorithms must define their own type of this
1400 field if they are used.
1403 All fields in the public key are in MSB (most significant byte first)
1408 3.12 SILC Version Detection
1410 The version detection of both client and server is performed at the
1411 connection phase while executing the SILC Key Exchange protocol. The
1412 version identifier is exchanged between initiator and responder. The
1413 version identifier is of the following format:
1416 SILC-<protocol version>-<software version>
1419 The version strings are of the following format:
1422 protocol version = <major>.<minor>
1423 software version = <major>[.<minor>[.<build or vendor string>]]
1426 Protocol version MAY provide both major and minor version. Currently
1427 implementations MUST set the protocol version and accept at least the
1428 protocol version as SILC-1.1-<software version>. If new protocol version
1429 causes incompatibilities with older version the <minor> version number
1430 MUST be incremented. The <major> is incremented if new protocol version
1431 is fully incompatible.
1433 Software version MAY provide major, minor and build (vendor) version.
1434 The software version MAY be freely set and accepted. The version string
1435 MUST consist of printable US-ASCII characters.
1438 Thus, the version strings could be, for example:
1443 SILC-1.1-1.0.VendorXYZ
1444 SILC-1.1-2.4.5 Vendor Limited
1451 Backup routers may exist in the cell in addition of the primary router.
1452 However, they must not be active routers and act as routers in the cell.
1453 Only one router may be acting as primary router in the cell. In the case
1454 of failure of the primary router may one of the backup routers become
1455 active. The purpose of backup routers are in case of failure of the
1456 primary router to maintain working connections inside the cell and outside
1457 the cell and to avoid netsplits.
1459 Backup routers are normal servers in the cell that are prepared to take
1460 over the tasks of the primary router if needed. They need to have at
1461 least one direct and active connection to the primary router of the cell.
1462 This communication channel is used to send the router information to
1463 the backup router. When the backup router connects to the primary router
1464 of the cell it MUST present itself as router server in the Connection
1465 Authentication protocol, even though it is normal server as long as the
1466 primary router is available. Reason for this is that the configuration
1467 needed in the responder end requires usually router connection level
1468 configuration. The responder, however must understand and treat the
1469 connection as normal server (except when feeding router level data to
1472 Backup router must know everything that the primary router knows to be
1473 able to take over the tasks of the primary router. It is the primary
1474 router's responsibility to feed the data to the backup router. If the
1475 backup router does not know all the data in the case of failure some
1476 connections may be lost. The primary router of the cell must consider
1477 the backup router being actual router server when it feeds the data to
1480 In addition of having direct connection to the primary router of the
1481 cell, the backup router must also have connection to the same router
1482 the primary router of the cell is connected. However, it must not be
1483 active router connection meaning that the backup router must not use
1484 that channel as its primary route and it must not notify the router
1485 about having connected servers, channels and clients behind it. It
1486 merely connects to the router. This sort of connection is later
1487 referred as being passive connection. Some keepalive actions may be
1488 needed by the router to keep the connection alive.
1490 It is required that other normal servers have passive connections to
1491 the backup router(s) in the cell. Some keepalive actions may be needed
1492 by the server to keep the connection alive. After they notice the
1493 failure of the primary router they must start using the connection to
1494 the first backup router as their primary route.
1496 Also, if any other router in the network is using the cell's primary
1497 router as its own primary router, it must also have passive connection
1498 to the cell's backup router. It too is prepared to switch to use the
1499 backup router as its new primary router as soon as the orignal primary
1500 router becomes unresponsive.
1502 All of the parties of this protocol knows which one is the backup router
1503 of the cell from their local configuration. Each of the entity must
1504 be configured accordingly and care must be taken when configuring the
1505 backup routers, servers and other routers in the network.
1507 It must be noted that some of the channel messages and private messages
1508 may be lost during the switch to the backup router. The announcements
1509 assures that the state of the network is not lost during the switch.
1511 It is RECOMMENDED that there would be at least one backup router in
1512 the cell. It is NOT RECOMMENDED to have all servers in the cell acting
1513 as backup routers as it requires establishing several connections to
1514 several servers in the cell. Large cells can easily have several
1515 backup routers in the cell.
1517 The order of the backup routers are decided at the configuration phase.
1518 All the parties of this protocol must be configured accordingly to
1519 understand the order of the backup routers. It is not required that
1520 the backup server is actually active server in the cell. Backup router
1521 may be a spare server in the cell that does not accept normal client
1522 connections at all. It may be reserved purely for the backup purposes.
1523 These, however, are cell management issues.
1525 If also the first backup router is down as well and there is another
1526 backup router in the cell then it will start acting as the primary
1527 router as described above.
1531 3.13.1 Switching to Backup Router
1533 When the primary router of the cell becomes unresponsive, for example
1534 by sending EOF to the connection, all the parties of this protocol MUST
1535 replace the old connection to the primary router with first configured
1536 backup router. The backup router usually needs to do local modifications
1537 to its database in order to update all the information needed to maintain
1538 working routes. The backup router must understand that clients that
1539 were orignated from the primary router are now originated from some of
1540 the existing server connections and must update them accordingly. It
1541 must also remove those clients that were owned by the primary router
1542 since those connections were lost when the primary router became
1545 All the other parties of the protocol must also update their local
1546 database to understand that the route to the primary router will now go
1547 to the backup router.
1549 The servers connected to the backup router must announce their clients,
1550 channels, channel users, channel user modes and channel modes to the
1551 backup router. This is to assure that none of the important notify
1552 packets were lost during the switch to the backup router. The backup
1553 router must check which of these announced entities it already have
1554 and distribute the new ones to the primary route.
1556 The backup router too must announce its servers, clients, channels
1557 and other information to the new primary router. The primary router
1558 of the backup router too must announce its informations to the backup
1559 router. Both must process only the ones they do not know about. If
1560 any of the announced modes does not match then they are enforced in
1561 normal manner defined later in this specification.
1565 3.13.2 Resuming Primary Router
1567 Usually the primary router is unresponsive only a short period of time
1568 and it is intended that the original router of the cell will reassume
1569 its position as primary router when it comes back online. The backup
1570 router that is now acting as primary router of the cell must constantly
1571 try to connect to the original primary router of the cell. It is
1572 RECOMMENDED that it would try to reconnect in 30 second intervals to
1575 When the connection is established to the primary router the backup
1576 resuming protocol is executed. The protocol is advanced as follows:
1578 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1579 value 1 the primary router that came back online. The packet
1580 will indicate the primary router has been replaced by the backup
1581 router. After sending the packet the backup router will announce
1582 all of its channels, channel users, modes etc. to the primary
1585 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type
1586 value 2 to its current primary router to indicate that it will
1587 resign as being primary router. Then, backup router sends the
1588 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all
1589 connected servers to also indicate that it will resign as being
1592 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with
1593 type value 2 to the router that is using the backup router
1594 currently as its primary router.
1596 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1597 with type value 1 or 2 must reconnect immediately to the
1598 primary router of the cell that came back online. After they
1599 have created the connection they MUST NOT use that connection
1600 as active primary route but still route all packets to the
1601 backup router. After the connection is created they MUST send
1602 SILC_PACKET_RESUME_ROUTER with type value 3 back to the
1603 backup router. The session ID value found in the first packet
1604 MUST be set in this packet.
1606 5. Backup router MUST wait for all packets with type value 3 before
1607 it continues with the protocol. It knows from the session ID values
1608 set in the packet when it have received all packets. The session
1609 value should be different in all packets it have send earlier.
1610 After the packets is received the backup router sends the
1611 SILC_PACKET_RESUME_ROUTER packet with type value 4 to the
1612 primary router that came back online. This packet will indicate
1613 that the backup router is now ready to resign as being primary
1614 router. The session ID value in this packet MUST be the same as
1615 in first packet sent to the primary router. During this time
1616 the backup router should still route all packets it is receiving
1617 from server connections.
1619 6. The primary router receives the packet and send the
1620 SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers
1621 including the backup router. It also sends the packet with type
1622 value 6 to its primary router, and to the router that is using
1623 it as its primary router. The Session ID value in this packet
1626 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER
1627 with type value 5 or 6 must switch their primary route to the
1628 new primary router and remove the route for the backup router, since
1629 it is not anymore the primary router of the cell. They must also
1630 update their local database to understand that the clients are
1631 not originated from the backup router but from the locally connected
1632 servers. After that they MUST announce their channels, channel
1633 users, modes etc. to the primary router. They must not use the
1634 backup router connection after this and the connection is considered
1635 to be passive connection. The implementations SHOULD be able
1636 to disable the connection without closing the actual link.
1638 After this protocol is executed the backup router is now again normal
1639 server in the cell that has the backup link to the primary router. The
1640 primary router feeds the router specific data again to the backup router.
1641 All server connections in the backup router are considered passive
1644 When the primary router of the cell comes back online and connects
1645 to its primary router, the remote primary router must send the
1646 SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the
1647 connection is not allowed since the router has been replaced by an
1648 backup router. The session ID value in this packet SHOULD be zero (0).
1649 When the router receives this packet it must not use the connection
1650 as active connection but to understand that it cannot act as primary
1651 router in the cell. It must wait that the backup router connects to
1652 it, and the backup resuming protocol is executed.
1654 The following type values has been defined for SILC_PACKET_RESUME_ROUTER
1657 1 SILC_SERVER_BACKUP_START
1658 2 SILC_SERVER_BACKUP_START_GLOBAL
1659 3 SILC_SERVER_BACKUP_START_CONNECTED
1660 4 SILC_SERVER_BACKUP_START_ENDING
1661 5 SILC_SERVER_BACKUP_START_RESUMED
1662 6 SILC_SERVER_BACKUP_START_GLOBAL
1663 20 SILC_SERVER_BACKUP_START_REPLACED
1665 If any other value is found in the type field the packet must be
1666 discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload
1667 is defined in [SILC2].
1671 3.13.3 Discussion on Backup Router Scheme
1673 It is clear that this backup router support is not able to handle all
1674 possible situations arrising in unreliable network environment. This
1675 scheme for example does not handle situation when the router actually
1676 does not go offline but the network link goes down temporarily. It would
1677 require some intelligence to figure out when it is best time to switch
1678 to the backup router. To make it even more complicated it is possible
1679 that the backup router may have not lost the network link to the primary
1682 Other possible situation is when the network link is lost temporarily
1683 between two primary routers in the SILC network. Unless the routers
1684 notice the link going down they cannot perhaps find alternative routes.
1685 Worst situation is when the link goes down only for a short period of
1686 time, thus causing lag. Should the routers or servers find alternative
1687 routes if they cannot get response from the router during the lag?
1688 When alternative routes are being found it must be careful not to
1689 mess up existing primary routes between routers in the network.
1691 It is suggested that the current backup router scheme is only temporary
1692 solution and existing backup router protocols are studied further. It
1693 is also suggested that the backup router specification will be separated
1694 from this SILC specification Internet-Draft and additional specification
1695 is written on the subject.
1701 This section describes various SILC procedures such as how the
1702 connections are created and registered, how channels are created and
1703 so on. The section describes the procedures only generally as details
1704 are described in [SILC2] and [SILC3].
1708 4.1 Creating Client Connection
1710 This section describes the procedure when client connects to SILC server.
1711 When client connects to server the server MUST perform IP address lookup
1712 and reverse IP address lookup to assure that the origin host really is
1713 who it claims to be. Client, host, connecting to server SHOULD have
1714 both valid IP address and fully qualified domain name (FQDN).
1716 After that the client and server performs SILC Key Exchange protocol
1717 which will provide the key material used later in the communication.
1718 The key exchange protocol MUST be completed successfully before the
1719 connection registration may continue. The SILC Key Exchange protocol
1720 is described in [SILC3].
1722 Typical server implementation would keep a list of connections that it
1723 allows to connect to the server. The implementation would check, for
1724 example, the connecting client's IP address from the connection list
1725 before the SILC Key Exchange protocol has been started. Reason for
1726 this is that if the host is not allowed to connect to the server there
1727 is no reason to perform the key exchange protocol.
1729 After successful key exchange protocol the client and server performs
1730 connection authentication protocol. The purpose of the protocol is to
1731 authenticate the client connecting to the server. Flexible
1732 implementation could also accept the client to connect to the server
1733 without explicit authentication. However, if authentication is
1734 desired for a specific client it may be based on passphrase or
1735 public key authentication. If authentication fails the connection
1736 MUST be terminated. The connection authentication protocol is described
1739 After successful key exchange and authentication protocol the client
1740 registers itself by sending SILC_PACKET_NEW_CLIENT packet to the
1741 server. This packet includes various information about the client
1742 that the server uses to create the client. Server creates the client
1743 and sends SILC_PACKET_NEW_ID to the client which includes the created
1744 Client ID that the client MUST start using after that. After that
1745 all SILC packets from the client MUST have the Client ID as the
1746 Source ID in the SILC Packet Header, described in [SILC2].
1748 Client MUST also get the server's Server ID that is to be used as
1749 Destination ID in the SILC Packet Header when communicating with
1750 the server (for example when sending commands to the server). The
1751 ID may be resolved in two ways. Client can take the ID from an
1752 previously received packet from server that MUST include the ID,
1753 or to send SILC_COMMAND_INFO command and receive the Server ID as
1756 Server MAY choose not to use the information received in the
1757 SILC_PACKET_NEW_CLIENT packet. For example, if public key or
1758 certificate were used in the authentication, server MAY use those
1759 informations rather than what it received from client. This is suitable
1760 way to get the true information about client if it is available.
1762 The nickname of client is initially set to the username sent in the
1763 SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more
1764 suitable by sending SILC_COMMAND_NICK command. However, this is not
1765 required as part of registration process.
1767 Server MUST also distribute the information about newly registered
1768 client to its router (or if the server is router, to all routers in
1769 the SILC network). More information about this in [SILC2].
1773 4.2 Creating Server Connection
1775 This section describes the procedure when server connects to its
1776 router (or when router connects to other router, the cases are
1777 equivalent). The procedure is very much alike when client connects
1778 to the server thus it is not repeated here.
1780 One difference is that server MUST perform connection authentication
1781 protocol with proper authentication. A proper authentication is based
1782 on passphrase or public key authentication.
1784 After server and router has successfully performed the key exchange
1785 and connection authentication protocol, the server register itself
1786 to the router by sending SILC_PACKET_NEW_SERVER packet. This packet
1787 includes the server's Server ID that it has created by itself and
1788 other relevant information about the server.
1790 After router has received the SILC_PACKET_NEW_SERVER packet it
1791 distributes the information about newly registered server to all routers
1792 in the SILC network. More information about this in [SILC2].
1794 As client needed to resolve the destination ID this MUST be done by the
1795 server that connected to the router, as well. The way to resolve it is
1796 to get the ID from previously received packet. The server MAY also
1797 use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start
1798 using its own Server ID as Source ID in SILC Packet Header and the
1799 router's Server ID as Destination when communicating with the router.
1803 4.2.1 Announcing Clients, Channels and Servers
1805 After server or router has connected to the remote router, and it already
1806 has connected clients and channels it MUST announce them to the router.
1807 If the server is router server, also all the local servers in the cell
1810 All clients are announced by compiling a list of ID Payloads into the
1811 SILC_PACKET_NEW_ID packet. All channels are announced by compiling a
1812 list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also,
1813 the channel users on the channels must be announced by compiling a
1814 list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into
1815 the SILC_PACKET_NOTIFY packet. The users' modes on the channel must
1816 also be announced by compiling list of Notify Payloads with the
1817 SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY
1820 The router MUST also announce the local servers by compiling list of
1821 ID Payloads into the SILC_PACKET_NEW_ID packet.
1823 Also, clients' modes (user modes in SILC) MUST be announced. This is
1824 done by compiling a list of Notify Payloads with the
1825 SILC_NOTIFY_UMODE_CHANGE nofity type into the SILC_PACKET_NOTIFY packet.
1827 Also, channel's topics MUST be announced by compiling a list of Notify
1828 Payloads with the SILC_NOTIFY_TOPIC_SET notify type into the
1829 SILC_PACKET_NOTIFY packet.
1831 The router which receives these lists MUST process them and broadcast
1832 the packets to its primary route.
1834 When processing the announced channels and channel users the router MUST
1835 check whether a channel exists already with the same name. If channel
1836 exists with the same name it MUST check whether the Channel ID is
1837 different. If the Channel ID is different the router MUST send the notify
1838 type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID
1839 change to the ID the router has. If the mode of the channel is different
1840 the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the
1841 server to force the mode change to the mode that the router has.
1843 The router MUST also generate new channel key and distribute it to the
1844 channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode
1847 If the channel has channel founder on the router the router MUST send
1848 the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force
1849 the mode change for the channel founder on the server. The channel
1850 founder privileges MUST be removed.
1852 The router processing the channels MUST also compile a list of
1853 Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the
1854 SILC_PACKET_NOTIFY and send the packet to the server. This way the
1855 server (or router) will receive the clients on the channel that
1860 4.3 Joining to a Channel
1862 This section describes the procedure when client joins to a channel.
1863 Client joins to channel by sending command SILC_COMMAND_JOIN to the
1864 server. If the receiver receiving join command is normal server the
1865 server MUST check its local list whether this channel already exists
1866 locally. This would indicate that some client connected to the server
1867 has already joined to the channel. If this is case the client is
1868 joined to the channel, new channel key is created and information about
1869 newly joined channel is sent to the router. The router is informed
1870 by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST
1871 also be sent to the local clients on the channel. The new channel key
1872 is also sent to the router and to local clients on the channel.
1874 If the channel does not exist in the local list the client's command
1875 MUST be sent to the router which will then perform the actual joining
1876 procedure. When server receives the reply to the command from the
1877 router it MUST be sent to the client which sent the command originally.
1878 Server will also receive the channel key from the server that it MUST
1879 send to the client which originally requested the join command. The
1880 server MUST also save the channel key.
1882 If the receiver of the join command is router it MUST first check its
1883 local list whether anyone in the cell has already joined to the channel.
1884 If this is the case the client is joined to the channel and reply is
1885 sent to the client. If the command was sent by server the command reply
1886 is sent to the server which sent it. Then the router MUST also create
1887 new channel key and distribute it to all clients on the channel and
1888 all servers that has clients on the channel. Router MUST also send
1889 the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel
1890 and to local servers that has clients on the channel.
1892 If the channel does not exist on the router's local list it MUST
1893 check the global list whether the channel exists at all. If it does
1894 the client is joined to the channel as described previously. If
1895 the channel does not exist the channel is created and the client
1896 is joined to the channel. The channel key is also created and
1897 distributed as previously described. The client joining to the created
1898 channel is made automatically channel founder and both channel founder
1899 and channel operator privileges is set for the client.
1901 If the router created the channel in the process, information about the
1902 new channel MUST be broadcasted to all routers. This is done by
1903 broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary
1904 route. When the router joins the client to the channel it MUST also
1905 send information about newly joined client to all routers in the SILC
1906 network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify
1907 type to the router's primary route.
1909 It is important to note that new channel key is created always when
1910 new client joins to channel, whether the channel has existed previously
1911 or not. This way the new client on the channel is not able to decrypt
1912 any of the old traffic on the channel. Client which receives the reply to
1913 the join command MUST start using the received Channel ID in the channel
1914 message communication thereafter. Client also receives the key for the
1915 channel in the command reply. Note that the channel key is never
1916 generated if the SILC_CMODE_PRIVKEY mode is set.
1920 4.4 Channel Key Generation
1922 Channel keys are created by router which creates the channel by taking
1923 enough randomness from cryptographically strong random number generator.
1924 The key is generated always when channel is created, when new client
1925 joins a channel and after the key has expired. Key could expire for
1928 The key MUST also be re-generated whenever some client leaves a channel.
1929 In this case the key is created from scratch by taking enough randomness
1930 from the random number generator. After that the key is distributed to
1931 all clients on the channel. However, channel keys are cell specific thus
1932 the key is created only on the cell where the client, which left the
1933 channel, exists. While the server or router is creating the new channel
1934 key, no other client may join to the channel. Messages that are sent
1935 while creating the new key are still processed with the old key. After
1936 server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start
1937 using the new key. If server creates the new key the server MUST also
1938 send the new key to its router. See [SILC2] on more information about
1939 how channel messages must be encrypted and decrypted when router is
1942 When client receives the SILC_PACKET_CHANNEL_KEY packet with the
1943 Channel Key Payload it MUST process the key data to create encryption
1944 and decryption key, and to create the HMAC key that is used to compute
1945 the MACs of the channel messages. The processing is as follows:
1947 channel_key = raw key data
1948 HMAC key = hash(raw key data)
1950 The raw key data is the key data received in the Channel Key Payload.
1951 The hash() function is the hash function used in the HMAC of the channel.
1952 Note that the server MUST also save the channel key.
1956 4.5 Private Message Sending and Reception
1958 Private messages are sent point to point. Client explicitly destines
1959 a private message to specific client that is delivered to only to that
1960 client. No other client may receive the private message. The receiver
1961 of the private message is destined in the SILC Packet Header as any
1962 other packet as well.
1964 If the sender of a private message does not know the receiver's Client
1965 ID, it MUST resolve it from server. There are two ways to resolve the
1966 client ID from server; it is RECOMMENDED that client implementations
1967 send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client
1968 MAY also send SILC_COMMAND_WHOIS command to receive the Client ID.
1969 If the sender has received earlier a private message from the receiver
1970 it should have cached the Client ID from the SILC Packet Header.
1972 If server receives a private message packet which includes invalid
1973 destionation Client ID the server MUST send SILC_COMMAND_IDENTIFY
1974 command reply packet destined to the client with error status.
1976 See [SILC2] for description of private message encryption and decryption
1981 4.6 Private Message Key Generation
1983 Private message MAY be protected by the key generated by the client.
1984 The key may be generated and sent to the other client by sending packet
1985 SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network
1986 and is secured by session keys. After that the private message key
1987 is used in the private message communication between those clients.
1989 Other choice is to entirely use keys that are not sent through
1990 the SILC network at all. This significantly adds security. This key
1991 would be pre-shared-key that is known by both of the clients. Both
1992 agree about using the key and starts sending packets that indicate
1993 that the private message is secured using private message key.
1995 The key material used as private message key is implementation issue.
1996 However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate
1997 the key material. If the key is normal pre-shared-key or randomly
1998 generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then
1999 the key material SHOULD be processed as defined in the [SILC3]. In
2000 the processing, however, the HASH, as defined in [SILC3] MUST be
2001 ignored. After processing the key material it is employed as defined
2002 in [SILC3], however, the HMAC key material MUST be discarded.
2004 If the key is pre-shared-key or randomly generated the implementations
2005 should use the SILC protocol's mandatory cipher as the cipher. If the
2006 SKE was used to negotiate key material the cipher was negotiated as well.
2009 4.7 Channel Message Sending and Reception
2011 Channel messages are delivered to group of users. The group forms a
2012 channel and all clients on the channel receives messages sent to the
2015 Channel messages are destined to channel by specifying the Channel ID
2016 as Destination ID in the SILC Packet Header. The server MUST then
2017 distribute the message to all clients on the channel by sending the
2018 channel message destined explicitly to a client on the channel.
2020 See the [SILC2] for description of channel messege routing for router
2023 See [SILC2] for description of channel message encryption and decryption
2028 4.8 Session Key Regeneration
2030 Session keys MUST be regenerated periodically, say, once in an hour.
2031 The re-key process is started by sending SILC_PACKET_REKEY packet to
2032 other end, to indicate that re-key must be performed. The initiator
2033 of the connection SHOULD initiate the re-key.
2035 If perfect forward secrecy (PFS) flag was selected in the SILC Key
2036 Exchange protocol [SILC3] the re-key MUST cause new key exchange with
2037 SKE protocol. In this case the protocol is secured with the old key
2038 and the protocol results to new key material. See [SILC3] for more
2039 information. After the SILC_PACKET_REKEY packet is sent the sender
2040 will perform the SKE protocol.
2042 If PFS flag was set the resulted key material is processed as described
2043 in the section Processing the Key Material in [SILC3]. The difference
2044 with re-key in the processing is that the initial data for the hash
2045 function is just the resulted key material and not the HASH as it
2046 is not computed at all with re-key. Other than that, the key processing
2047 it equivalent to normal SKE negotiation.
2049 If PFS flag was not set, which is the default case, then re-key is done
2050 without executing SKE protocol. In this case, the new key is created by
2051 providing the current sending encryption key to the SKE protocol's key
2052 processing function. The process is described in the section Processing
2053 the Key Material in [SILC3]. The difference in the processing is that
2054 the initial data for the hash function is the current sending encryption
2055 key and not the SKE's KEY and HASH values. Other than that, the key
2056 processing is equivalent to normal SKE negotiation.
2058 After both parties has regenerated the session key, both MUST send
2059 SILC_PACKET_REKEY_DONE packet to each other. These packets are still
2060 secured with the old key. After these packets, the subsequent packets
2061 MUST be protected with the new key.
2065 4.9 Command Sending and Reception
2067 Client usually sends the commands in the SILC network. In this case
2068 the client simply sends the command packet to server and the server
2069 processes it and replies with command reply packet.
2071 However, if the server is not able to process the command, it is sent
2072 to the server's router. This is case for example with commands such
2073 as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there
2074 are other commands as well. For example, if client sends the WHOIS
2075 command requesting specific information about some client the server must
2076 send the WHOIS command to router so that all clients in SILC network
2077 are searched. The router, on the other hand, sends the WHOIS command
2078 further to receive the exact information about the requested client.
2079 The WHOIS command travels all the way to the server which owns the client
2080 and it replies with command reply packet. Finally, the server which
2081 sent the command receives the command reply and it must be able to
2082 determine which client sent the original command. The server then
2083 sends command reply to the client. Implementations should have some
2084 kind of cache to handle, for example, WHOIS information. Servers
2085 and routers along the route could all cache the information for faster
2086 referencing in the future.
2088 The commands sent by server may be sent hop by hop until someone is able
2089 to process the command. However, it is preferred to destine the command
2090 as precisely as it is possible. In this case, other routers en route
2091 MUST route the command packet by checking the true sender and true
2092 destination of the packet. However, servers and routers MUST NOT route
2093 command reply packets to clients coming from other server. Client
2094 MUST NOT accept command reply packet originated from anyone else but
2095 from its own server.
2099 4.10 Closing Connection
2101 When remote client connection is closed the server MUST send the notify
2102 type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels
2103 the client was joined. The server MUST also save the client's information
2104 for a period of time for history purposes.
2106 When remote server or router connection is closed the server or router
2107 MUST also remove all the clients that was behind the server or router
2108 from the SILC Network. The server or router MUST also send the notify
2109 type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all
2110 local clients that are joined on the same channels with the remote
2111 server's or router's clients.
2115 4.11 Detaching and Resuming a Session
2117 SILC protocol provides a possibility for a client to detach itself from
2118 the network without actually signing off from the network. The client
2119 connection to the server is closed but the client remains as valid client
2120 in the network. The client may then later resume its session back from
2121 any server in the network.
2123 When client wishes to detach from the network it MUST send the
2124 SILC_COMMAND_DETACH command to its server. The server then MUST set
2125 SILC_UMODE_DETACHED mode to the client and send SILC_NOTIFY_UMODE_CHANGE
2126 notify to its primary router, which will then MUST broadcast it further
2127 to other routers in the network. This user mode indicates that the
2128 client is detached from the network. Implementations MUST NOT use
2129 the SILC_UMODE_DETACHED flag to determine whether a packet can be sent
2130 to the client. All packets MUST still be sent to the client even if
2131 client is detached from the network. Only the server that originally
2132 had the active client connection is able to make the decision after it
2133 notices that the network connection is not active. In this case the
2134 default case is to discard the packet.
2136 The SILC_UMODE_DETACHED flag cannot be set by client itself directly
2137 with SILC_COMMAND_UMODE command, but only implicitly by sending the
2138 SILC_COMMAND_DETACH command. The flag also cannot be unset by the
2139 client, server or router with SILC_COMMAND_UMODE command, but only
2140 implicitly by sending and receiving the SILC_PACKET_RESUME_CLIENT
2143 When the client wishes to resume its session in the SILC Network it
2144 connects to a server in the network, which MAY also be a different
2145 from the original server, and performs normal procedures regarding
2146 creating a connection as described in section 4.1. After the SKE
2147 and the Connection Authentication protocols has been successfully
2148 completed the client MUST NOT send SILC_PACKET_NEW_CLIENT packet, but
2149 MUST send SILC_PACKET_RESUME_CLIENT packet. This packet is used to
2150 perform the resuming procedure. The packet MUST include the detached
2151 client's Client ID, which the client must know. It also includes
2152 Authentication Payload which includes signature made with the client's
2153 private key. The signature is computed as defined in the section
2154 3.9.1. Thus, the authentication method MUST be based in public key
2157 When server receives the SILC_PACKET_RESUME_CLIENT packet it MUST
2158 verify that the Client ID is valid client and that it has the
2159 SILC_UMODE_DETACHED mode set. It then MUST verify the Authentication
2160 Payload with the detached client's public key. If it does not have
2161 the public key it MUST retrieve it by sending SILC_COMMAND_GETKEY
2162 command to the server that has the public key from the original
2163 client connection. The server MUST NOT use the public key received
2164 in the SKE protocol for this connection. If the signature is valid
2165 the server MUST unset the SILC_UMODE_DETACHED flag, and send the
2166 SILC_PACKET_RESUME_CLIENT packet to its primary router. The routers
2167 MUST broadcast the packet and unset the SILC_UMODE_DETACHED flag
2168 when the packet is received. If the server is router server it also
2169 MUST send the SILC_PACKET_RESUME_CLIENT packet to the original server
2170 whom owned the detached client.
2172 The servers and routers that receives the SILC_PACKET_RESUME_CLIENT
2173 packet MUST know whether the packet already has been received for
2174 the client. It is protocol error to attempt to resume the client
2175 session from more than one server. The implementations could set
2176 internal flag that indicates that the client is resumed. If router
2177 receive SILC_PACKET_RESUME_CLIENT packet for client that is already
2178 resumed the client MUST be killed from the network. This would
2179 indicate that the client is attempting to resume the session more
2180 than once which is protocol error. In this case the router sends
2181 SILC_NOTIFY_TYPE_KILLED to the client. All routers that detect
2182 the same situation MUST also send the notify for the client.
2184 The servers and routers that receive the SILC_PACKET_RESUME_CLIENT
2185 must also understand that the client may not be found behind the
2186 same server that it originally came from. They must update their
2187 caches according this. The server that now owns the client session
2188 MUST check whether the Client ID of the resumed client is based
2189 on the server's Server ID. If it is not it MUST create new Client
2190 ID and send SILC_NOTIFY_TYPE_NICK_CHANGE to the network. It MUST
2191 also send the channel keys of all channels that the client is
2192 joined to the client since it does not have them. Whether the
2193 Client ID was changed or not the server MUST send SILC_PACKET_NEW_ID
2194 packet to the client. Only after this the client is resumed back
2195 to the network and may start sending packets and messages.
2197 It is also possible that the server does not know about the channels
2198 that the client has joined. In this case it MUST join client internally
2199 to the channels, generate new channel keys and distribute the keys
2200 to the channels as described in section 4.4.
2202 It is implementation issue for how long servers keep detached client
2203 sessions. It is RECOMMENDED that the detached sessions would be
2204 persistent as long as the server is running.
2208 5 Security Considerations
2210 Security is central to the design of this protocol, and these security
2211 considerations permeate the specification. Common security considerations
2212 such as keeping private keys truly private and using adequate lengths for
2213 symmetric and asymmetric keys must be followed in order to maintain the
2214 security of this protocol.
2216 Special attention must also be paid on the servers and routers that are
2217 running the SILC service. The SILC protocol's security depends greatly
2218 on the security and the integrity of the servers and administrators that
2219 are running the service. It is recommended that some form of registration
2220 is required by the server and router administrator prior acceptance to
2221 the SILC Network. Even though, the SILC protocol is secure in a network
2222 of mutual distrust between clients, servers, routers and adminstrators
2223 of the servers, the client should be able to trust the servers they are
2224 using if they whish to do so.
2226 It however must be noted that if the client requires absolute security
2227 by not trusting any of the servers or routers in the SILC Network, it can
2228 be accomplished by negotiating private keys outside the SILC Network,
2229 either using SKE or some other key exchange protocol, or to use some
2230 other external means for distributing the keys. This applies for all
2231 messages, private messages and channel messages.
2233 It is important to note that SILC, like any other security protocol is
2234 not full proof system and cannot secure from insecure environment; the
2235 SILC servers and routers could very well be compromised. However, to
2236 provide acceptable level of security and usability for end user the
2237 protocol use many times session keys or other keys generated by the
2238 servers to secure the messages. This is intentional design feature to
2239 allow ease of use for end user. This way the network is still usable,
2240 and remains encrypted even if the external means of distributing the
2241 keys is not working. The implementation, however, may like to not
2242 follow this design feature, and always negotiate the keys outside SILC
2243 network. This is acceptable solution and many times recommended. The
2244 implementation still must be able to work with the server generated keys.
2246 If this is unacceptable for the client or end user, the private keys
2247 negotiatied outside the SILC Network should always be used. In the end
2248 it is always implementor's choice whether to negotiate private keys by
2249 default or whether to use the keys generated by the servers.
2251 It is also recommended that router operators in the SILC Network would
2252 form a joint forum to discuss the router and SILC Network management
2253 issues. Also, router operators along with the cell's server operators
2254 should have a forum to discuss the cell management issues.
2260 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
2263 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
2264 Protocols", Internet Draft, April 2001.
2266 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001.
2268 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
2271 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
2274 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
2277 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
2280 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
2283 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
2286 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
2289 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
2292 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
2293 Infrastructure, Certificate and CRL Profile", RFC 2459,
2296 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
2297 John Wiley & Sons, New York, NY, 1996.
2299 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
2302 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
2303 RFC 2412, November 1998.
2305 [ISAKMP] Maughan D., et al, "Internet Security Association and
2306 Key Management Protocol (ISAKMP)", RFC 2408, November
2309 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
2310 (IKE)", RFC 2409, November 1998.
2312 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
2313 Authentication", RFC 2104, February 1997.
2315 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
2316 Specifications, Version 2.0", RFC 2437, October 1998.
2318 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
2319 Requirement Levels", BCP 14, RFC 2119, March 1997.
2321 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
2322 10646", RFC 2279, January 1998.
2331 Snellmanninkatu 34 A 15
2335 EMail: priikone@iki.fi
2337 This Internet-Draft expires XXX