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5 <title>SILC Protocol White Paper</title>
6 <link rev=made href="mailto:priikone@silcnet.org">
7 <meta name="Author" content="Pekka Riikonen - SILC Project">
8 <meta name="Description"
9 content="SILC - Secure Internet Live Conferencing Protocol">
10 <meta name="Created" content="Version 1.0 / 04 Aug 2001">
12 <body bgcolor="#ffffff">
14 <font face="Helvetica">
16 <font size="6"><b>SILC Protocol White Paper</b></font><br>
17 <font size="2">Version 1.0 / 04 Aug 2001</font>
22 Chat protocols are very popular on the Internet. They have actually
23 been very popular since the very first chat protocols appeared on the net.
24 The Internet Relay Chat (IRC) was one of the first chat protocols, and quickly
25 gained the status of being the most popular chat on the net. Today, IRC
26 has several competitors from various other so called Instant Messaging (IM)
27 protocols, such as ICQ. However, all of these different chat protocols
28 have something in common; they are all insecure.
31 The security is important feature in applications and protocols in
32 contemporary network environment. The older chat protocols, however have
33 failed to meet the growing security requirements on the Internet.
34 It is not anymore enough to just provide services, like for example
35 chat services. Now, they need to be secure services.
38 The Secure Internet Live Conferencing (SILC) protocol is a new generation
39 chat protocol which provides full featured conferencing services, just
40 like any other contemporary chat protocol provides. In addition, it
41 provides security by encrypting and authenticating the messages in
42 the network. The security has been the primary goal of the SILC protocol
43 and the protocol has been designed from the day one security in mind.
44 All packets and messages travelling in the SILC Network are always
45 encrypted and authenticated. The network topology is also different
46 from for example IRC network. The SILC network topology attempts to be
47 more powerful and scalable than the IRC network. The basic purpose
48 of the SILC protocol is to provide secure conferencing services.
51 The SILC Protocol have been developed as Open Source project. The
52 protocol specifications are freely available and they have been submitted to
53 the IETF. The very first implementations of the protocol are also already
57 <h1>About This White Paper</h1>
59 The purpose of this white paper is to give short but deep enough introduction
60 to the SILC Protocol. The document describes the purpose of the protocol
61 and how the protocol works in practice. This document is intended for all
62 audience. This document should be easy to understand for non-technical
63 person and still be detailed enough for technically oriented person. See
64 the section <a href="#terms">Terms and Abbreviations</a> for terms used
69 (c) Copyright 2001 Pekka Riikonen
70 (<a href="mailto:priikone at silcnet.org">priikone at silcnet.org</a>)
72 This document is free document; you can redistribute it and/or modify
73 it under the terms of the GNU General Public License as published by
74 the Free Software Foundation; either version 2 of the License, or
75 (at your option) any later version. This document is distributed in
76 the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
77 the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
78 See the GNU General Public License for more details.
82 <h1>SILC Protocol</h1>
85 The Secure Internet Live Conferencing (SILC) protocol provides secure
86 conferencing services over insecure network channel. The SILC is IRC
87 like protocol, however it does not support IRC. Strong cryptographic
88 methods are used to protect SILC packets inside the SILC network. SILC
89 provides all the common conferencing services like channels, channel
90 messages, private messages, nicknames and various commands. Difference
91 to other chat protocol is in the design of the protocol. The SILC
92 protocol has been designed from the day one security in mind and it
93 shows in the protocol design.
96 The network topology is also different to various other chat protocol,
97 like for example IRC. IRC has tree style network where SILC has so
98 called cellular network. A cell consists of a router, servers and clients.
99 The cell can also have backup routers in case the private router becomes
103 <img src="silc_network.png" alt="SILC Network" align="center" border"0">
106 The diagram above illustrates a portion of the SILC network. It shows
107 two cells that both has several servers, and backup routers and several
108 clients. Clients can connect to server and routers if they want to.
109 The following sections will describe the entities of the SILC Network
117 A client is a piece of software connecting to SILC server. The software
118 is usually run by the end user, a real person that is. The purpose of the
119 clients is to provide the end user an interface to the SILC services.
120 They are used to actually engage the conversations on the SILC Network,
121 and they can be used to execute various SILC commands.
124 The clients are distinquished from other clients by unique Client ID.
125 There cannot be multiple same Client IDs in the SILC Network at the same time.
126 The end user, however does not use Client IDs. The end users usually selects
127 a perferred nickname they want to use, and identifies themself with that
128 nickname to other users on the network. The nicknames are not unique in
129 the SILC Network. There can be multiple same nicknames at the same time
130 on the network. The maximum length for the nickname is 128 characters.
133 Most of the other chat protocols have unique nicknames. This is where SILC
134 differs from most of the other chat protocols. The purpose of this
135 feature is to make IRC style nickname wars obsolete, as no one owns their
136 nickname; there can always be somene else with the same nickname.
139 When client connects to the server the SILC Key Exchange (SKE) protocol and
140 SILC Connection Authentication protocol are executed. The result of the
141 SKE protocol is the session key that the client and server use to secure
142 their communication. All commands, for example, that the client sends
143 to the server are secured with the session key. The session key expires
144 periodically and the rekey process can be executed with or without the
145 Perfect Forward Secrecy (PFS). The connection authentication protocol is
146 used to authenticate the client to the server. The server may allow the
147 client to connect without authentication, or it may require a passphrase or
148 public key based authentication.
155 Servers forms the basis for the SILC Network, by providing a point to which
156 clients may connect. There are two kinds of servers in SILC; normal servers
157 and router servers. The next section describes the function of router
161 Normal servers connect to router server. Normal servers cannot directly
162 connect to other normal servers. Messages that are destined outside the
163 local server are always sent to the router for further routing.
164 The clients usually connect to the normal server, however, clients may
165 connect to router servers as well. The SILC Network diagram above
166 illustrates how normal servers connects to the router server.
169 The servers are distinquished by other servers in the network by unique
170 Server ID. There cannot be multiple same Server IDs in the SILC Network
171 at the same time. The servers keep track of local information. It knows
172 all locally connected clients and it knows all channels that its clients
173 have joined. However, it does not know any global information. It
174 usually does not keep track of global clients, however, it may cache
175 that information if it was queried. The reason for this is that the
176 server does not need to keep global information up to date and thus
177 makes the server faster (and in the end the entire network faster).
178 They can always query the information from the router.
181 When server connects to its router the SILC Key Exchange (SKE) protocol
182 and the SILC Connection Authentication protocol are executed, just like
183 when client connects to server. The SKE results in to the session key
184 that is used to secure the communication between the server and the
185 router. The connection authentication protocol is used to authenticate
186 the server to the router. The authentication is always based in either
187 passphrase or public key.
194 The router servers are servers that actually handles the message routing
195 in the network. They are, however also normal servers and they do accept
196 client connections. Each of the router in the network is called a cell.
197 A cell can have only one active router and it may have several servers
198 and several clients. The cell, however may have backup routers that can
199 take over the tasks of the primary router if it becomes unresponsive.
200 The switch to the backup router should be transparent and only local
201 connections to the primary router are lost. Other connections in the
202 cell are intact, and clients and servers merely experience some lag in
203 the network connection during the switch to the backup router.
206 The normal server knows only local information. Router server on the
207 other hand knows local information and global information. It considers
208 the cell as local and outside cells as global. It knows all the clients
209 connected to the network, all created channels, and all routers and servers
210 in the network. The server may query the global information if it is needed.
211 For example, when client sends WHOIS command, the server may query the
212 information from the router. If the router does not know all the details
213 that the WHOIS command requires it can query the information from a router
214 or a server which knows all the details. It may then cache that information.
217 The primary purpose of the router server is to route the messages to
218 local servers and local clients, and messages that are destined to outside
219 the cell are routed to the primary route or some other secondary
220 route if it is a faster route. The routers in the network forms a ring.
221 Each router has a primary route to other router in the network. Finally
222 the ring is closed by the last router using the first router in the
223 network as its primary route.
226 <img src="silc_routers.png" alt="SILC Routers" align="center" border"0">
229 The diagram above illustrates how the routers forms a ring in the network.
230 A router may have several secondary routes which it may use when it
234 When routers connect to its primary router the SKE and the SILC Connection
235 Authentication protocols are executed just like when normal server connects
236 to its router. The session key is used to secure the communication between
237 the routers. All the secondary routes also have their own session keys.
241 <h1>SILC Packet Protocol</h1>
244 The basis of SILC protocol relies in the SILC packets and they are with
245 out a doubt the most important part of the protocol. The SILC Packet
246 protocol is a binary packet protocol. The protocol provides secure
247 binary packets and assures that the contents of the packets are secured
251 Packets are used in the SILC protocol all the time to send for example
252 channel messages, private messages, commands and other information. All
253 packets in SILC network are always encrypted and their integrity is
254 assured by computed Message Authentication Codes (MAC). The protocol
255 defines several packet types and packet payloads. Each packet type
256 usually has a specific packet payload that actually defines the contents
257 of the packet. Hence, the actual data in the packet is the packet payload
258 defined in the protocol.
261 <img src="silc_packet.png" alt="Typical SILC Packet" align="center" border"0">
264 As the diagram above illustrates the SILC packet is constructed from the
265 SILC Packet Header that is included in all SILC packets, data area that
266 includes the packet payloads, and MAC area which assures the integrity of the
267 packet. Entire SILC packet is always encrypted, except for the MAC area
268 which is never encrypted. The encryption process and the key used,
269 however depends on the packet payload. Some of the payloads are encrypted
270 with the session key and some are encrypted with other keys, for example
271 with channel message keys. The SILC Packet Header is always encrypted with
272 the session key. The MAC is computed from the SILC Packet Header and the
273 data area before encrypting the packet.
277 <h1>SILC Key Exchange Protocol</h1>
280 SILC Key Exchange Protocol (SKE) is used to exchange shared secret
281 between connecting entities. The result of this protocol is a key material
282 used to secure the communication channel. This protocol is executed when,
283 for example client connects to server. It is also executed when server
284 connects to router. And, there is no reason why it could not be executed
285 between two clients too, if two clients would need to create secret key.
286 The purpose of the SKE protocol is to create session keys to be used
287 in current SILC session. The SKE is based on the Diffie-Hellman key
288 exchange algorithm, and is immune to man-in-the-middle attack.
291 This is the first protocol that is executed when creating connection to,
292 for example SILC server. All the other protocols are always executed
293 after this protocol. This way all the other protocols are secured since
294 the SKE creates the session key that is used to secure all subsequent
295 packets. The session keys created in the SKE are valid only for some
296 period of time (usually an hour) or at most until the session ends.
297 The rekey process can be executed with or without the Perfect Forward
301 The security properties that are used in the SILC session are also
302 negotiated during the SKE. The protocol has initiator and responder.
303 The initator is the one who starts the SKE negotiation and responder is
304 the one who receives the SKE negotiation. When the protocol is started
305 initiator sends a list of security properties that it supports. The
306 responder then selects the security properties it supports and sends
307 its reply to the initiator. The security properties includes ciphers,
308 hash functions, public key algorithms, HMAC functions and other
309 security properties. The responder can always choose the properties
313 After the security properties are selected the protocol continues
314 by performing the Diffie-Hellman key exchange algorithm. At the same
315 time the intiator and responder also sends their public keys or
316 certificates to each other. The responder also computes a signature
317 that the initiator will verify. It is also possible to perform a
318 mutual authentication when both of the parties computes a signature
319 which are verified by each other independently. If any of the phases
320 of the protocol are to fail the connection is closed immeadiately.
323 The public key or certificate that is received during the SKE protocol
324 must be verified. If it is not verified it would be possible to
325 execute a man-in-the-middle attack against the SKE protocol. If
326 certificates are used they can be verified by a third party Certification
327 Authority (CA). Verifying a public key requires either confirming
328 a fingerprint of the public key over phone or email, or the server
329 can for example publish the fingerprint (and the public key) on some
330 website. In real life systems accepting the public key without
331 verification, however is often desired. In many security protocols,
332 such as in SSH2, the public key is accepted without verification
333 in the first time when the connection is created. The public key is
334 then cached on local hard disk. When connecting next time to the
335 server the public key on local cache is verified against the public
336 key server sent. In real life this works most of the time. However,
337 if client (or server) cannot trust this, it must find some other way
338 to verify the received public key or certificate.
342 <h1>SILC Connection Authentication Protocol</h1>
345 Purpose of SILC Connection Authentication protocol is to authenticate the
346 connecting party with server or router. This protocol is executed when
347 for example client connects to server. It is also executed when server
348 connects to router. Its other purpose is to provide information for the
349 server about which type of connection it is. The type of the connection
350 defines whether it is client, server or router. If it is client then
351 the server will create a new Client ID for the client. If it is server
352 then it will except the server to send its Server ID. Server IDs are
353 created by the servers and routers itself.
356 Since the SILC Connection Authentication protocol is always executed after
357 the SKE protocol, session keys has been established already. This means
358 that all packets sent in the connection authentication protocol are encrypted
362 The authentication may be based either in passphrase or public key
363 encryption. It is also possible to not require authentication at all.
364 If the authentication is based to passphrase the passphrase is sent
365 to the server. As the packet sent by, for example client, is entirely
366 encrypted it is safe to send the passphrase inside the packet.
369 If the authentication is based to public key then, for example the client,
370 signs data with its private key and sends it to the server. The server
371 then verifies this signature by using the client's public key. The
372 packet is also encrypted in the case of public key authentication.
375 If the authentication is to fail the connection to the server or router
376 will be refused. If it is succesful the connection is granted. After
377 this the client is ready to communicate in the SILC Network.
384 A channel is a named group of one or more clients which will all receive
385 messages addressed to that channel. The channel is created when first
386 client joins to it, and the channel ceases to exist when the last client
387 leaves it. When channel exists, any client can reference it using the
388 name of the channel. Channel is a place where group of people can engage
392 Channel names are unique in the SILC Network. There cannot be multiple
393 same channels in the network at the same time. However, channel has also
394 a Channel ID which is actually used to reference the channel in the
395 SILC Network. The maximum length for the channel name is 256 characters.
398 Channels can have operators that can administrate the channel and operate
399 all of its modes. There are two types of operators on the channel:
400 channel founder and channel operator.
403 The channel founder is the client which created the channel. Channel
404 founder is channel operator with some more privileges. Channel founder
405 can operate all of the channel's modes. Furthermore, channel founder
406 privileges cannot be removed by any other operator on channel and channel
407 founder cannot be removed from the channel by force. It is also possible
408 for the channel founder to regain its privileges at later time, even if
409 they have left the channel.
412 Channel operator is operator that can operate most of the channel's
413 modes and administrate the channel. However, it cannot operate all
414 modes which are strictly reserved for channel founder. Channel operator
415 is, however able to adminstrate the channel, set some modes on the
416 channel, remove a badly behaving client from the channel, and promote
417 other clients to become channel operator.
421 <h2>Channel Message Delivery</h2>
424 All clients that have joined the channel can send messages to the channel.
425 All channel messages are secured and authenticated by channel key. The
426 channel key is generated by the server when the channel is created,
427 a client joins the channel, or a client leaves the channel. The channel
428 key is also regenerated periodically. The reason for the regeneration
429 of channel key everytime someone joins or leaves the channel is that
430 it prevents new clients joining the channel, and old clients leaving the
431 channel, to encrypt or decrypt old or new messages. They can encrypt
432 and decrypt channel messages only when they have joined on the channel.
435 Channel keys are cell specific in the SILC Network. Each cell that
436 have clients joined on a particular channel have also own key for the
437 channel. That key is not shared by other cells in the network. Inside
438 the cell the channel key is known by the router and all servers that
439 have clients on the channel and all clients that have joined the channel.
442 <OBJECT data="silc_channel.jpg" type="application/postscript">
443 <img src="silc_channel.png" alt="Channel Message Delivery" align="center" border"0">
447 The diagram above illustrates typical delivery of channel messages inside
448 a cell and between two cells. Both of the cells have their own channel
449 key. Both cells knows all clients joined on the channel. When message
450 is sent to the channel by an client, it is encrypted with the current
451 channel key in that cell. The servers and the router in the local cell
452 then routes the message to all local clients who have joined the channel.
453 If the channel has clients that belong to other cell in the network the
454 router will route the channel message to that cell. When channel
455 messages are sent between routers they are first decrypted with the
456 current channel key, and then re-encrypted with the session key shared
457 between the two routers. The router who receives the channel message
458 then decrypts it with the session and re-encrypts it with the
459 current channel key in that cell. It then distributes the channel message
460 to all clients on the channel. The clients who have joined the channel
461 always knows the current channel key and can decrypt all channel messages
462 they receive. Note that normal servers in the SILC network never decrypt
463 the channel messages even though the have the key. There is no reason
464 for servers to decrypt the message. The router decrypts the message
465 only when sending it between two routers.
468 This method of channel message delivery is the the default way to send
469 channel messages in the SILC Network. However, this is not perfect
470 solution on all circumstances. If the clients joined on a particular
471 channel cannot trust, or do not want to trust the servers and routers
472 in the SILC Network they can consider the fact, that servers and routers
473 knows the channel key is actually a breach of security.
476 If the clients on the other hand can trust their servers and routers
477 in the SILC Network this is the recommended way of sending channel
478 messages. This method is the simplest method for end user since it
479 does not require any special settings before engaging the conversation
480 on the channel. The client merely joins the channel, receives the
481 channel key from the server and can start the conversation on the
485 In addition of encrypting channel messages it also possible to digitally
486 sign all sent channel messages. The receiver could the verify the
487 signature of each of the message using the sender's public key.
491 <h2>Channel Message Delivery With Channel Private Key</h2>
494 If the clients cannot trust the servers and routers in the SILC Network
495 they should not use the default way of sending the channel messages.
496 Instead, they should use channel private keys to encrypt and decrypt
497 the channel messages. Channel private keys are keys that are known
498 only by the clients who have joined the channel. All servers and
499 routers do not know the key and cannot decrypt the messages. When
500 message is sent between two routers they are merely re-encrypted with
501 the session key but not decrypted since the router do not have the
505 The clients who have joined the channel must first agree on the channel
506 private key they are going to use. The key may generally be anything.
507 It may be a passphrase or a random string, or the key may negotiated
508 using some key exchange protocol which provides negotiating the
509 key for multiple clients at the same time.
512 As the channel private key is actually entirely local setting in the
513 client, it is possible to set several channel private keys for one
514 channel. It is possible to have multiple channel private keys that
515 are not known by all channel members. When encrypting messages with
516 one channel private key only the clients who have that key can decrypt
517 the message. The other key could be shared for example by all clients
518 on the channel and thus all clients can decrypt messages encrypted with
519 that key. In this way it is actually possible to have a private group
520 conversation inside the channel while having global conversation at the
525 <h1>Private Messages</h1>
527 Private messages are messages that are sent from one client to another
528 through the SILC Network. They are private because they are not sent to
529 anyone else except to the true receiver of the message. Private messages
530 can be used to engage private conversation with another client if channels
534 As all messages in SILC the private message are also encrypted and
535 authenticated. There are several ways to secure private messages. By
536 default private messages are encrypted using the session keys established
537 in the SKE protocol. It is also possible to negotiate a private message
538 key between the two clients and encrypt the messages with that key. It
539 is even possible to encrypt the messages with public key cryptosystem,
540 if desired. The next sections will describe all these private message
544 The SILC protocol provides these three methods of delivering private messages
545 because none of the methods alone can satisfy the security requirements
546 of all people. The end user should decide the acceptable level of risk,
547 the required level of security and other security and usability aspects when
548 deciding what way of sending private message suites for them.
551 In addition of encrypting private messages it also possible to digitally
552 sign all sent private messages. The receiver could the verify the
553 signature of each of the message using the sender's public key.
557 <h2>Private Message Delivery With Session Keys</h2>
559 Sending private messages are by default secured with session keys established
560 in the SKE protocol. This means that the private message is always encrypted
561 with the session key of the next receiver of the message enroute to the
562 receiving client. This also means that the message is decrypted and
563 re-encrypted everytime it is sent further to the receiving client.
566 <img src="silc_priv1.png" alt="Basic Private Message Delivery" align="center" border"0">
569 As the above diagram shows the private messages sent by Client A to the
570 Client B travels through the SILC Network and is always decrypted and
571 re-encrypted with the session key of the next receiver. The Client B then
572 finally decrypts the private messages that is encrypted with the session
573 key shared between the Client B and the Server Y.
576 This way of securing private messages is not perfect and cannot be used
577 in all circumstances. If the clients having the conversation cannot trust
578 the servers and routers in the SILC Network they should not send private
579 messages that are secured in this manner. Messages secured in this manner
580 can be decrypted by the servers and routers that the clients may consider
584 If the clients on the other hand trust the servers and routers in their
585 SILC Network, or they do not care that servers can decrypt their messages,
586 sending private messages in this way is very simple from client's point
587 of view. For servers and routers this of course means that they need
588 to decrypt and re-encrypt each private message. Since this way of securing
589 private message cannot be used at all times the SILC protocol provides
590 other ways of securing private messages.
594 <h2>Private Message Delivery With Private Message Key</h2>
596 Private messages can be secured with private message key as well. This
597 key is known only by the sender of the message and the receiver of the
598 message. This way no one else except the sender and the receiver can encrypt
599 and decrypt the private messages. The message is encrypted by the sender
600 with the private message key and all the servers and routers pass the message
601 through enroute to the receiver. They cannot decrypt the message since
602 they do not have the key. When sending private messages in this way it
603 does not matter whether the clients trust or do not trust the servers and
604 routers in the SILC network.
607 <img src="silc_priv2.png" alt="Private Messages with Private Message Key" align="center" border"0">
610 As the above diagram shows the Client A encrypts the message with private
611 message key and sends the message to the SILC Network. All servers and
612 routers merely pass the message through since they cannot decrypt it.
613 The Client B then receives the message and decrypts it with the private
617 Sending private messages in this manner is always secure since the key is
618 shared only by the sender and the receiver. The problem of this method
619 is that the sender and the receiver must somehow agree about the key
620 they are going to use. The private message key can generally be anything.
621 It can be a passphrase that only the sender and the receiver knows. They
622 could have been agreed to use some word or phrase as the key sometime
623 earlier before they started the conversation. Or the key maybe from some
624 random string from a code book that only the sender and the receiver poses.
625 Or it can be a key that is negotiated using some key exchange protocol.
628 The problem however is fundamental. How to agree to use some key when
629 you cannot reach the other person over secure channel? The SILC protocol
630 solves this problem by providing a possiblity to negotiate the key
631 between two clients using the SKE protocol. One or both of the clients
632 can set up the SKE server running in their host and ask the other client
633 to connect to it. As a result of the SKE protocol the clients have
634 now shared secret that they can use as private message key. The key
635 is known only by the two clients that exexcuted the SKE protocol. They
636 can then use that key to secure all subsequent private messages.
639 Using this method of private messages delivery is recommended if the
640 clients cannot trust the servers and routers in the SILC Network. The
641 drawback is the extra phase of setting the private message key before
642 starting the conversation. However, using the SKE protocol is the
643 recommended way to negotiate the private message key since it can be
644 automatized and does not cause any extra tasks for end user.
648 <h2>Private Message Delivery With Public Key Encryption</h2>
650 If the clients cannot trust the servers and routers in the SILC Network
651 they can use the private message key. As described in the previous section
652 it is easy to set up with the SKE protocol. However, sometimes the two
653 clients do not want to use any passphrases as private message key or
654 negotiate the key with SKE, or perhaps they are unable to negotiate the
655 key because of some other external problem. The SILC protocol provides
656 yet another way of securing the private messages. This way does not
657 require setting or negotiating private message key. And, in this method
658 also it does not matter whether the clients trust or do not trust the
659 servers and routers in the SILC Network. The method is public key
660 encryption. The clients can encrypt the private messages with the
661 receiver's public key and send the message to the network. The servers
662 and routers cannot decrypt the messages since they do not have the
663 receiver's private key. The receiver on the other hand has the private
664 key which it uses to decrypt the message.
667 <img src="silc_priv3.png" alt="Private Messges with Public Key Cryptosystem" align="center" border"0">
670 As the above diagram shows the Client A has the Client B's public key.
671 It will encrypt the message with that key and sends the message to the
672 SILC Network. All servers and routers pass the message through since
673 they cannot decrypt it. The Client B then uses its private key to
674 decrypt the message. The Client B has also the Client A's public key
675 that it can use to encrypt messages that it will send to Client A.
678 Even this method of private message delivery is not perfect. The drawbacks
679 of this method is that the public key encryption process, as being
680 asymmetric cryptosystem, is much slower than encryption process with
681 symmetric cryptosystems. This is not probably problem with short messages
682 but may be inconvenient with long messages. The other drawback is that the
683 sender must first assure that the public key it is using in the encryption
684 is actually the receiver's public key. This is a absolute requirement
685 in this method. If the sender cannot authenticate the receiver's public
686 key this method of private message delivery should not be used. In SILC
687 protocol clients can fetch other clients public keys from servers.
688 However, the servers may not have authenticated the fetched public key so
689 that should not be fully trusted. Use of certificates can solve the
690 problem. The receiver's certificate could be authenticated by a third
691 party Certification Authority (CA).
694 Usually verifying the public key is not a problem since the receiver
695 can provide the public key on some website, or verify the fingerprint of
696 the key over email, or phone call. The clients can also fetch the
697 public keys from SILC servers if they trust that the keys are authentic.
698 If both of the clients trust that the public keys are authentic using this
699 method of private message delivery is very simple and recommended.
705 The Secure Internet Live Conferencing (SILC) protocol is a new generation
706 chat protocol that provides all the common conferencing services with
707 strong support for security. It has wide range of security properties
708 that should meet the highest levels of security requirements, while not
709 forgetting easy of use. The network topology offers new architectural
710 solution with better scalability over traditional chat protocols.
714 <h1>Further Information</h1>
716 More detailed information about the SILC protocol is available in the
717 SILC protocol specification documents. There exists currently four
718 Internet Drafts that defines the protocol in great detail. The Internet
719 Drafts are available from the following sources but also from the
720 <a href="http://www.ietf.org">IETF website</a>.
723 - <a href="http://silcnet.org/docs/draft-riikonen-silc-spec-03.txt">
724 Secure Internet Live Conferencing (SILC), Protocol Specification</a>
727 - <a href="http://silcnet.org/docs/draft-riikonen-silc-pp-03.txt">
728 SILC Packet Protocol</a>
731 - <a href="http://silcnet.org/docs/draft-riikonen-silc-ke-auth-03.txt">
732 SILC Key Exchange and Authentication Protocols</a>
735 - <a href="http://silcnet.org/docs/draft-riikonen-silc-commands-01.txt">
739 For comprehensive introduction to cryptography refer to the
740 <a href="http://www.ssh.com/tech/crypto/">Cryptography A-2-Z document</a>.
744 <h1>Terms and Abbreviations</h1>
747 - Asymmetric cryptosystem
749 Asymmetric cryptosystem provides public encryption. It has two keys,
750 one public key and one private key (also called as secret key). The public
751 key is publicly available allowing anyone to encrypt messages with the
752 public key. Only the posessor of the private key can decrypt those messages.
753 Difference to symmetric cryptosystem is that symmetric cryptosystem use only
754 one key, and the key is usually used to both encryption and decryption. The
755 asymmetric cryptosystem is also called as public key encryption, public key
756 cryptosystem or public key algorithm. SILC supports RSA and DSS asymmetric
762 The verification of the identity of a person, host or process in order
763 to gain access to a service or prove identity. In data communications
764 it also means verifying the origin of a message.
769 Certificate is a digital document which can be used to verify the
770 identity of a person or host. In SILC certificates can be used to prove
771 identity of clients, servers and routers. Basically certificate is a public
772 key with subject name. SILC supports X.509, OpenPGP and SPKI certificates.
773 Supported public keys are SILC style public key and SSH2 style public
777 - Certification Authority (CA)
779 A third party entity that can verify identity of a person or host. CA
780 is usually external company that provides certificates and their
781 verification services.
784 - Diffie-Hellman key exchange
786 First public key algorithm ever invented. It is used to generate a secret
787 key between two or more parties. It gets its security from the difficulty
788 of calculating discrete lograrithms.
793 A mechanism (usually mathematical) to transfer plaintext (or cleartext)
794 to ciphertext to provide confidentiality. A process to transfer
795 the ciphertext back to plaintext is called decryption.
800 The verification of data to detect any modifications. If data is
801 modified enroute from the sender to the receiver, the modification will
807 Hash Message Authentication Code. Also called as keyed hash function.
808 It is a secret key authentication algorithm which proves that the message
809 is not modified and that the HMAC was computed by the sender of the
813 - Man-in-the-middle attack
815 An attack against two connecting entities where the attacker executes
816 key exchange protocol with both of the parties indepently without
817 their knowledge. Both of the connecting entities will end up having secret
818 key with the attacker, and the attacker can encrypt and decrypt all the
819 messages that goes between the two entities.
822 - Message Authentication Code (MAC)
824 MAC provides message integrity by computing the MAC using a secret
825 key authentication algorithm (HMAC).
828 - Perfect Forward Secrecy (PFS)
830 A property of rekey (or key re-generation) which defines whether the
831 new key is derived from the old key. If Perfect Forward Secrecy is
832 selected the new key is never dependent of the old key which means
833 that if the old key would get compromised at later time it will not
834 compromise the new key. In SILC setting PFS in the SKE protocol means
835 executing the SKE protocol again. If PFS is not selected the new key
836 is always derived from the old key.
841 A key re-generation process where the old key has expired or is not
842 secure anymore to use. In this case rekey is performed and new key
846 - Symmetric cryptosystem
848 Symmetric cryptosystem is one key cryptosystem where one key is used
849 usually to both encryption and decryption process. The symmetric
850 cryptosystems are usually significantly faster than asymmetric cryptosystems.
851 DES, AES, Twofish and Blowfish are examples of symmetric cryptosystems.
852 SILC supports all the common symmetric cryptosystems including AES.
853 SILC does not support DES as it is insecure and 3DES as it is too slow.
projects / silc.git / blob