8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-silc-ke-auth-05.txt 15 May 2002
20 Expires: 15 November 2002
25 SILC Key Exchange and Authentication Protocols
26 <draft-riikonen-silc-ke-auth-05.txt>
31 This document is an Internet-Draft and is in full conformance with
32 all provisions of Section 10 of RFC 2026. Internet-Drafts are
33 working documents of the Internet Engineering Task Force (IETF), its
34 areas, and its working groups. Note that other groups may also
35 distribute working documents as Internet-Drafts.
37 Internet-Drafts are draft documents valid for a maximum of six months
38 and may be updated, replaced, or obsoleted by other documents at any
39 time. It is inappropriate to use Internet-Drafts as reference
40 material or to cite them other than as "work in progress."
42 The list of current Internet-Drafts can be accessed at
43 http://www.ietf.org/ietf/1id-abstracts.txt
45 The list of Internet-Draft Shadow Directories can be accessed at
46 http://www.ietf.org/shadow.html
48 The distribution of this memo is unlimited.
54 This memo describes two protocols used in the Secure Internet Live
55 Conferencing (SILC) protocol, specified in the Secure Internet Live
56 Conferencing, Protocol Specification internet-draft [SILC1]. The
57 SILC Key Exchange (SKE) protocol provides secure key exchange between
58 two parties resulting into shared secret key material. The protocol
59 is based on Diffie-Hellman key exchange algorithm and its functionality
60 is derived from several key exchange protocols. SKE uses best parts
61 of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol
62 and the OAKLEY Key Determination protocol [OAKLEY].
64 The SILC Connection Authentication protocol provides user level
65 authentication used when creating connections in SILC network. The
66 protocol is transparent to the authentication data which means that it
67 can be used to authenticate the user with, for example, passphrase
68 (pre-shared-secret) or public key (and certificate).
76 1 Introduction .................................................. 2
77 1.1 Requirements Terminology .................................. 3
78 2 SILC Key Exchange Protocol .................................... 3
79 2.1 Key Exchange Payloads ..................................... 4
80 2.1.1 Key Exchange Start Payload .......................... 4
81 2.1.2 Key Exchange Payload ................................ 8
82 2.2 Key Exchange Procedure .................................... 10
83 2.3 Processing the Key Material ............................... 12
84 2.4 SILC Key Exchange Groups .................................. 14
85 2.4.1 diffie-hellman-group1 ............................... 14
86 2.4.2 diffie-hellman-group2 ............................... 14
87 2.5 Key Exchange Status Types ................................. 15
88 3 SILC Connection Authentication Protocol ....................... 16
89 3.1 Connection Auth Payload ................................... 18
90 3.2 Connection Authentication Types ........................... 19
91 3.2.1 Passphrase Authentication ........................... 19
92 3.2.2 Public Key Authentication ........................... 20
93 3.3 Connection Authentication Status Types .................... 20
94 4 Security Considerations ....................................... 21
95 5 References .................................................... 21
96 6 Author's Address .............................................. 22
103 Figure 1: Key Exchange Start Payload
104 Figure 2: Key Exchange Payload
105 Figure 3: Connection Auth Payload
111 This memo describes two protocols used in the Secure Internet Live
112 Conferencing (SILC) protocol specified in the Secure Internet Live
113 Conferencing, Protocol Specification Internet-Draft [SILC1]. The
114 SILC Key Exchange (SKE) protocol provides secure key exchange between
115 two parties resulting into shared secret key material. The protocol
116 is based on Diffie-Hellman key exchange algorithm and its functionality
117 is derived from several key exchange protocols. SKE uses best parts
118 of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol
119 and the OAKLEY Key Determination protocol.
121 The SILC Connection Authentication protocol provides user level
122 authentication used when creating connections in SILC network. The
123 protocol is transparent to the authentication data which means that it
124 can be used to authenticate the user with, for example, passphrase
125 (pre-shared- secret) or public key (and certificate).
127 The basis of secure SILC session requires strong and secure key exchange
128 protocol and authentication. The authentication protocol is entirely
129 secured and no authentication data is ever sent in the network without
130 encrypting and authenticating it first. Thus, authentication protocol
131 may be used only after the key exchange protocol has been successfully
134 This document refers constantly to other SILC protocol specification
135 Internet Drafts that are a must read for those who wants to understand
136 the function of these protocols. The most important references are
137 the Secure Internet Live Conferencing, Protocol Specification [SILC1]
138 and the SILC Packet Protocol [SILC2] Internet Drafts.
140 The protocol is intended to be used with the SILC protocol thus it
141 does not define own framework that could be used. The framework is
142 provided by the SILC protocol.
146 1.1 Requirements Terminology
148 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
149 MAY, and OPTIONAL, when they appear in this document, are to be
150 interpreted as described in [RFC2119].
154 2 SILC Key Exchange Protocol
156 SILC Key Exchange Protocol (SKE) is used to exchange shared secret
157 between connecting entities. The result of this protocol is a key
158 material used to secure the communication channel. The protocol uses
159 Diffie-Hellman key exchange algorithm and its functionality is derived
160 from several key exchange protocols. SKE uses best parts of the SSH2
161 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY
162 Key Determination protocol. The protocol does not claim any conformance
163 to any of these protocols, they were merely used as a reference when
164 designing this protocol.
166 The purpose of SILC Key Exchange protocol is to create session keys to
167 be used in current SILC session. The keys are valid only for some period
168 of time (usually an hour) or at most until the session ends. These keys
169 are used to protect packets like commands, command replies and other
170 communication between two entities. If connection is server to router
171 connection, the keys are used to protect all traffic between those
172 servers. In client connections usually all the packets are protected
173 with this key except channel messages; channels has their own keys and
174 they are not exchanged with this protocol.
176 The Diffie-Hellman implementation used in the SILC SHOULD be compliant
181 2.1 Key Exchange Payloads
183 During the key exchange procedure public data is sent between initiator
184 and responder. This data is later used in the key exchange procedure.
185 There are several payloads used in the key exchange. As for all SILC
186 packets, SILC Packet Header, described in [SILC2], is at the start of
187 all packets. The same is done with these payloads as well. All the
188 fields in the payloads are always in MSB (most significant byte first)
189 order. Following descriptions of these payloads.
193 2.1.1 Key Exchange Start Payload
195 The key exchange between two entities MUST be started by sending the
196 SILC_PACKET_KEY_EXCHANGE packet containing Key Exchange Start Payload.
197 Initiator sends the Key Exchange Start Payload to the responder filled
198 with all security properties it supports. The responder then checks
199 whether it supports the security properties.
201 It then sends a Key Exchange Start Payload to the initiator filled with
202 security properties it selected from the original payload. The payload
203 sent by responder MUST include only one chosen property per list. The
204 character encoding for the security property values as defined in [SILC1]
205 SHOULD be UTF-8 [RFC2279].
207 The Key Exchange Start Payload is used to tell connecting entities what
208 security properties and algorithms should be used in the communication.
209 The Key Exchange Start Payload is sent only once per session. Even if
210 the PFS (Perfect Forward Secrecy) flag is set the Key Exchange Start
211 Payload is not re-sent. When PFS is desired the Key Exchange Payloads
212 are sent to negotiate new key material. The procedure is equivalent to
213 the very first negotiation except that the Key Exchange Start Payload
216 As this payload is used only with the very first key exchange the payload
217 is never encrypted, as there are no keys to encrypt it with.
219 A cookie is also sent in this payload. A cookie is used to randomize the
220 payload so that none of the key exchange parties can determine this
221 payload before the key exchange procedure starts. The cookie MUST be
222 returned to the original sender by the responder.
224 Following diagram represents the Key Exchange Start Payload. The lists
225 mentioned below are always comma (`,') separated and the list MUST NOT
226 include white spaces (` ').
232 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
233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
234 | RESERVED | Flags | Payload Length |
235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
243 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
244 | Version String Length | |
245 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
249 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
250 | Key Exchange Grp Length | |
251 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
253 ~ Key Exchange Groups ~
255 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
256 | PKCS Alg Length | |
257 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
261 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
262 | Encryption Alg Length | |
263 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
265 ~ Encryption Algorithms ~
267 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
268 | Hash Alg Length | |
269 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
273 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
275 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
279 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
280 | Compression Alg Length | |
281 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
283 ~ Compression Algorithms ~
285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
289 Figure 1: Key Exchange Start Payload
294 o RESERVED (1 byte) - Reserved field. Sender fills this with
297 o Flags (1 byte) - Indicates flags to be used in the key
298 exchange. Several flags can be set at once by ORing the
299 flags together. The following flags are reserved for this
304 In this case the field is ignored.
308 If set the receiver of the payload does not reply to
313 Perfect Forward Secrecy (PFS) to be used in the
314 key exchange protocol. If not set, re-keying
315 is performed using the old key. See the [SILC1]
316 for more information on this issue. When PFS is
317 used, re-keying and creating new keys for any
318 particular purpose MUST cause new key exchange.
319 In this key exchange only the Key Exchange Payload
320 is sent and the Key Exchange Start Payload MUST
321 NOT be sent. When doing PFS the Key Exchange
322 Payloads are encrypted with the old keys.
324 Mutual Authentication 0x04
326 Both of the parties will perform authentication
327 by providing signed data for the other party to
328 verify. By default, only responder will provide
329 the signature data. If this is set then the
330 initiator must also provide it. Initiator MAY
331 set this but also responder MAY set this even if
332 initiator did not set it.
334 Rest of the flags are reserved for the future and
337 o Payload Length (2 bytes) - Length of the entire Key Exchange
338 Start payload, not including any other field.
340 o Cookie (16 bytes) - Cookie that randomize this payload so
341 that each of the party cannot determine the payload before
344 o Version String Length (2 bytes) - The length of the Version
345 String field, not including any other field.
347 o Version String (variable length) - Indicates the version of
348 the sender of this payload. Initiator sets this when sending
349 the payload and responder sets this when it replies by sending
350 this payload. See [SILC1] for definition of the version
353 o Key Exchange Grp Length (2 bytes) - The length of the
354 key exchange group list, not including any other field.
356 o Key Exchange Group (variable length) - The list of
357 key exchange groups. See the section 2.4 SILC Key Exchange
358 Groups for definitions of these groups.
360 o PKCS Alg Length (2 bytes) - The length of the PKCS algorithms
361 list, not including any other field.
363 o PKCS Algorithms (variable length) - The list of PKCS
366 o Encryption Alg Length (2 bytes) - The length of the encryption
367 algorithms list, not including any other field.
369 o Encryption Algorithms (variable length) - The list of
370 encryption algorithms.
372 o Hash Alg Length (2 bytes) - The length of the Hash algorithm
373 list, not including any other field.
375 o Hash Algorithms (variable length) - The list of Hash
376 algorithms. The hash algorithms are mainly used in the
379 o HMAC Length (2 bytes) - The length of the HMAC list, not
380 including any other field.
382 o HMACs (variable length) - The list of HMACs. The HMAC's
383 are used to compute the Message Authentication Codes (MAC)
386 o Compression Alg Length (2 bytes) - The length of the
387 compression algorithms list, not including any other field.
389 o Compression Algorithms (variable length) - The list of
390 compression algorithms.
395 2.1.2 Key Exchange Payload
397 Key Exchange payload is used to deliver the public key (or certificate),
398 the computed Diffie-Hellman public value and possibly signature data
399 from one party to the other. When initiator is using this payload
400 and the Mutual Authentication flag is not set then the initiator MUST
401 NOT provide the signature data. If the flag is set then the initiator
402 MUST provide the signature data so that the responder can verify it.
404 The Mutual Authentication flag is usually used when a separate
405 authentication protocol will not be executed for the initiator of the
406 protocol. This is case for example when the SKE is performed between
407 two SILC clients. In normal case, where client is connecting to a
408 server, or server is connecting to a router the Mutual Authentication
409 flag may be omitted. However, if the connection authentication protocol
410 for the connecting entity is not based on public key authentication (it
411 is based on passphrase) then the Mutual Authentication flag SHOULD be
412 enabled. This way the connecting entity has to provide proof of
413 possession of the private key for the public key it will provide in
414 SILC Key Exchange protocol.
416 When performing re-key with PFS selected this is the only payload that
417 is sent in the SKE protocol. The Key Exchange Start Payload MUST NOT
418 be sent at all. However, this payload does not have all the fields
419 present. In the re-key with PFS the public key and a possible signature
420 data SHOULD NOT be present. If they are present they MUST be ignored.
421 The only field that is present is the Public Data that is used to create
422 the new key material. In the re-key the Mutual Authentication flag, that
423 may be set in the initial negotiation, MUST also be ignored.
425 This payload is sent inside SILC_PACKET_KEY_EXCHANGE_1 and inside
426 SILC_PACKET_KEY_EXCHANGE_2 packet types. The initiator uses the
427 SILC_PACKET_KEY_EXCHANGE_1 and the responder the latter.
429 The following diagram represent the Key Exchange Payload.
435 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
436 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
437 | Public Key Length | Public Key Type |
438 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
440 ~ Public Key of the party (or certificate) ~
442 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
443 | Public Data Length | |
444 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
448 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
449 | Signature Length | |
450 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
454 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
458 Figure 2: Key Exchange Payload
462 o Public Key Length (2 bytes) - The length of the Public Key
463 (or certificate) field, not including any other field.
465 o Public Key Type (2 bytes) - The public key (or certificate)
466 type. This field indicates the type of the public key in
467 the packet. Following types are defined:
469 1 SILC style public key (mandatory)
470 2 SSH2 style public key (optional)
471 3 X.509 Version 3 certificate (optional)
472 4 OpenPGP certificate (optional)
473 5 SPKI certificate (optional)
475 The only required type to support is type number 1. See
476 [SILC1] for the SILC public key specification. See
477 SSH public key specification in [SSH-TRANS]. See X.509v3
478 certificate specification in [PKIX-Part1]. See OpenPGP
479 certificate specification in [PGP]. See SPKI certificate
480 specification in [SPKI]. If this field includes zero (0)
481 or unsupported type number the protocol MUST be aborted
482 sending SILC_PACKET_FAILURE message and the connection SHOULD
483 be closed immediately.
485 o Public Key (or certificate) (variable length) - The
486 public key or certificate of the party. This public key
487 is used to verify the digital signature. The public key
488 or certificate in this field is encoded in the manner as
489 defined in their respective definitions; see previous field.
491 o Public Data Length (2 bytes) - The length of the Public Data
492 field, not including any other field.
494 o Public Data (variable length) - The public data to be
495 sent to the receiver (Diffie-Hellman public values). See
496 section 2.2 Key Exchange Procedure for detailed description
497 how this field is computed. This value is binary encoded.
499 o Signature Length (2 bytes) - The length of the signature,
500 not including any other field.
502 o Signature Data (variable length) - The signature signed
503 by the sender. The receiver of this signature MUST
504 verify it. The verification is done using the sender's
505 public key. See section 2.2 Key Exchange Procedure for
506 detailed description how to produce the signature. If
507 the Mutual Authentication flag is not set then initiator
508 MUST NOT provide this field and the Signature Length field
509 MUST be set to zero (0) value. If the flag is set then
510 also the initiator MUST provide this field. The responder
511 MUST always provide this field.
516 2.2 Key Exchange Procedure
518 The key exchange begins by sending SILC_PACKET_KEY_EXCHANGE packet with
519 Key Exchange Start Payload to select the security properties to be used
520 in the key exchange and later in the communication.
522 After Key Exchange Start Payload has been processed by both of the
523 parties the protocol proceeds as follows:
526 Setup: p is a large and public safe prime. This is one of the
527 Diffie Hellman groups. q is order of subgroup (largest
528 prime factor of p). g is a generator and is defined
529 along with the Diffie Hellman group.
531 1. Initiator generates a random number x, where 1 < x < q,
532 and computes e = g ^ x mod p. The result e is then
533 encoded into Key Exchange Payload, with the public key
534 (or certificate) and sent to the responder.
536 If the Mutual Authentication flag is set then initiator
537 MUST also produce signature data SIGN_i which the responder
538 will verify. The initiator MUST compute a hash value
539 HASH_i = hash(Initiator's Key Exchange Start Payload |
540 public key (or certificate) | e). It then signs the HASH_i
541 value with its private key resulting a signature SIGN_i.
543 2. Responder generates a random number y, where 1 < y < q,
544 and computes f = g ^ y mod p. It then computes the
545 shared secret KEY = e ^ y mod p, and, a hash value
546 HASH = hash(Initiator's Key Exchange Start Payload |
547 public key (or certificate) | Initiator's public key
548 (or certificate) | e | f | KEY). It then signs
549 the HASH value with its private key resulting a signature
552 It then encodes its public key (or certificate), f and
553 SIGN into Key Exchange Payload and sends it to the
556 If the Mutual Authentication flag is set then the responder
557 SHOULD verify that the public key provided in the payload
558 is authentic, or if certificates are used it verifies the
559 certificate. The responder MAY accept the public key without
560 verifying it, however, doing so may result to insecure key
561 exchange (accepting the public key without verifying may be
562 desirable for practical reasons on many environments. For
563 long term use this is never desirable, in which case
564 certificates would be the preferred method to use). It then
565 computes the HASH_i value the same way initiator did in the
566 phase 1. It then verifies the signature SIGN_i from the
567 payload with the hash value HASH_i using the received public
570 3. Initiator verifies that the public key provided in
571 the payload is authentic, or if certificates are used
572 it verifies the certificate. The initiator MAY accept
573 the public key without verifying it, however, doing
574 so may result to insecure key exchange (accepting the
575 public key without verifying may be desirable for
576 practical reasons on many environments. For long term
577 use this is never desirable, in which case certificates
578 would be the preferred method to use).
580 Initiator then computes the shared secret KEY =
581 f ^ x mod p, and, a hash value HASH in the same way as
582 responder did in phase 2. It then verifies the
583 signature SIGN from the payload with the hash value
584 HASH using the received public key.
587 If any of these phases is to fail the SILC_PACKET_FAILURE MUST be sent
588 to indicate that the key exchange protocol has failed, and the connection
589 SHOULD be closed immediately. Any other packets MUST NOT be sent or
590 accepted during the key exchange except the SILC_PACKET_KEY_EXCHANGE_*,
591 SILC_PACKET_FAILURE and SILC_PACKET_SUCCESS packets.
593 The result of this protocol is a shared secret key material KEY and
594 a hash value HASH. The key material itself is not fit to be used as
595 a key, it needs to be processed further to derive the actual keys to be
596 used. The key material is also used to produce other security parameters
597 later used in the communication. See section 2.3 Processing the Key
598 Material for detailed description how to process the key material.
600 If the Mutual Authentication flag was set the protocol produces also
601 a hash value HASH_i. This value, however, must be discarded.
603 After the keys are processed the protocol is ended by sending the
604 SILC_PACKET_SUCCESS packet. Both entities send this packet to
605 each other. After this both parties will start using the new keys.
609 2.3 Processing the Key Material
611 Key Exchange protocol produces secret shared key material KEY. This
612 key material is used to derive the actual keys used in the encryption
613 of the communication channel. The key material is also used to derive
614 other security parameters used in the communication. Key Exchange
615 protocol produces a hash value HASH as well.
617 The keys MUST be derived from the key material as follows:
620 Sending Initial Vector (IV) = hash(0x0 | KEY | HASH)
621 Receiving Initial Vector (IV) = hash(0x1 | KEY | HASH)
622 Sending Encryption Key = hash(0x2 | KEY | HASH)
623 Receiving Encryption Key = hash(0x3 | KEY | HASH)
624 Sending HMAC Key = hash(0x4 | KEY | HASH)
625 Receiving HMAC Key = hash(0x5 | KEY | HASH)
629 The Initial Vector (IV) is used in the encryption when doing for
630 example CBC mode. As many bytes as needed are taken from the start of
631 the hash output for IV. Sending IV is for sending key and receiving IV
632 is for receiving key. For receiving party, the receiving IV is actually
633 sender's sending IV, and, the sending IV is actually sender's receiving
634 IV. Initiator uses IV's as they are (sending IV for sending and
635 receiving IV for receiving).
637 The Encryption Keys are derived as well from the hash(). If the hash()
638 output is too short for the encryption algorithm more key material MUST
639 be produced in the following manner:
642 K1 = hash(0x2 | KEY | HASH)
643 K2 = hash(KEY | HASH | K1)
644 K3 = hash(KEY | HASH | K1 | K2) ...
646 Sending Encryption Key = K1 | K2 | K3 ...
649 K1 = hash(0x3 | KEY | HASH)
650 K2 = hash(KEY | HASH | K1)
651 K3 = hash(KEY | HASH | K1 | K2) ...
653 Receiving Encryption Key = K1 | K2 | K3 ...
657 The key is distributed by hashing the previous hash with the original
658 key material. The final key is a concatenation of the hash values.
659 For Receiving Encryption Key the procedure is equivalent. Sending key
660 is used only for encrypting data to be sent. The receiving key is used
661 only to decrypt received data. For receiving party, the receive key is
662 actually sender's sending key, and, the sending key is actually sender's
663 receiving key. Initiator uses generated keys as they are (sending key
664 for sending and receiving key for receiving).
666 The HMAC keys are used to create MAC values to packets in the
667 communication channel. As many bytes as needed are taken from the start
668 of the hash output to generate the MAC keys.
670 These procedures are performed by all parties of the key exchange
671 protocol. This MUST be done before the protocol has been ended by
672 sending the SILC_PACKET_SUCCESS packet.
674 This same procedure is used in the SILC in some other circumstances
675 as well. Any changes to this procedure is mentioned separately when
676 this procedure is needed. See the [SILC1] and the [SILC2] for these
681 2.4 SILC Key Exchange Groups
683 The Following groups may be used in the SILC Key Exchange protocol.
684 The first group diffie-hellman-group1 is REQUIRED, other groups MAY be
685 negotiated to be used in the connection with Key Exchange Start Payload
686 and SILC_PACKET_KEY_EXCHANGE packet. However, the first group MUST be
687 proposed in the Key Exchange Start Payload regardless of any other
688 requested group (however, it does not have to be the first in the list).
692 2.4.1 diffie-hellman-group1
694 The length of this group is 1024 bits. This is REQUIRED group.
695 The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
700 179769313486231590770839156793787453197860296048756011706444
701 423684197180216158519368947833795864925541502180565485980503
702 646440548199239100050792877003355816639229553136239076508735
703 759914822574862575007425302077447712589550957937778424442426
704 617334727629299387668709205606050270810842907692932019128194
708 Its hexadecimal value is
711 FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
712 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
713 EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
714 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
715 EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
720 The generator used with this prime is g = 2. The group order q is
723 This group was taken from the OAKLEY specification.
727 2.4.2 diffie-hellman-group2
729 The length of this group is 1536 bits. This is OPTIONAL group.
730 The prime is 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }.
738 241031242692103258855207602219756607485695054850245994265411
739 694195810883168261222889009385826134161467322714147790401219
740 650364895705058263194273070680500922306273474534107340669624
741 601458936165977404102716924945320037872943417032584377865919
742 814376319377685986952408894019557734611984354530154704374720
743 774996976375008430892633929555996888245787241299381012913029
744 459299994792636526405928464720973038494721168143446471443848
745 8520940127459844288859336526896320919633919
748 Its hexadecimal value is
751 FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
752 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
753 EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
754 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
755 EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
756 C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
757 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
758 670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
761 The generator used with this prime is g = 2. The group order q is
764 This group was taken from the OAKLEY specification.
768 2.5 Key Exchange Status Types
770 This section defines all key exchange protocol status types that may
771 be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets
772 to indicate the status of the protocol. Implementations may map the
773 status types to human readable error message. All types except the
774 SILC_SKE_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet.
775 The length of status is 32 bits (4 bytes). The following status types
781 Protocol were executed successfully.
784 1 SILC_SKE_STATUS_ERROR
786 Unknown error occurred. No specific error type is defined.
789 2 SILC_SKE_STATUS_BAD_PAYLOAD
791 Provided KE payload were malformed or included bad fields.
794 3 SILC_SKE_STATUS_UNSUPPORTED_GROUP
796 None of the provided groups were supported.
799 4 SILC_SKE_STATUS_UNSUPPORTED_CIPHER
801 None of the provided ciphers were supported.
804 5 SILC_SKE_STATUS_UNSUPPORTED_PKCS
806 None of the provided public key algorithms were supported.
809 6 SILC_SKE_STATUS_UNSUPPORTED_HASH_FUNCTION
811 None of the provided hash functions were supported.
814 7 SILC_SKE_STATUS_UNSUPPORTED_HMAC
816 None of the provided HMACs were supported.
819 8 SILC_SKE_STATUS_UNSUPPORTED_PUBLIC_KEY
821 Provided public key type is not supported.
824 9 SILC_SKE_STATUS_INCORRECT_SIGNATURE
826 Provided signature was incorrect.
829 10 SILC_SKE_STATUS_BAD_VERSION
831 Provided version string was not acceptable.
834 11 SILC_SKE_STATUS_INVALID_COOKIE
836 The cookie in the Key Exchange Start Payload was malformed,
837 because responder modified the cookie.
842 3 SILC Connection Authentication Protocol
844 Purpose of Connection Authentication protocol is to authenticate the
845 connecting party with server. Usually connecting party is client but
846 server may connect to router server as well. Its other purpose is to
847 provide information for the server about which type of connection this
848 is. The type defines whether this is client, server or router
849 connection. Server uses this information to create the ID for the
852 After the authentication protocol has been successfully completed
853 SILC_PACKET_NEW_ID must be sent to the connecting client by the server.
854 See the [SILC1] for the details of the connecting procedure.
856 Server MUST verify the authentication data received and if it is to fail
857 the authentication MUST be failed by sending SILC_PACKET_FAILURE packet.
858 If everything checks out fine the protocol is ended by server by sending
859 SILC_PACKET_SUCCESS packet.
861 The protocol is executed after the SILC Key Exchange protocol. It MUST
862 NOT be executed in any other time. As it is performed after key exchange
863 protocol all traffic in the connection authentication protocol is
864 encrypted with the exchanged keys.
866 The protocol MUST be started by the connecting party by sending the
867 SILC_PACKET_CONNECTION_AUTH packet with Connection Auth Payload,
868 described in the next section. This payload MUST include the
869 authentication data. The authentication data is set according
870 authentication method that MUST be known by both parties. If connecting
871 party does not know what is the mandatory authentication method it MAY
872 request it from the server by sending SILC_PACKET_CONNECTION_AUTH_REQUEST
873 packet. This packet is not part of this protocol and is described in
874 section Connection Auth Request Payload in [SILC2]. However, if
875 connecting party already knows the mandatory authentication method
876 sending the request is not necessary.
878 See [SILC1] and section Connection Auth Request Payload in [SILC2] also
879 for the list of different authentication methods. Authentication method
880 MAY also be NONE, in which case the server does not require
881 authentication at all. However, in this case the protocol still MUST be
882 executed; the authentication data just is empty indicating no
883 authentication is required.
885 If authentication method is passphrase the authentication data is
886 plaintext passphrase. As the payload is entirely encrypted it is safe
887 to have plaintext passphrase. It is also provided as plaintext passphrase
888 because the receiver may need to pass the entire passphrase into a
889 passphrase checker, and hash digest of the passphrase would prevent this.
890 See the section 3.2.1 Passphrase Authentication for more information.
892 If authentication method is public key authentication the authentication
893 data is a signature of the hash value of hash HASH plus Key Exchange
894 Start Payload, established by the SILC Key Exchange protocol. This
895 signature MUST then be verified by the server. See the section 3.2.2
896 Public Key Authentication for more information.
898 The connecting client of this protocol MUST wait after successful execution
899 of this protocol for the SILC_PACKET_NEW_ID packet where it will receive
900 the ID it will be using in the SILC network. The connecting client cannot
901 start normal SILC session (sending messages or commands) until it has
902 received its ID. The ID's are always created by the server except
903 for server to router connection where servers create their own ID's.
907 3.1 Connection Auth Payload
909 Client sends this payload to authenticate itself to the server. Server
910 connecting to another server also sends this payload. Server receiving
911 this payload MUST verify all the data in it and if something is to fail
912 the authentication MUST be failed by sending SILC_PACKET_FAILURE packet.
914 The payload may only be sent with SILC_PACKET_CONNECTION_AUTH packet.
915 It MUST NOT be sent in any other packet type. The following diagram
916 represent the Connection Auth Payload.
922 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
923 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
924 | Payload Length | Connection Type |
925 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
927 ~ Authentication Data ~
929 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
933 Figure 3: Connection Auth Payload
937 o Payload Length (2 bytes) - Length of the entire Connection
940 o Connection Type (2 bytes) - Indicates the type of the
941 connection. See section Connection Auth Request Payload
942 in [SILC2] for the list of connection types. This field MUST
943 include valid connection type or the packet MUST be discarded
944 and authentication MUST be failed.
946 o Authentication Data (variable length) - The actual
947 authentication data. Contents of this depends on the
948 authentication method known by both parties. If no
949 authentication is required this field does not exist.
956 3.2 Connection Authentication Types
958 SILC supports two authentication types to be used in the connection
959 authentication protocol; passphrase or public key based authentication.
960 The following sections defines the authentication methods. See [SILC2]
961 for defined numerical authentication method types.
965 3.2.1 Passphrase Authentication
967 Passphrase authentication or pre-shared-key based authentication is
968 simply an authentication where the party that wants to authenticate
969 itself to the other end sends the passphrase that is required by
970 the other end, for example server. The plaintext passphrase is put
971 to the payload, that is then encrypted. The plaintext passphrase
972 MUST be in UTF-8 [RFC2279] encoding. If the passphrase is in the
973 sender's system in some other encoding it MUST be UTF-8 encoded
974 before transmitted. The receiver MAY change the encoding of the
975 passphrase to its system's default character encoding before verifying
978 If the passphrase matches with the one in the server's end the
979 authentication is successful. Otherwise SILC_PACKET_FAILURE MUST be
980 sent to the sender and the protocol execution fails.
982 This is REQUIRED authentication method to be supported by all SILC
985 When password authentication is used it is RECOMMENDED that maximum
986 amount of padding is applied to the SILC packet. This way it is not
987 possible to approximate the length of the password from the encrypted
993 3.2.2 Public Key Authentication
995 Public key authentication may be used if passphrase based authentication
996 is not desired. The public key authentication works by sending a
997 signature as authentication data to the other end, say, server. The
998 server MUST then verify the signature by the public key of the sender,
999 which the server has received earlier in SKE protocol.
1001 The signature is computed using the private key of the sender by signing
1002 the HASH value provided by the SKE protocol previously, and the Key
1003 Exchange Start Payload from SKE protocol that was sent to the server.
1004 These are concatenated and hash function is used to compute a hash value
1005 which is then signed.
1007 auth_hash = hash(HASH | Key Exchange Start Payload);
1008 signature = sign(auth_hash);
1010 The hash() function used to compute the value is the hash function
1011 negotiated in the SKE protocol. The server MUST verify the data, thus
1012 it must keep the HASH and the Key Exchange Start Payload saved during
1013 SKE and authentication protocols.
1015 If the verified signature matches the sent signature, the authentication
1016 were successful and SILC_PACKET_SUCCESS is sent. If it failed the
1017 protocol execution is stopped and SILC_PACKET_FAILURE is sent.
1019 This is REQUIRED authentication method to be supported by all SILC
1024 3.3 Connection Authentication Status Types
1026 This section defines all connection authentication status types that
1027 may be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets
1028 to indicate the status of the protocol. Implementations may map the
1029 status types to human readable error message. All types except the
1030 SILC_AUTH_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet.
1031 The length of status is 32 bits (4 bytes). The following status types
1036 Protocol was executed successfully.
1041 Authentication failed.
1047 4 Security Considerations
1049 Security is central to the design of this protocol, and these security
1050 considerations permeate the specification. Common security considerations
1051 such as keeping private keys truly private and using adequate lengths for
1052 symmetric and asymmetric keys must be followed in order to maintain the
1053 security of this protocol.
1060 [SILC1] Riikonen, P., "Secure Internet Live Conferencing (SILC),
1061 Protocol Specification", Internet Draft, May 2002.
1063 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
1066 [SILC4] Riikonen, P., "SILC Commands", Internet Draft, May 2002.
1068 [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
1071 [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
1074 [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC
1077 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
1080 [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
1083 [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol",
1086 [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440,
1089 [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
1092 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
1093 Infrastructure, Certificate and CRL Profile", RFC 2459,
1096 [Schneier] Schneier, B., "Applied Cryptography Second Edition",
1097 John Wiley & Sons, New York, NY, 1996.
1099 [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography",
1102 [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol",
1103 RFC 2412, November 1998.
1105 [ISAKMP] Maughan D., et al, "Internet Security Association and
1106 Key Management Protocol (ISAKMP)", RFC 2408, November
1109 [IKE] Harkins D., and Carrel D., "The Internet Key Exchange
1110 (IKE)", RFC 2409, November 1998.
1112 [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
1113 Authentication", RFC 2104, February 1997.
1115 [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
1116 Specifications, Version 2.0", RFC 2437, October 1998.
1118 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
1119 Requirement Levels", BCP 14, RFC 2119, March 1997.
1121 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
1122 10646", RFC 2279, January 1998.
1130 Snellmaninkatu 34 A 15
1134 EMail: priikone@iki.fi
1136 This Internet-Draft expires 15 November 2002