8 .ds RF FORMFEED[Page %]
11 .ds RH 2 December 2003
17 Network Working Group P. Riikonen
19 draft-riikonen-flags-payloads-04.txt 2 December 2003
25 SILC Message Flag Payloads
26 <draft-riikonen-flags-payloads-04.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 the data payloads associated with the SILC Message
55 Flags, as defined in the SILC Packet Protocol specification [SILC2]. The
56 purpose of the Message Flags is to augment the function of the Message
57 Payload used to send both private and channel messages, by allowing the
58 sender to tell the receiver what type of data the payload includes, and
59 how the data should be processed. Some of the Message Flags may define
60 additional payloads to be associated with the flag, and this memo
61 describes these payloads.
74 1 Introduction .................................................. 2
75 1.1 Requirements Terminology .................................. 2
76 2 SILC Message Flags ............................................ 3
77 3 SILC Message Flag Payloads .................................... 3
78 3.1 SILC_MESSAGE_FLAG_REQUEST ................................. 3
79 3.2 SILC_MESSAGE_FLAG_REPLY ................................... 4
80 3.3 SILC_MESSAGE_FLAG_SIGNED .................................. 4
81 3.4 SILC_MESSAGE_FLAG_DATA .................................... 7
82 3.5 SILC_MESSAGE_FLAG_ACK ..................................... 8
83 4 Security Considerations ....................................... 9
84 5 References .................................................... 9
85 6 Author's Address .............................................. 10
86 7 Full Copyright Statement ...................................... 10
92 The Secure Internet Live Conferencing [SILC1] supports sending binary
93 messages between users in the network. To make the data sending, and
94 processing at the receiver's end as simple as possible the SILC defines
95 Message Flags to the Message Payload [SILC2] that is used to send private
96 and channel messages, which can help the receiver to decide how the data
97 is encoded, and how it should be interpreted. Some of the Message Flags
98 may define additional payloads to be associated with the flag, but the
99 [SILC2] does not define them. This memo defines the payloads for those
100 Message Flags that was marked to include additional payloads in [SILC2].
102 By defining the payloads for the Message Flags the Message Payload
103 can be augmented to support any kind of data, which can be easily
104 interpreted at the receiver end. For example, it would be possible to
105 send audio stream, video stream, image files and HTML pages as messages,
106 and the receiver can either choose to ignore the message or to process
107 it, or to perhaps pass the message to some application for processing.
108 Without specific payloads for Message Flags it is almost impossible for
109 the receiver to interpret binary data from the payload.
113 1.1 Requirements Terminology
115 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
116 MAY, and OPTIONAL, when they appear in this document, are to be
117 interpreted as described in [RFC2119].
125 The Message Flags was added to the SILC protocol for the reason that SILC
126 provides sending binary data as messages between users, and entities in
127 the network, and interpreting pure binary data is almost impossible.
128 With the Message Flags the purpose, the reason, and the method for how
129 the message must be interpreted can be told to the recipient. Other
130 conferencing protocols which are usually ASCII based protocols do not have
131 such problems since they do not generally support sending of binary data
132 at all, or require specific encoding of the data before it can be sent
135 The Message Payload in SILC can have flags that can augment the function
136 of the payload. The flags can tell for example that the message is a
137 request, or a reply to an earlier received request. They can tell that
138 the message is some action that the sender is performing, or they can tell
139 that the message is an auto reply, or that it is explicitly digitally
140 signed by the sender.
142 The problem of Message Flags is that the space for flags mask is only 16
143 bits, so there is a limited number of flags available. For this reason
144 having a flag that defines a generic way of sending any kind of data as
145 a message, and can be easily interpreted at the receiver's end is important.
146 For this reason the flag SILC_MESSAGE_FLAG_DATA was added to the protocol
147 which can represent any data. This memo describe how this flag is used
148 and how the associated payload is constructed and processed. This memo
149 also describes payloads for all the other flags that can have associated
154 3 SILC Message Flag Payloads
156 The [SILC2] defines the flags which may have associated payloads. This
157 section will list these flags and define the payloads.
161 3.1 SILC_MESSAGE_FLAG_REQUEST
163 Currently this flag can be used in the context of application specific,
164 service specific or vendor specific requests, and the data payload type is
165 dependent of this context. Therefore, payload is not defined for this
166 flag in this memo. This flag may also be masked with some other flag in
167 the message payload, including with some other flag that defines
174 3.2 SILC_MESSAGE_FLAG_REPLY
176 Currently this flag can be used in the context of application specific,
177 service specific or vendor specific replies, and the data payload type is
178 dependent of this context. Therefore, payload is not defined for this
179 flag in this memo. This flag may also be masked with some other flag in
180 the message payload, including with some other flag that defines
185 3.3 SILC_MESSAGE_FLAG_SIGNED
187 This flag is used to tell the recipient that the sent message is
188 digitally signed by the sender, and that the recipient should verify
189 the signature to verify the true authenticity of the received message.
190 All message payloads in SILC provides message authentication code (MAC)
191 which can be used to verify that the sender produced and sent the message.
192 Even so, signing messages digitally can be used to verify the authenticity
193 of the message when recipient trusts the sender and to provide
196 This flag defines a payload which is used to deliver the actual message,
197 sender's public key and the digital signature. The payload for
198 SILC_MESSAGE_FLAG_SIGNED is as follows:
200 (*) indicates that the field is not encrypted.
226 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
227 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
229 ~ Start of Message Payload ~
231 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
233 ~ Public Key Payload * ~
235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
236 | Signature Data Length * | |
237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
241 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
245 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
249 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
253 Figure 1: SILC_MESSAGE_FLAG_SIGNED Payload
257 o Start of Message Payload (variable length) - This is the
258 start of the Message Payload without the IV and MAC fields,
259 since those fields are appended at the end of this payload.
261 o Public Key Payload (variable length) - This includes the
262 Public Key Payload [SILC2] which can be used to deliver the
263 sender's public key (or certificate). It also indicates the
264 type of the public key (or certificate) which the recipient
265 use to identify how the signature must be verified. This
266 payload must always be present but it is not required to
267 include the public key data. The Public Key Type field in
268 the Public Key Payload MUST be set to the correct type of
269 the key, even if the actual public key data is not included.
270 This field is not encrypted but is authenticated.
272 o Signature Data Length (2 bytes) - Indicates the length of
273 the Signature Data field not including any other field.
274 This field is not encrypted but is authenticated.
276 o Signature Data (variable length) - Includes the actual
277 signature data. The signature computation and encoding
278 is key type specific. See [SILC3] for all key types, and
279 their respective references for how to compute and encode
280 the signature. This field is not encrypted but is
283 o Initial Vector (variable length) - the IV of the Message
284 Payload as defined in [SILC2]. This field is not encrypted
285 but is authenticated.
287 o MAC (variable length) - the MAC of the Message Payload as
288 defined in [SILC2]. The MAC is computed after encryption
289 and after signature computation. All data in the Message
290 Payload and this payload, including the IV field are
291 included in the MAC computation. This field is not
295 How the data is processed before it is signed is key type specific.
296 The actual data that to be signed MUST be the plaintext message
297 payload before encryption. The data to be signed is concatenation
298 of the Start of Message Payload field and the Public Key Payload,
299 in that order. Any other fields are not included for signature data.
300 Before signing, the data is always processed, usually hashed. The
301 hash function to be used is defined in the key type specific
302 definitions. See the key type specific references in [SILC3].
304 If the public key of the sender is included in the payload the
305 recipient SHOULD verify it before accepting the public key. Recipient
306 SHOULD verify the signature before accepting and caching the public key.
307 With certificates the certificate verification may be done before
308 verifying the signature. If the signature verification fails the
309 message should still be displayed. The end user should also be
310 notified about the result of the signature verification.
312 To make the packet size smaller implementations may not want to
313 include the actual public key in all signed messages. Sending the
314 public key in the first message is usually sufficient. Subsequent
315 messages may include empty Public Key Payload with an indication of
318 Implementations that do not support this flag can still process the
319 message payload in normal manner. These implementations merely parse
320 the decrypted payload in normal manner and ignore the extra data in
321 the payload. They can do this by extracting the MAC and the IV from
322 the end of the data buffer and thus ignoring the data between start of
323 the Message Payload and the Initial Vector field.
325 This flag MAY be masked with any other Message Flag including those that
326 define additional payloads. As long as the defined payload resides in
327 the data area of the message payload this flag may be masked with the
332 3.4 SILC_MESSAGE_FLAG_DATA
334 This flag is used to represent any data as a message in the way that it
335 can be easily interpreted by the recipient. This flag is used to send
336 MIME objects as messages from the sender to the receiver. The MIME as
337 defined in [RFC2045], [RFC2046], [RFC2047], [RFC2048] and [RFC2049] is
338 well established protocol for sending different kind of data with many
339 applications and protocols. It support dozens of different media types
340 and encodings, and for this reason is ideal for sending data in SILC
341 message payloads as well.
343 When the receiver has checked that the message payload includes the
344 SILC_MESSAGE_FLAG_DATA flag, it may then start parsing the MIME header.
345 It would also be possible to pass the message to some application which
346 can already interpret MIME objects. If the receiver does not support the
347 media type received in the MIME header, it SHOULD be treated as
348 "application/octet-stream". The receiver MAY also ignore and discard
349 messages that it does not support.
351 The MIME header MUST be at the start of the data area of the Message
352 Payload. The MIME header received in the data area of the payload SHOULD
353 have the MIME-Version field at first and then Content-Type field. The
354 MIME-Version field is not required to be present in each body part of
355 multipart entity. Additionally the header MAY also include any other
356 MIME compliant headers. The character encoding for the MIME Header
357 strings inside the message payload is US-ASCII, as defined in [RFC2045].
358 The actual MIME object may define additional character sets or encodings
359 for the data it delivers.
361 Hence, the MIME Header in the message payload may be as follows:
365 MIME-Version: 1.0\\r\\n
366 Content-Type: discrete/composite\\r\\n
367 Content-Transfer-Encoding: binary\\r\\n
371 The Content-Transfer-Encoding field behaves as defined in [RFC2045] and
372 defines the encoding of the data in the MIME object. The preferred data
373 encoding with SILC is "binary". However, many MIME media types defines
374 their preferred encoding and they may be used if binary encoding is not
377 When sending large amounts of traffic or large files as MIME objects the
378 limits of the SILC Packet needs to be taken into consideration. The
379 maximum length of SILC Packet is 2^16 bytes, and larger messages would
380 need to be fragmented. MIME provides way of fragmenting and reassembling
381 messages, and it is to be done with SILC as defined in [RFC2046]. The
382 MIME fragmentation is defined for gateway usage, but in case of SILC the
383 sender (for example, a client) may also start sending fragmented MIME
386 This flag SHOULD NOT be masked with some other Message Flag that defines
387 payloads for message data. Generally this sort of setting would be
388 impossible for the receiver to interpret. However, flags that does not
389 define any specific payloads MAY be masked with this flag as well. For
390 example, this flag could be masked also with SILC_MESSAGE_FLAG_REQUEST flag.
391 It also can be masked with SILC_MESSAGE_FLAG_SIGNED flag since it does not
392 define data specific payload.
396 3.5 SILC_MESSAGE_FLAG_ACK
398 This flag is used to send acknowledgement messages. When sender of a
399 message requires the recipient to acknowledge the received message, the
400 sender MUST set the SILC_MESSAGE_FLAG_ACK and MUST NOT set the
401 SILC_MESSAGE_FLAG_NOREPLY. When a message with this flag set is received
402 an acknowledgement message MUST be sent back. In the acknowledgement
403 message the sender MUST set the SILC_MESSAGE_FLAG_ACK,
404 SILC_MESSAGE_FLAG_AUTOREPLY and SILC_MESSAGE_FLAG_NOREPLY flags. The
405 receiver MUST NOT acknowledge the acknowledgement message. This flag
406 MUST NOT be used with channel messages, and MUST be ignored if received
407 in a channel message.
409 The construction of the acknowledgement reply message is normal Message
410 Payload where the Message Data field includes a computed MAC of the
411 original received Message Payload MAC. Hence, the MAC is computed as
415 ack_mac = mac(key, MAC);
418 Where the 'key' is the MAC key used to compute MACs for the Message
419 Payload, and the 'MAC' is the MAC taken from the received Message Payload.
420 The 'ack_mac' is placed in the Message Data field in a new Message
421 Payload, and the payload is encrypted in normal manner. After this the
422 message is sent back to the original sender of the message.
424 The receiver of the acknowledgement reply message SHOULD verify the MAC
425 from the Message Data field to assure that acknowledgement was received to
426 an earlier sent message. Implementation needs to keep the old message
427 MACs stored until acknowledgement is received. It is left for
428 implementation to decide any possible retransmission strategy if
429 acknowledgement messages are not received.
433 4 Security Considerations
435 In case of SILC_MESSAGE_FLAG_DATA the implementors should pay special
436 attention to the security implications of any media type that can cause
437 the remote execution of any actions in the receiver's environment. The
438 [RFC2046] and [RFC2048] discusses more MIME specific security
439 considerations. Even though SILC provides secured messages, in case of
440 MIME which can be used to transfer files and documents which are stored in
441 the receiver's local environment, securing separately the MIME object may
442 be desired. For example, augmenting the MIME support in SILC messages to
443 support S/MIME may be desired in some implementations.
449 [SILC1] Riikonen, P., "Secure Internet Live Conferencing (SILC),
450 Protocol Specification", Internet Draft, June 2003.
452 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
455 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
456 Protocols", Internet Draft, June 2003.
458 [RFC2045] Freed, N., et al., "Multipurpose Internet Mail Extensions
459 (MIME) Part One: Format of Internet Message Bodies",
460 Standards Track, RFC 2045, November 1996.
462 [RFC2046] Freed, N., et al., "Multipurpose Internet Mail Extensions
463 (MIME) Part Two: Media Types", Standards Track, RFC 2045,
466 [RFC2047] Moore K., "MIME (Multipurpose Internet Mail Extensions)
467 Part Three: Message Header Extensions for Non-ASCII Text"
468 Standards Track, RFC 2047, November 1996.
470 [RFC2048] Freed, N., et al., "Multipurpose Internet Mail Extensions
471 (MIME) Part Four: Registration Procedures", Standards
472 Track, RFC 2048, November 1996.
474 [RFC2049] Freed, N., et al., "Multipurpose Internet Mail Extensions
475 (MIME) Part Five: Conformance Criteria and Examples",
476 Standards Track, RFC 2049, November 1996.
478 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
479 Requirement Levels", BCP 14, RFC 2119, March 1997.
487 Snellmaninkatu 34 A 15
491 EMail: priikone@iki.fi
496 7 Full Copyright Statement
498 Copyright (C) The Internet Society (2003). All Rights Reserved.
500 This document and translations of it may be copied and furnished to
501 others, and derivative works that comment on or otherwise explain it
502 or assist in its implementation may be prepared, copied, published
503 and distributed, in whole or in part, without restriction of any
504 kind, provided that the above copyright notice and this paragraph are
505 included on all such copies and derivative works. However, this
506 document itself may not be modified in any way, such as by removing
507 the copyright notice or references to the Internet Society or other
508 Internet organizations, except as needed for the purpose of
509 developing Internet standards in which case the procedures for
510 copyrights defined in the Internet Standards process must be
511 followed, or as required to translate it into languages other than
514 The limited permissions granted above are perpetual and will not be
515 revoked by the Internet Society or its successors or assigns.
517 This document and the information contained herein is provided on an
518 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
519 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
520 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
521 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
522 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.