8 .ds RF FORMFEED[Page %]
17 Network Working Group P. Riikonen
19 draft-riikonen-flags-payloads-03.txt 17 June 2003
20 Expires: 17 December 2003
25 SILC Message Flag Payloads
26 <draft-riikonen-flags-payloads-03.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 ............................................ 2
77 3 SILC Message Flag Payloads .................................... 3
78 3.1 SILC_MESSAGE_FLAG_REQUEST ................................. 3
79 3.2 SILC_MESSAGE_FLAG_REPLY ................................... 3
80 3.3 SILC_MESSAGE_FLAG_SIGNED .................................. 4
81 3.4 SILC_MESSAGE_FLAG_DATA .................................... 6
82 4 Security Considerations ....................................... 7
83 5 References .................................................... 8
84 6 Author's Address .............................................. 8
85 7 Full Copyright Statement ...................................... 9
91 The Secure Internet Live Conferencing [SILC1] supports sending binary
92 messages between users in the network. To make the data sending, and
93 processing at the receiver's end as simple as possible the SILC defines
94 Message Flags to the Message Payload [SILC2] that is used to send private
95 and channel messages, which can help the receiver to decide how the data
96 is encoded, and how it should be interpreted. Some of the Message Flags
97 may define additional payloads to be associated with the flag, but the
98 [SILC2] does not define them. This memo defines the payloads for those
99 Message Flags that was marked to include additional payloads in [SILC2].
101 By defining the payloads for the Message Flags the Message Payload
102 can be augmented to support any kind of data, which can be easily
103 interpreted at the receiver end. For example, it would be possible to
104 send audio stream, video stream, image files and HTML pages as messages,
105 and the receiver can either choose to ignore the message or to process
106 it, or to perhaps pass the message to some application for processing.
107 Without specific payloads for Message Flags it is almost impossible for
108 the receiver to interpret binary data from the payload.
112 1.1 Requirements Terminology
114 The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
115 MAY, and OPTIONAL, when they appear in this document, are to be
116 interpreted as described in [RFC2119].
122 The Message Flags was added to the SILC protocol for the reason that SILC
123 provides sending binary data as messages between users, and entities in
124 the network, and interpreting pure binary data is almost impossible.
125 With the flags the purpose, the reason, and the way the message is
126 supposed to be interpreted can be told to the recipient. Other
127 conferencing protocols which are usually ASCII based protocols do not have
128 such problems since they do not generally support sending of binary data
129 at all, or require encoding of the data before it can be sent over the
132 The Message Payload in SILC can have flags that can augment the function
133 of the payload. The flags can tell for example that the message is a
134 request, or a reply to an earlier received request. They can tell that
135 the message is some action that the sender is performing, or they can tell
136 that the message is an auto reply, or that it is explicitly digitally
137 signed by the sender.
139 The problem of Message Flags is that the space for flags mask is only 16
140 bits, so there is a limited number of flags available. For this reason
141 having a flag that defines a generic way of sending any kind of data as
142 a message, and can be easily interpreted at the receiver's end is important.
143 For this reason the flag SILC_MESSAGE_FLAG_DATA was added to the protocol
144 which can represent any data. This memo describe how this flag is used
145 and how the associated payload is constructed and processed. This memo
146 also describes payloads for all the other flags that can have associated
151 3 SILC Message Flag Payloads
153 The [SILC2] defines the flags which may have associated payloads. This
154 section will list these flags and define the payloads.
158 3.1 SILC_MESSAGE_FLAG_REQUEST
160 Currently this flag can be used in the context of application specific,
161 service specific or vendor specific requests, and the data payload type is
162 dependent of this context. Therefore, payload is not defined for this
163 flag in this memo. This flag may also be masked with some other flag in
164 the message payload, including with some other flag that defines
169 3.2 SILC_MESSAGE_FLAG_REPLY
171 Currently this flag can be used in the context of application specific,
172 service specific or vendor specific replies, and the data payload type is
173 dependent of this context. Therefore, payload is not defined for this
174 flag in this memo. This flag may also be masked with some other flag in
175 the message payload, including with some other flag that defines
180 3.3 SILC_MESSAGE_FLAG_SIGNED
182 This flag is used to tell the recipient that the sent message is
183 digitally signed by the sender, and that the recipient should verify
184 the signature to verify the true authenticity of the received message.
185 All message payloads in SILC provides message authentication code (MAC)
186 which can be used to verify that the sender produced and sent the message.
187 Even so, signing messages digitally can be used to verify the authenticity
188 of the message when recipient trusts the sender and to provide
191 This flag defines a payload which is used to deliver the actual message,
192 sender's public key and the digital signature. The payload for
193 SILC_MESSAGE_FLAG_SIGNED is as follows:
198 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
199 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
201 ~ Start of Message Payload ~
203 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
205 ~ Public Key Payload ~
207 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
208 | Signature Data Length | |
209 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
221 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
225 Figure 1: SILC_MESSAGE_FLAG_SIGNED Payload
229 o Start of Message Payload (variable length) - This is the
230 start of the Message Payload without the IV and MAC fields,
231 since those fields are appended at the end of this payload.
233 o Public Key Payload (variable length) - This includes the
234 Public Key Payload [SILC2] which can be used to deliver the
235 sender's public key (or certificate). It also indicates the
236 type of the public key (or certificate) which the recipient
237 use to identify how the signature must be verified. This
238 payload must always be present but it is not required to
239 include the public key data. The Public Key Type field in
240 the Public Key Payload MUST be set to the correct type of
241 the key, even if the actual public key data is not included.
243 o Signature Data Length (2 bytes) - Indicates the length of
244 the Signature Data field not including any other field.
246 o Signature Data (variable length) - Includes the actual
247 signature data. The signature computation and encoding
248 is key type specific. See [SILC3] for all key types, and
249 their respective references for how to compute and encode
252 o Initial Vector (variable length) - the IV of the Message
253 Payload as defined in [SILC2]. This field is not encrypted.
255 o MAC (variable length) - the MAC of the Message Payload as
256 defined in [SILC2]. The MAC is computed after encryption
257 and after signature computation. All data in the Message
258 Payload and this payload, including the IV field are
259 included in the MAC computation. This field is not
263 How the data is processed before it is signed is key type specific.
264 The actual data that to be signed MUST be the plaintext message
265 payload before encryption. The data to be signed is concatenation
266 of the Start of Message Payload field and the Public Key Payload,
267 in that order. Any other fields are not included for signature data.
268 Before signing, the data is always processed, usually hashed. The
269 hash function to be used is defined in the key type specific
270 definitions. See the key type specific references in [SILC3].
272 If the public key of the sender is included in the payload the
273 recipient SHOULD verify it before accepting the public key. Recipient
274 SHOULD verify the signature before accepting and caching the public key.
275 With certificates the certificate verification may be done before
276 verifying the signature. If the signature verification fails the
277 message should still be displayed. The end user should also be
278 notified about the result of the signature verification.
280 To make the packet size smaller implementations may not want to
281 include the actual public key in all signed messages. Sending the
282 public key in the first message is usually sufficient. Subsequent
283 messages may include empty Public Key Payload with an indication of
286 Implementations that do not support this flag can still process the
287 message payload in normal manner. These implementations merely parse
288 the decrypted payload in normal manner and ignore the extra data in
289 the payload. They can do this by extracting the MAC and the IV from
290 the end of the data buffer and thus ignoring the data between start of
291 the Message Payload and the Initial Vector field.
293 This flag MAY be masked with any other Message Flag including those that
294 define additional payloads. As long as the defined payload resides in
295 the data area of the message payload this flag may be masked with the
300 3.4 SILC_MESSAGE_FLAG_DATA
302 This flag is used to represent any data as a message in the way that it
303 can be easily interpreted by the recipient. This flag is used to send
304 MIME objects as messages from the sender to the receiver. The MIME as
305 defined in [RFC2045], [RFC2046], [RFC2047], [RFC2048] and [RFC2049] is
306 well established protocol for sending different kind of data with many
307 applications and protocols. It support dozens of different media types
308 and encodings, and for this reason is ideal for sending data in SILC
309 message payloads as well.
311 When the receiver has checked that the message payload includes the
312 SILC_MESSAGE_FLAG_DATA flag, it may then start parsing the MIME header.
313 It would also be possible to pass the message to some application which
314 can already interpret MIME objects. If the receiver does not support the
315 media type received in the MIME header, it SHOULD be treated as
316 "application/octet-stream". The receiver MAY also ignore and discard
317 messages that it does not support.
319 The MIME header MUST be at the start of the data area of the Message
320 Payload. The MIME header received in the data area of the payload SHOULD
321 have the MIME-Version field at first and then Content-Type field. The
322 MIME-Version field is not required to be present in each body part of
323 multipart entity. Additionally the header MAY also include any other
324 MIME compliant headers. The character encoding for the MIME Header
325 strings inside the message payload is US-ASCII, as defined in [RFC2045].
326 The actual MIME object may define additional character sets or encodings
327 for the data it delivers.
329 Hence, the MIME Header in the message payload may be as follows:
333 MIME-Version: 1.0\\r\\n
334 Content-Type: discrete/composite\\r\\n
335 Content-Transfer-Encoding: binary\\r\\n
339 The Content-Transfer-Encoding field behaves as defined in [RFC2045] and
340 defines the encoding of the data in the MIME object. The preferred data
341 encoding with SILC is "binary". However, many MIME media types defines
342 their preferred encoding and they may be used if binary encoding is not
345 When sending large amounts of traffic or large files as MIME objects the
346 limits of the SILC Packet needs to be taken into consideration. The
347 maximum length of SILC Packet is 2^16 bytes, and larger messages would
348 need to be fragmented. MIME provides way of fragmenting and reassembling
349 messages, and it is to be done with SILC as defined in [RFC2046]. The
350 MIME fragmentation is defined for gateway usage, but in case of SILC the
351 sender (for example, a client) may also start sending fragmented MIME
354 This flag SHOULD NOT be masked with some other Message Flag that defines
355 payloads for message data. Generally this sort of setting would be
356 impossible for the receiver to interpret. However, flags that does not
357 define any specific payloads MAY be masked with this flag as well. For
358 example, this flag could be masked also with SILC_MESSAGE_FLAG_REQUEST flag.
359 It also can be masked with SILC_MESSAGE_FLAG_SIGNED flag since it does not
360 define data specific payload.
364 4 Security Considerations
366 In case of SILC_MESSAGE_FLAG_DATA the implementors should pay special
367 attention to the security implications of any media type that can cause
368 the remote execution of any actions in the receiver's environment. The
369 [RFC2046] and [RFC2048] discusses more MIME specific security
370 considerations. Even though SILC provides secured messages, in case of
371 MIME which can be used to transfer files and documents which are stored in
372 the receiver's local environment, securing separately the MIME object may
373 be desired. For example, augmenting the MIME support in SILC messages to
374 support S/MIME may be desired in some implementations.
382 [SILC1] Riikonen, P., "Secure Internet Live Conferencing (SILC),
383 Protocol Specification", Internet Draft, June 2003.
385 [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft,
388 [SILC3] Riikonen, P., "SILC Key Exchange and Authentication
389 Protocols", Internet Draft, June 2003.
391 [RFC2045] Freed, N., et al., "Multipurpose Internet Mail Extensions
392 (MIME) Part One: Format of Internet Message Bodies",
393 Standards Track, RFC 2045, November 1996.
395 [RFC2046] Freed, N., et al., "Multipurpose Internet Mail Extensions
396 (MIME) Part Two: Media Types", Standards Track, RFC 2045,
399 [RFC2047] Moore K., "MIME (Multipurpose Internet Mail Extensions)
400 Part Three: Message Header Extensions for Non-ASCII Text"
401 Standards Track, RFC 2047, November 1996.
403 [RFC2048] Freed, N., et al., "Multipurpose Internet Mail Extensions
404 (MIME) Part Four: Registration Procedures", Standards
405 Track, RFC 2048, November 1996.
407 [RFC2049] Freed, N., et al., "Multipurpose Internet Mail Extensions
408 (MIME) Part Five: Conformance Criteria and Examples",
409 Standards Track, RFC 2049, November 1996.
411 [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate
412 Requirement Levels", BCP 14, RFC 2119, March 1997.
420 Snellmaninkatu 34 A 15
424 EMail: priikone@iki.fi
430 7 Full Copyright Statement
432 Copyright (C) The Internet Society (2003). All Rights Reserved.
434 This document and translations of it may be copied and furnished to
435 others, and derivative works that comment on or otherwise explain it
436 or assist in its implementation may be prepared, copied, published
437 and distributed, in whole or in part, without restriction of any
438 kind, provided that the above copyright notice and this paragraph are
439 included on all such copies and derivative works. However, this
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441 the copyright notice or references to the Internet Society or other
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