





Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-flags-payloads-04.txt                     2 December 2003
Expires: 2 June 2004


                        SILC Message Flag Payloads
                  <draft-riikonen-flags-payloads-04.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The distribution of this memo is unlimited.


Abstract

   This memo describes the data payloads associated with the SILC Message
   Flags, as defined in the SILC Packet Protocol specification [SILC2].  The
   purpose of the Message Flags is to augment the function of the Message
   Payload used to send both private and channel messages, by allowing the
   sender to tell the receiver what type of data the payload includes, and
   how the data should be processed.  Some of the Message Flags may define
   additional payloads to be associated with the flag, and this memo
   describes these payloads.










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Table of Contents

   1 Introduction ..................................................  2
     1.1 Requirements Terminology ..................................  2
   2 SILC Message Flags ............................................  3
   3 SILC Message Flag Payloads ....................................  3
     3.1 SILC_MESSAGE_FLAG_REQUEST .................................  3
     3.2 SILC_MESSAGE_FLAG_REPLY ...................................  4
     3.3 SILC_MESSAGE_FLAG_SIGNED ..................................  4
     3.4 SILC_MESSAGE_FLAG_DATA ....................................  7
     3.5 SILC_MESSAGE_FLAG_ACK .....................................  8
   4 Security Considerations .......................................  9
   5 References ....................................................  9
   6 Author’s Address .............................................. 10
   7 Full Copyright Statement ...................................... 10


1. Introduction

   The Secure Internet Live Conferencing [SILC1] supports sending binary
   messages between users in the network.  To make the data sending, and
   processing at the receiver’s end as simple as possible the SILC defines
   Message Flags to the Message Payload [SILC2] that is used to send private
   and channel messages, which can help the receiver to decide how the data
   is encoded, and how it should be interpreted.  Some of the Message Flags
   may define additional payloads to be associated with the flag, but the
   [SILC2] does not define them.  This memo defines the payloads for those
   Message Flags that was marked to include additional payloads in [SILC2].

   By defining the payloads for the Message Flags the Message Payload
   can be augmented to support any kind of data, which can be easily
   interpreted at the receiver end.  For example, it would be possible to
   send audio stream, video stream, image files and HTML pages as messages,
   and the receiver can either choose to ignore the message or to process
   it, or to perhaps pass the message to some application for processing.
   Without specific payloads for Message Flags it is almost impossible for
   the receiver to interpret binary data from the payload.


1.1 Requirements Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED,
   MAY, and OPTIONAL, when they appear in this document, are to be
   interpreted as described in [RFC2119].







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2 SILC Message Flags

   The Message Flags was added to the SILC protocol for the reason that SILC
   provides sending binary data as messages between users, and entities in
   the network, and interpreting pure binary data is almost impossible.
   With the Message Flags the purpose, the reason, and the method for how
   the message must be interpreted can be told to the recipient.  Other
   conferencing protocols which are usually ASCII based protocols do not have
   such problems since they do not generally support sending of binary data
   at all, or require specific encoding of the data before it can be sent
   over the network.

   The Message Payload in SILC can have flags that can augment the function
   of the payload.  The flags can tell for example that the message is a
   request, or a reply to an earlier received request.  They can tell that
   the message is some action that the sender is performing, or they can tell
   that the message is an auto reply, or that it is explicitly digitally
   signed by the sender.

   The problem of Message Flags is that the space for flags mask is only 16
   bits, so there is a limited number of flags available.  For this reason
   having a flag that defines a generic way of sending any kind of data as
   a message, and can be easily interpreted at the receiver’s end is important.
   For this reason the flag SILC_MESSAGE_FLAG_DATA was added to the protocol
   which can represent any data.  This memo describe how this flag is used
   and how the associated payload is constructed and processed.  This memo
   also describes payloads for all the other flags that can have associated
   payloads.


3 SILC Message Flag Payloads

   The [SILC2] defines the flags which may have associated payloads.  This
   section will list these flags and define the payloads.


3.1 SILC_MESSAGE_FLAG_REQUEST

   Currently this flag can be used in the context of application specific,
   service specific or vendor specific requests, and the data payload type is
   dependent of this context.  Therefore, payload is not defined for this
   flag in this memo.  This flag may also be masked with some other flag in
   the message payload, including with some other flag that defines
   additional payload.







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3.2 SILC_MESSAGE_FLAG_REPLY

   Currently this flag can be used in the context of application specific,
   service specific or vendor specific replies, and the data payload type is
   dependent of this context.  Therefore, payload is not defined for this
   flag in this memo.  This flag may also be masked with some other flag in
   the message payload, including with some other flag that defines
   additional payload.


3.3 SILC_MESSAGE_FLAG_SIGNED

   This flag is used to tell the recipient that the sent message is
   digitally signed by the sender, and that the recipient should verify
   the signature to verify the true authenticity of the received message.
   All message payloads in SILC provides message authentication code (MAC)
   which can be used to verify that the sender produced and sent the message.
   Even so, signing messages digitally can be used to verify the authenticity
   of the message when recipient trusts the sender and to provide
   non-repudiation.

   This flag defines a payload which is used to deliver the actual message,
   sender’s public key and the digital signature.  The payload for
   SILC_MESSAGE_FLAG_SIGNED is as follows:

   (*) indicates that the field is not encrypted.

























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                          1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                   Start of Message Payload                    ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                      Public Key Payload *                     ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Signature Data Length *   |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                        Signature Data *                       ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                       Initial Vector *                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                              MAC *                            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1:  SILC_MESSAGE_FLAG_SIGNED Payload


      o Start of Message Payload (variable length) - This is the
        start of the Message Payload without the IV and MAC fields,
        since those fields are appended at the end of this payload.

      o Public Key Payload (variable length) - This includes the
        Public Key Payload [SILC2] which can be used to deliver the
        sender’s public key (or certificate).  It also indicates the
        type of the public key (or certificate) which the recipient
        use to identify how the signature must be verified.  This
        payload must always be present but it is not required to
        include the public key data.  The Public Key Type field in
        the Public Key Payload MUST be set to the correct type of
        the key, even if the actual public key data is not included.
        This field is not encrypted but is authenticated.

      o Signature Data Length (2 bytes) - Indicates the length of
        the Signature Data field not including any other field.
        This field is not encrypted but is authenticated.




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      o Signature Data (variable length) - Includes the actual
        signature data.  The signature computation and encoding
        is key type specific.  See [SILC3] for all key types, and
        their respective references for how to compute and encode
        the signature.  This field is not encrypted but is
        authenticated.

      o Initial Vector (variable length) - the IV of the Message
        Payload as defined in [SILC2].  This field is not encrypted
        but is authenticated.

      o MAC (variable length) - the MAC of the Message Payload as
        defined in [SILC2].  The MAC is computed after encryption
        and after signature computation.  All data in the Message
        Payload and this payload, including the IV field are
        included in the MAC computation.  This field is not
        encrypted.

   How the data is processed before it is signed is key type specific.
   The actual data that to be signed MUST be the plaintext message
   payload before encryption.  The data to be signed is concatenation
   of the Start of Message Payload field and the Public Key Payload,
   in that order.  Any other fields are not included for signature data.
   Before signing, the data is always processed, usually hashed.  The
   hash function to be used is defined in the key type specific
   definitions.  See the key type specific references in [SILC3].

   If the public key of the sender is included in the payload the
   recipient SHOULD verify it before accepting the public key.  Recipient
   SHOULD verify the signature before accepting and caching the public key.
   With certificates the certificate verification may be done before
   verifying the signature.  If the signature verification fails the
   message should still be displayed.  The end user should also be
   notified about the result of the signature verification.

   To make the packet size smaller implementations may not want to
   include the actual public key in all signed messages.  Sending the
   public key in the first message is usually sufficient.  Subsequent
   messages may include empty Public Key Payload with an indication of
   the public key type.

   Implementations that do not support this flag can still process the
   message payload in normal manner.  These implementations merely parse
   the decrypted payload in normal manner and ignore the extra data in
   the payload.  They can do this by extracting the MAC and the IV from
   the end of the data buffer and thus ignoring the data between start of
   the Message Payload and the Initial Vector field.




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   This flag MAY be masked with any other Message Flag including those that
   define additional payloads.  As long as the defined payload resides in
   the data area of the message payload this flag may be masked with the
   other flags.


3.4 SILC_MESSAGE_FLAG_DATA

   This flag is used to represent any data as a message in the way that it
   can be easily interpreted by the recipient.  This flag is used to send
   MIME objects as messages from the sender to the receiver.  The MIME as
   defined in [RFC2045], [RFC2046], [RFC2047], [RFC2048] and [RFC2049] is
   well established protocol for sending different kind of data with many
   applications and protocols.  It support dozens of different media types
   and encodings, and for this reason is ideal for sending data in SILC
   message payloads as well.

   When the receiver has checked that the message payload includes the
   SILC_MESSAGE_FLAG_DATA flag, it may then start parsing the MIME header.
   It would also be possible to pass the message to some application which
   can already interpret MIME objects.  If the receiver does not support the
   media type received in the MIME header, it SHOULD be treated as
   "application/octet-stream".  The receiver MAY also ignore and discard
   messages that it does not support.

   The MIME header MUST be at the start of the data area of the Message
   Payload.  The MIME header received in the data area of the payload SHOULD
   have the MIME-Version field at first and then Content-Type field.  The
   MIME-Version field is not required to be present in each body part of
   multipart entity.  Additionally the header MAY also include any other
   MIME compliant headers.  The character encoding for the MIME Header
   strings inside the message payload is US-ASCII, as defined in [RFC2045].
   The actual MIME object may define additional character sets or encodings
   for the data it delivers.

   Hence, the MIME Header in the message payload may be as follows:

        MIME-Version: 1.0\r\n
        Content-Type: discrete/composite\r\n
        Content-Transfer-Encoding: binary\r\n
        \r\n

   The Content-Transfer-Encoding field behaves as defined in [RFC2045] and
   defines the encoding of the data in the MIME object.  The preferred data
   encoding with SILC is "binary".  However, many MIME media types defines
   their preferred encoding and they may be used if binary encoding is not
   suitable.




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   When sending large amounts of traffic or large files as MIME objects the
   limits of the SILC Packet needs to be taken into consideration.  The
   maximum length of SILC Packet is 2^16 bytes, and larger messages would
   need to be fragmented.  MIME provides way of fragmenting and reassembling
   messages, and it is to be done with SILC as defined in [RFC2046].  The
   MIME fragmentation is defined for gateway usage, but in case of SILC the
   sender (for example, a client) may also start sending fragmented MIME
   objects.

   This flag SHOULD NOT be masked with some other Message Flag that defines
   payloads for message data.  Generally this sort of setting would be
   impossible for the receiver to interpret.  However, flags that does not
   define any specific payloads MAY be masked with this flag as well.  For
   example, this flag could be masked also with SILC_MESSAGE_FLAG_REQUEST flag.
   It also can be masked with SILC_MESSAGE_FLAG_SIGNED flag since it does not
   define data specific payload.


3.5 SILC_MESSAGE_FLAG_ACK

   This flag is used to send acknowledgement messages.  When sender of a
   message requires the recipient to acknowledge the received message, the
   sender MUST set the SILC_MESSAGE_FLAG_ACK and MUST NOT set the
   SILC_MESSAGE_FLAG_NOREPLY.  When a message with this flag set is received
   an acknowledgement message MUST be sent back.  In the acknowledgement
   message the sender MUST set the SILC_MESSAGE_FLAG_ACK,
   SILC_MESSAGE_FLAG_AUTOREPLY and SILC_MESSAGE_FLAG_NOREPLY flags.  The
   receiver MUST NOT acknowledge the acknowledgement message.  This flag
   MUST NOT be used with channel messages, and MUST be ignored if received
   in a channel message.

   The construction of the acknowledgement reply message is normal Message
   Payload where the Message Data field includes a computed MAC of the
   original received Message Payload MAC.  Hence, the MAC is computed as
   follows:

        ack_mac = mac(key, MAC);

   Where the ’key’ is the MAC key used to compute MACs for the Message
   Payload, and the ’MAC’ is the MAC taken from the received Message Payload.
   The ’ack_mac’ is placed in the Message Data field in a new Message
   Payload, and the payload is encrypted in normal manner.  After this the
   message is sent back to the original sender of the message.

   The receiver of the acknowledgement reply message SHOULD verify the MAC
   from the Message Data field to assure that acknowledgement was received to
   an earlier sent message.  Implementation needs to keep the old message
   MACs stored until acknowledgement is received.  It is left for



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   implementation to decide any possible retransmission strategy if
   acknowledgement messages are not received.


4 Security Considerations

   In case of SILC_MESSAGE_FLAG_DATA the implementors should pay special
   attention to the security implications of any media type that can cause
   the remote execution of any actions in the receiver’s environment.  The
   [RFC2046] and [RFC2048] discusses more MIME specific security
   considerations.  Even though SILC provides secured messages, in case of
   MIME which can be used to transfer files and documents which are stored in
   the receiver’s local environment, securing separately the MIME object may
   be desired.  For example, augmenting the MIME support in SILC messages to
   support S/MIME may be desired in some implementations.


5 References

   [SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
                Protocol Specification", Internet Draft, June 2003.

   [SILC2]      Riikonen, P., "SILC Packet Protocol", Internet Draft,
                June 2003.

   [SILC3]      Riikonen, P., "SILC Key Exchange and Authentication
                Protocols", Internet Draft, June 2003.

   [RFC2045]    Freed, N., et al., "Multipurpose Internet Mail Extensions
                (MIME) Part One: Format of Internet Message Bodies",
                Standards Track, RFC 2045, November 1996.

   [RFC2046]    Freed, N., et al., "Multipurpose Internet Mail Extensions
                (MIME) Part Two: Media Types", Standards Track, RFC 2045,
                November 1996.

   [RFC2047]    Moore K., "MIME (Multipurpose Internet Mail Extensions)
                Part Three: Message Header Extensions for Non-ASCII Text"
                Standards Track, RFC 2047, November 1996.

   [RFC2048]    Freed, N., et al., "Multipurpose Internet Mail Extensions
                (MIME) Part Four: Registration Procedures", Standards
                Track, RFC 2048, November 1996.

   [RFC2049]    Freed, N., et al., "Multipurpose Internet Mail Extensions
                (MIME) Part Five: Conformance Criteria and Examples",
                Standards Track, RFC 2049, November 1996.




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   [RFC2119]    Bradner, S., "Key Words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.



6 Author’s Address

   Pekka Riikonen
   Snellmaninkatu 34 A 15
   70100 Kuopio
   Finland

   EMail: priikone@iki.fi



7 Full Copyright Statement

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Riikonen                                                       [Page 10]
