Internet DRAFT - draft-riikonen-silc-flags-payloads
draft-riikonen-silc-flags-payloads
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
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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
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
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or assist in its implementation may be prepared, copied, published
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The limited permissions granted above are perpetual and will not be
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