[silc.git] / doc / draft-riikonen-silc-pp-01.nroff

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Network Working Group                                      P. Riikonen
Internet-Draft
draft-riikonen-silc-pp-01.txt                           6 October 2000
Expires: 6 Jun 2001

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.ce 2
SILC Packet Protocol
<draft-riikonen-silc-pp-01.txt>

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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   
distribute working documents as Internet-Drafts.   

Internet-Drafts are draft documents valid for a maximum of six months   
and may be updated, replaced, or obsoleted by other documents at any   
time.  It is inappropriate to use Internet-Drafts as reference   
material or to cite them other than as "work in progress."   

The list of current Internet-Drafts can be accessed at   
http://www.ietf.org/ietf/1id-abstracts.txt   

The list of Internet-Draft Shadow Directories can be accessed at   
http://www.ietf.org/shadow.html   

The distribution of this memo is unlimited.  


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Abstract

This memo describes a Packet Protocol used in the Secure Internet Live
Conferencing (SILC) protocol specified in the Secure Internet Live
Conferencing, Protocol Specification Internet Draft [SILC1].  This
protocol describes the packet types and packet payloads which defines
the contents of the packets.  The protocol provides secure binary packet
protocol that assures that the contents of the packets are secured and
authenticated.









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Table of Contents

.nf
1 Introduction ..................................................  3
2 SILC Packet Protocol ..........................................  4
  2.1 SILC Packet ...............................................  4
  2.2 SILC Packet Header ........................................  5
  2.3 SILC Packet Types .........................................  7
      2.3.1 SILC Packet Payloads ................................ 15
      2.3.2 Generic payloads .................................... 16
            2.3.2.1 ID Payload .................................. 16
            2.3.2.2 Argument Payload ............................ 16
      2.3.3 Disconnect Payload .................................. 17
      2.3.4 Success Payload ..................................... 18
      2.3.5 Failure Payload ..................................... 18
      2.3.6 Reject Payload ...................................... 19
      2.3.7 Notify Payload ...................................... 20
      2.3.8 Error Payload ....................................... 21
      2.3.9 Channel Message Payload ............................. 22
      2.3.10 Channel Key Payload ................................ 24
      2.3.11 Private Message Payload ............................ 26
      2.3.12 Private Message Key Payload ........................ 27
      2.3.13 Command Payload .................................... 28
      2.3.14 Command Reply Payload .............................. 29
      2.3.15 Connection Auth Request Payload .................... 29
      2.3.16 New ID Payload ..................................... 30
      2.3.17 New Client Payload ................................. 31
      2.3.18 New Server Payload ................................. 32
      2.3.19 New Channel Payload ................................ 33
      2.3.20 Key Agreement Payload .............................. XXX
  2.4 SILC ID Types ............................................. 39
  2.5 Packet Encryption And Decryption .......................... 39
      2.5.1 Normal Packet Encryption And Decryption ............. 39
      2.5.2 Channel Message Encryption And Decryption ........... 40
      2.5.3 Private Message Encryption And Decryption ........... 41
  2.6 Packet MAC Generation ..................................... 41
  2.7 Packet Padding Generation ................................. 42
  2.8 Packet Compression ........................................ 42
  2.9 Packet Sending ............................................ 43
  2.10 Packet Reception ......................................... 43
  2.11 Packet Routing ........................................... 44
  2.12 Packet Broadcasting ...................................... 45
  2.13 Packet Tunneling ......................................... 45
3 Security Considerations ....................................... 46
4 References .................................................... 46
5 Author's Address .............................................. 47

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List of Figures

.nf
Figure 1:   Typical SILC Packet
Figure 2:   SILC Packet Header
Figure 3:   ID Payload
Figure 4:   Argument Payload
Figure 5:   Disconnect Payload
Figure 6:   Success Payload
Figure 7:   Failure Payload
Figure 8:   Reject Payload
Figure 9:   Notify Payload
Figure 10:  Error Payload
Figure 11:  Channel Message Payload
Figure 12:  Channel Key Payload
Figure 13:  Private Message Payload
Figure 14:  Private Message Key Payload
Figure 15:  Command Payload
Figure 16:  Connection Auth Request Payload
Figure 17:  New Client Payload
Figure 18:  New Server Payload
Figure 19:  New Channel Payload


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1. Introduction

This document describes a Packet Protocol used in the Secure Internet
Live Conferencing (SILC) protocol specified in the Secure Internet Live
Conferencing, Protocol Specification Internet Draft [SILC1].  This
protocol describes the packet types and packet payloads which defines
the contents of the packets.  The protocol provides secure binary packet
protocol that assures that the contents of the packets are secured and
authenticated.

The basis of SILC protocol relies in the SILC packets and it is with
out a doubt the most important part of the protocol.  It is also probably
the most complicated part of the protocol.  Packets are used all the
time in the SILC network to send messages, commands and other information.
All packets in SILC network are always encrypted and their integrity
is assured by computed MACs.  The protocol defines several packet types
and packet payloads.  Each packet type usually has a specific packet
payload that actually defines the contents of the packet.  Each packet
also includes a default SILC Packet Header that provides sufficient
information about the origin of the packet and destination of the
packet.


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2 SILC Packet Protocol

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2.1 SILC Packet

SILC packets deliver messages from sender to receiver securely by
encrypting important fields of the packet.  The packet consists of
default SILC Packet Header, Padding, Packet Payload data, and, packet 
MAC.

The following diagram illustrates typical SILC packet.


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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|   n bytes   | 1 - n bytes |      n bytes       |  n bytes       
| SILC Header |   Padding   |    Data Payload    |    MAC    
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.in 3

.ce
Figure 1:  Typical SILC Packet


SILC Header is always the first part of the packet and its purpose
is to provide information about the packet.  It provides for example
the packet type, origin of the packet and the destination of the packet.
The header is variable in length and first two (2) bytes of the
header (thus first two bytes of the packet) are not encrypted.  The
first two (2) bytes are the length of the packet which is not encrypted.
See The following section for description of SILC Packet header.  Packets
without SILC header or with malformed SILC header must be dropped.

Padding follows the packet header.  The purpose of the padding is to
make the packet multiple by eight (8) or by the block size of the
cipher used in the encryption, which ever is larger.  The maximum
length of padding is currently 16 bytes.  The padding is always
encrypted.

Data payload area follows padding and it is the actual data of the
packet.  The packet data is the packet payloads defined in this
protocol.  The data payload area is always encrypted.

The last part of SILC packet is the packet MAC that assures the
integrity of the packet.  The MAC is always computed from the packet
before the encryption is applied to the packet.  If compression is used
in the packet the MAC is computed after the compression has been
applied.  The compression, on the other hand, is always applied before
encryption.

All fields in all packet payloads are always in MSB (most significant
byte first) order.


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2.2 SILC Packet Header

The default SILC packet header is applied to all SILC packets and it is
variable in length.  The purpose of SILC Packet header is to provide
detailed information about the packet.  The receiver of the packet uses
the packet header to parse the packet and gain other relevant parameters
of the packet.

The following diagram represents the default SILC header format.
(*) indicates that this field is never encrypted.  Other fields are
always 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Payload Length *       |     Flags     |  Packet Type  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Source ID Length       |     Destination ID Length     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Src ID Type  |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                           Source ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Dst ID Type  |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                         Destination ID                        ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 2:  SILC Packet Header


.in 6
o Payload Length (2 bytes) - Is the length of the packet
  not including the padding of the packet.  This field must
  not be encrypted but must always be authenticated.

o Flags (1 byte) - Indicates flags to be used in packet
  processing.  Several flags may be set by ORing the flags
  together.

  The following flags are reserved for this field:


     No flags                  0x00

       In this case the field is ignored.


     Private Message Key       0x01

       Indicates that the packet must include private
       message that is encrypted using private key set by
       client.  Servers does not know anything about this
       key and this causes that the private message is
       not handled by the server at all, it is just
       passed along.  See section 2.5.3 Private Message
       Encryption And Decryption for more information.


     List                      0x02
  
       Indicates that the packet consists of list of
       packet payloads indicated by the Packet Type field.
       The payloads are added one after the other.  Note that
       there are packet types that must not be used as
       list.  Parsing of list packet is done by calculating
       the length of each payload and parsing them one by
       one.


     Broadcast                 0x04

       Marks the packet to be broadcasted.  Client cannot
       send broadcast packet and normal server cannot send
       broadcast packet.  Only router server may send broadcast
       packet.  The router receiving of packet with this flag 
       set must send (broadcast) the packet to its primary
       route.  If router has several router connections the
       packet may be sent only to the primary route.  See
       section 2.13 Packet Broadcasting for description of 
       packet broadcasting.


     Tunneled                  0x08

       Marks that the packet is tunneled.  Tunneling means
       that extra SILC Packet Header has been applied to the
       original packet.  The outer header has this flag
       set.  See section 2.14 Packet Tunneling for more
       information.
.in 3



o Packet Type (1 byte) - Is the type of the packet. Receiver 
  uses this field to parse the packet.  See section 2.3
  SILC Packets for list of defined packet types.

o Source ID Length (2 bytes) - Indicates the length of the
  Source ID field in the header, not including this or any
  other fields.

o Destination ID Length (2 bytes) - Indicates the length of the
  Destination ID field in the header, not including this or
  any other fields.

o Src ID Type (1 byte) - Indicates the type of ID in the
  Source ID field.  See section 2.4 SILC ID Types for
  defined ID types.

o Source ID (variable length) - The actual source ID that
  indicates who is the original sender of the packet.

o Dst ID Type (1 byte) - Indicates the type of ID in the
  Destination ID field.  See section 2.4 SILC ID Types for
  defined ID types.

o Destination ID (variable length) - The actual source ID that
  indicates who is the end receiver of the packet.


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2.3 SILC Packet Types

SILC packet types defines the contents of the packet and it is used by
the receiver to parse the packet.  The packet type is 8 bits, as a one
byte, in length.  The range for the packet types are from 0 - 255,
where 0 is never sent and 255 is currently reserved for future
extensions and must not be defined to any other purpose.  Every SILC
specification compliant implementation should support all of these packet
types.

The below list of the SILC Packet types includes reference to the packet
payload as well.  Packet payloads are the actual packet, that is, the data
that the packet consists of.  Each packet type defines packet payload 
which usually may only be sent with the specific packet type.

Most of the packets are packets that must be destined directly to entity
that is connected to the sender.  It is not allowed, for example, for
router to send disconnect packet to client that is not directly connected
to the router.  However, there are some special packet types that may
be destined to some entity that the sender has not direct connection
with.  These packets are for example private message packets, channel
message packets, command packets and some other packets that may be
broadcasted in the SILC network.  If the packet is allowed to be sent to
indirectly connected entity it is mentioned separately in the packet
description (unless it is obvious as in private and channel message
packets).  Other packets must not be sent or accepted, if sent, to
indirectly connected entities.

List of SILC Packet types are defined as follows.

.in 1
     0    SILC_PACKET_NONE

          This type is reserved and it is never sent.         


     1    SILC_PACKET_DISCONNECT

          This packet is sent to disconnect the remote end.  Reason of
          the disconnection is sent inside the packet payload.  Client
          usually does not send this packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.3 Disconnect Payload


     2    SILC_PACKET_SUCCESS

          This packet is sent upon successful execution of some protocol.
          The status of the success is sent in the packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.4 Success Payload


     3    SILC_PACKET_FAILURE

          This packet is sent upon failure of some protocol.  The status
          of the failure is sent in the packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.5 Failure Payload


     4    SILC_PACKET_REJECT

          This packet may be sent upon rejection of some protocol.
          The status of the rejection is sent in the packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.6 Reject Payload


     5    SILC_PACKET_NOTIFY

          This packet is used to send notify message, usually from
          server to client, although it may be sent from server to another
          server as well.  Client never sends this packet.  Server may
          send this packet to channel as well when the packet is 
          distributed to all clients on the channel.

          Payload of the packet:  See section 2.3.7 Notify Payload.


     6    SILC_PACKET_ERROR

          This packet is sent when an error occurs.  Server may
          send this packet.  Client never sends this packet.  The
          client may entirely ignore the packet, however, server is
          most likely to take action anyway.  This packet may be sent
          to entity that is indirectly connected to the sender.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.8 Error Payload.


     7    SILC_PACKET_CHANNEL_MESSAGE

          This packet is used to send messages to channels.  The packet
          includes Channel ID of the channel and the actual message to
          the channel.  Messages sent to the channel are always protected
          by channel specific keys.  Channel Keys are distributed by
          SILC_PACKET_CHANNEL_KEY packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.9 Channel Message 
                                  Payload


     8    SILC_PACKET_CHANNEL_KEY

          This packet is used to distribute new key for particular
          channel.  Each channel has their own independent keys that
          is used to protect the traffic on the channel.  Only server
          may send this packet.  This packet may be sent to entity
          that is indirectly connected to the sender.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.10 Channel Key Payload


     9    SILC_PACKET_PRIVATE_MESSAGE

          This packet is used to send private messages from client
          to another client.  By default, private messages are protected
          by session keys established by normal key exchange protocol.
          However, it is possible to use specific key to protect private
          messages.  SILC_PACKET_PRIVATE_MESSAGE_KEY packet is used to 
          agree the key with the remote client.  Pre-shared key may be 
          used as well if both of the client knows it, however, it needs 
          to be agreed outside SILC.  See more of this in [SILC1].

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.11 Private Message
                                  Payload


     10   SILC_PACKET_PRIVATE_MESSAGE_KEY

          This packet is used to agree about a key to be used to protect
          the private messages between two clients.  If this is not sent
          the normal session key is used to protect the private messages
          inside SILC network.  Agreeing to use specific key to protect
          private messages adds security, as no server between the two
          clients will be able to decrypt the private message.  However,
          servers inside SILC network are considered to be trusted, thus
          using normal session key to protect private messages does not
          degree security.  Whether to agree to use specific keys by
          default or to use normal session keys by default, is 
          implementation specific issue.  See more of this in [SILC1].

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.12 Private Message
                                  Key Payload


     11   SILC_PACKET_COMMAND

          This packet is used to send commands from client to server.
          Server may send this packet to other servers as well.  All
          commands are listed in their own section SILC Command Types
          in [SILC1].  The contents of this packet is command specific.
          This packet may be sent to entity that is indirectly connected
          to the sender.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.13 Command Payload


     12   SILC_PACKET_COMMAND_REPLY

          This packet is send as reply to the SILC_PACKET_COMMAND packet.
          The contents of this packet is command specific.  This packet
          maybe sent to entity that is indirectly connected to the sender.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.14 Command Reply 
                                  Payload and section 2.3.13 Command
                                  Payload


     13   SILC_PACKET_KEY_EXCHANGE

          This packet is used to start SILC Key Exchange Protocol, 
          described in detail in [SILC3].

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  Payload of this packet is described
                                  in the section SILC Key Exchange
                                  Protocol and its sub sections in
                                  [SILC3].


     14   SILC_PACKET_KEY_EXCHANGE_1

          This packet is used as part of the SILC Key Exchange Protocol.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  Payload of this packet is described
                                  in the section SILC Key Exchange
                                  Protocol and its sub sections in
                                  [SILC3].


     15   SILC_PACKET_KEY_EXCHANGE_2

          This packet is used as part of the SILC Key Exchange Protocol.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  Payload of this packet is described
                                  in the section SILC Key Exchange
                                  Protocol and its sub sections in
                                  [SILC3].


     16   SILC_PACKET_CONNECTION_AUTH_REQUEST

          This packet is used to request the authentication method to
          be used in the SILC Connection Authentication Protocol.  If 
          initiator of the protocol does not know the mandatory 
          authentication method this packet is used to determine it.

          The party receiving this payload must respond with the same
          packet including the mandatory authentication method.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.15 Connection Auth
                                  Request Payload


     17   SILC_PACKET_CONNECTION_AUTH

          This packet is used to start and perform the SILC Connection
          Authentication Protocol.  This protocol is used to authenticate
          the connecting party.  The protocol is described in detail in
          [SILC3].

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  Payload of this packet is described
                                  in the section SILC Authentication
                                  Protocol and it sub sections in [SILC].


     18   SILC_PACKET_NEW_ID

          This packet is used to distribute new ID's from server to
          router and from router to all routers in the SILC network.
          This is used when for example new client is registered to
          SILC network.  The newly created ID's of these operations are
          distributed by this packet.  Only server may send this packet,
          however, client must be able to receive this packet.

          Payload of the packet:  See section 2.3.16 New ID Payload


     19   SILC_PACKET_NEW_CLIENT

          This packet is used by client to register itself to the   
          SILC network.  This is sent after key exchange and  
          authentication protocols has been completed.  Client sends
          various information about itself in this packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.17 New Client Payload


     20   SILC_PACKET_NEW_SERVER

          This packet is used by server to register itself to the
          SILC network.  This is sent after key exchange and 
          authentication protocols has been completed.  Server sends
          this to the router it connected to, or, if router was
          connecting, to the connected router.  Server sends
          its Server ID and other information in this packet.
          Client must not send or receive this packet.

          This packet must not be sent as list and the List flag must
          not be set.

          Payload of the packet:  See section 2.3.18 New Server Payload


     21   SILC_PACKET_NEW_CHANNEL

          This packet is used to notify routers about newly created
          channel.  Channels are always created by the router and it must
          notify other routers about the created channel.  Router sends
          this packet to its primary route.  Client must not send this
          packet.  This packet maybe sent to entity that is indirectly
          connected to the sender.

          Payload of the packet:  See section 2.3.19 New Channel Payload


     22   SILC_PACKET_REKEY

          This packet is used to indicate that re-key must be performed
          for session keys.  See section Session Key Regeneration in
          [SILC1] for more information.  This packet does not have
          a payload.

          This packet must not be sent as list and the List flag must
          not be set.


     23   SILC_PACKET_REKEY_DONE

          This packet is used to indicate that re-key is performed and
          new keys must be used hereafter.  This is sent only if re-key
          was done without PFS option.  If PFS is set, this is not sent
          as SILC Key Exchange protocol is executed.  This packet does
          not have a payload.

          This packet must not be sent as list and the List flag must
          not be set.

     
     24   SILC_PACKET_HEARTBEAT

          This packet is used by clients, servers and routers to keep the
          connection alive.  It is recommended that all servers implement
          keepalive actions and perform it to both direction in a link.
          This packet does not have a payload.

          This packet must not be sent as list and the List flag must
          not be set.


     25   SILC_PACKET_KEY_AGREEMENT

          This packet is used by clients to request key negotiation 
          between another client in the SILC network.  If the negotiation
          is started it is performed using the SKE protocol.  The result of
          the negotiation, the secret key material, can be used for
          example as private message key.  The server and router must not
          send this packet.

          Payload of the packet:  See section 2.3.20 Key Agreement Payload


    26    SILC_PACKET_CELL_ROUTERS

          This packet is used by primary router in the cell to notify its
          primary router what other routers (backup routers) exist in the
          cell.  In case of failure of the primary router in the cell the
          first router in the list will act as primary router of the cell.
          This packet may be sent at anytime after connection has been
          registered to the primary router.  The client must not send this
          packet.

          Payload of the packet:  See section 2.3.21 Cell Routers Payload


     27 - 199

         Currently undefined commands.


     200 - 254

         These packet types are reserved for private use and they will not
         be defined by this document.


     255 SILC_PACKET_MAX

         This type is reserved for future extensions and currently it 
         is not sent.
.in 3


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2.3.1 SILC Packet Payloads

All payloads resides in the main data area of the SILC packet.  However
all payloads must be at the start of the data area after the default
SILC packet header and padding.  All fields in the packet payload are
always encrypted, as, they reside in the data area of the packet which
is always encrypted.

Payloads described in this section are common payloads that must be
accepted anytime during SILC session.  Most of the payloads may only
be sent with specific packet type which is defined in the description
of the payload.

There are a lot of other payloads in the SILC as well.  However, they
are not common in the sense that they could be sent at any time. 
These payloads are not described in this section.  These are payloads
such as SILC Key Exchange payloads and so on.  These are described
in [SILC1] and [SILC3].


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2.3.2 Generic payloads

This section describes generic payloads that are not associated to any
specific packet type.  They can be used for example inside some other
packet payloads.


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2.3.2.1 ID Payload

This payload can be used to send an ID.  ID's are variable length thus
this payload provides a way to send variable length ID's.

The following diagram represents the ID Payload.

.in 5
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             ID Type           |           ID Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                           ID Data                             ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 3:  ID Payload


.in 6
o ID Type (2 bytes) - Indicates the type of the ID.  See 
  section 2.4 SILC ID Types for list of defined ID types.

o ID Length (2 bytes) - Length of the ID Data area not 
  including the length of any other fields in the payload.

o ID Data (variable length) - The actual ID data.
.in 3


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2.3.2.2 Argument Payload

Argument Payload is used to set arguments for any packet payload that
needs and supports arguments, such as commands.  Number of arguments
associated with a packet must be indicated by the packet payload who
needs the arguments. Argument Payloads must always reside right after
the packet payload needing the arguments.  Incorrect amount of argument
payloads must cause rejection of the packet.  The following diagram represents
the Argument Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        | Argument Type |               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
|                                                               |
~                        Argument Data                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 4:  Argument Payload


.in 6
o Payload Length (2 bytes) - Length of the argument payload data 
  area not including the length of any other fields in the 
  payload.

o Argument Type (1 byte) - Indicates the type of the argument.  
  Every argument may have a specific type that must be defined
  by the packet payload needing the argument.  For example
  every command specify a number for each argument that maybe 
  associated with the command.  By using this number the receiver 
  of the packet knows what type of argument this is.  If there is
  no specific argument type this field is set to zero (0).

o Argument Data (variable length) - Argument data.
.in 3


.ti 0
2.3.3 Disconnect Payload

Disconnect payload is sent upon disconnection.  The payload is simple;
reason of disconnection is sent to the disconnected party.

The payload may only be sent with SILC_PACKET_DISCONNECT packet.  It
must not be sent in any other packet type.  The following diagram represents
the Disconnect Payload.







.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                      Disconnect Message                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 5:  Disconnect Payload




.in 6
o Disconnect Message (variable length) - Human readable
  reason of the disconnection.
.in 3


.ti 0
2.3.4 Success Payload

Success payload is sent when some protocol execution is successfully
completed.  The payload is simple; indication of the success is sent.
This maybe any data, including binary or human readable data.

.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                      Success Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 6:  Success Payload


.in 6
o Success Indication (variable length) - Indication of
  the success.  This maybe for example some flag that
  indicates the protocol and the success status or human
  readable success message.  The true length of this
  payload is available by calculating it from the SILC
  Packet Header.
.in 3


.ti 0
2.3.5 Failure Payload

This is opposite of Success Payload.  Indication of failure of
some protocol is sent in the payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                      Failure Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 7:  Failure Payload


.in 6
o Failure Indication (variable length) - Indication of
  the failure.  This maybe for example some flag that
  indicates the protocol and the failure status or human
  readable failure message.  The true length of this
  payload is available by calculating it from the SILC
  Packet Header.
.in 3


.ti 0
2.3.6 Reject Payload

This payload is sent when some protocol is rejected to be executed.
Other operations may send this as well that was rejected.  The
indication of the rejection is sent in the payload.  The indication
may be binary or human readable data.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                       Reject Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 8:  Reject Payload


.in 6
o Reject Indication (variable length) - Indication of
  the rejection.  This maybe for example some flag that
  indicates the protocol and the rejection status or human
  readable rejection message.  The true length of this
  payload is available by calculating it from the SILC
  Packet Header.
.in 3





.ti 0
2.3.7 Notify Payload

Notify payload is used to send notify messages.  The payload is usually
sent from server to client, however, server may send it to another
server as well.  This payload may also be sent to a channel.  Client must
not send this payload.  The receiver of this payload may totally ignore the
contents of the payload, however, notify message should be audited.

The payload may only be sent with SILC_PACKET_NOTIFY packet.  It must
not be sent in any other packet type.  The following diagram represents the
Notify Payload.

.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Notify Type          |        Payload Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Argument Nums |
+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 9:  Notify Payload


.in 6
o Notify Type (2 bytes) - Indicates the type of the notify
  message.

o Payload Length (2 bytes) - Length of the entire Notify Payload
  including any associated Argument Payloads.

o Argument Nums (2 bytes) - Indicates the number of Argument
  Payloads associated to this payload.  Notify types may define
  arguments to be send along the notify message.
.in 3

The following list of currently defined notify types.  The format for notify
arguments is same as in SILC commands described in [SILC1].  Also, all
ID's sent in arguments are sent inside ID Payload.

.in 6
0     SILC_NOTIFY_TYPE_NONE

      If no specific notify type apply for the notify message this type
      may be used.

      Max Arguments:  1
          Arguments:  (1) <message>

      The <message> is implementation specific free text string.  Receiver
      may ignore this message.


1     SILC_NOTIFY_TYPE_INVITE

      Sent when receiver has been invited to a channel.  This type must be
      sent directly to the invited client.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) <Channel ID>

      The <Client ID> is the client who invites the receiver of this type 
      to channel indicated by <Channel ID>.


2     SILC_NOTIFY_TYPE_JOIN

      Sent when client has joined to a channel.  The server must distribute
      this type only to the local clients on the channel and then send
      it to its primary router.  The router or server receiving the packet
      distributes this type to the local clients on the channel and
      broadcast it to the network.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) <Channel ID>

      The <Client ID> is the client that joined to the channel indicated
      by the <Channel ID>.


3     SILC_NOTIFY_TYPE_LEAVE

      Sent when client has left a channel.  The server must distribute
      this type only to the local clients on the channel and then send
      it to its primary router.  The router or server receiving the packet
      distributes this type to the local clients on the channel and
      broadcast it to the network.

      Max Arguments:  1
          Arguments:  (1) <Client ID>

      The <Client ID> is the client who left the channel.


4     SILC_NOTIFY_TYPE_SIGNOFF

      Sent when client signoffs from SILC network.  The server must
      distribute this type only to the local clients on the channel and
      then send it to its primary router.  The router or server receiving
      the packet distributes this type to the local clients on the channel
      and broadcast it to the network.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) <message>

      The <Client ID> is the client who left SILC network.  The <message>
      is free text string indicating the reason of signoff.


5     SILC_NOTIFY_TYPE_TOPIC_SET

      Sent when topic is set/changed on a channel.  This type must be sent
      only to the clients who is joined on the channel whose topic was
      set or changed.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) <topic>

      The <Client ID> is the client who set or changed the <topic>.


6     SILC_NOTIFY_TYPE_NICK_CHANGE

      Sent when client changes nick on a channel.  The server must
      distribute this type only to the local clients on the channel and
      then send it to its primary router. The router or server receiving
      the packet distributes this type to the local clients on the channel
      and broadcast it to the network.

      Max Arguments:  2
          Arguments:  (1) <Old Client ID>  (2) <New Client ID>

      The <Old Client ID> is the old ID of the client who changed the 
      nickname.  The <New Client ID> is the new ID generated by the change
      of the nickname.


7     SILC_NOTIFY_TYPE_CMODE_CHANGE

      Sent when channel mode has changed.  This type must be sent only to
      the clients who is joined on the channel whose mode was changed.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) <mode mask>

      The <Client ID> is the client who changed the mode.  The <mode mask>
      is the new mode mask of the channel.


8     SILC_NOTIFY_TYPE_CUMODE_CHANGE

      Sent when user mode on channel has changed.  This type must be sent
      only to the clients who is joined on the channel where the target 
      client is on.

      Max Arguments:  3
          Arguments:  (1) <Client ID>  (2) <mode mask>
                      (3) <Target Client ID>

      The <Client ID> is the client who changed the mode.  The <mode mask>
      is the new mode mask of the channel.  The <Target Client ID> is the
      client which mode was changed.


9     SILC_NOTIFY_TYPE_MOTD

      Sent when Message of the Day (motd) is sent to client.

      Max Arguments:  1
          Arguments:  (1) <motd>

      The <motd> is the Message of the Day.


10    SILC_NOTIFY_TYPE_CHANNEL_CHANGE

      Sent when channel's ID has changed for a reason or another.  This 
      is sent by noral server to the client.  Client must change the 
      old Channel ID to the new one.  This type must be sent only to the
      clients who is joined on the channel.

      Max Arguments:  2
          Arguments:  (1) <Old Channel ID>  (2) <New Channel ID>

      The <Old Channel ID> is the channel's old ID and the <New Channel ID>
      is the new one that must replace the old one.


11    SILC_NOTIFY_TYPE_SERVER_SIGNOFF

      Sent when server quits SILC network.  Those clients from this server
      that are on channels must be removed from the channel.

      Max Arguments:  1
          Arguments:  (1) <Server ID>

      The <Server ID> is the server's ID.


12    SILC_NOTIFY_TYPE_KICKED

      Sent when a client has been kicked from a channel.  This is sent 
      also to the client who was kicked from the channel.  The client
      who was kicked from the channel must be removed from the channel.
      This notify type is always destined to the channel.  The router or
      server receiving the packet distributes this type to the local
      clients on the channel and broadcast it to the network.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) [<comment>]

      The <Client ID> is the client who was kicked from the channel.
      The kicker may have set the <comment> to indicate the reason for
      the kicking.


13    SILC_NOTIFY_TYPE_KILLED

      Sent when a client has been killed from the network.  This is sent 
      also to the client who was killed from the network.  The client
      who was killed from the network must be removed from the network.
      This notify type is destined directly to the client who was killed
      and to channel if the client is on any channel.  The router or
      server receiving the packet distributes this type to the local
      clients on the channel and broadcast it to the network.

      Max Arguments:  2
          Arguments:  (1) <Client ID>  (2) [<comment>]

      The <Client ID> is the client who was killed from the network.
      The killer may have set the <comment> to indicate the reason for
      the killing.

.in 3

Notify types starting from 16384 are reserved for private notify
message types.


.ti 0
2.3.8 Error Payload

Error payload is sent upon error.  Error may occur in various
conditions when server sends this packet.  Client may not send this
payload but must be able to accept it.  However, client may
totally ignore the contents of the packet as server is going to
take action on the error anyway.  However, it is recommended
that the client takes error packet seriously.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                         Error Message                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 10:  Error Payload


.in 6
o Error Message (variable length) - Human readable error
  message.
.in 3


.ti 0
2.3.9 Channel Message Payload

Channel messages are the most common messages sent in the SILC.
Channel Message Payload is used to send message to channels.  These
messages can only be sent if client has joined to some channel.
Even though this packet is the most common in SILC it is still
special packet.  Some special handling on sending and reception
of channel message is required.

Padding must be applied into this payload since the payload is
encrypted separately from other parts of the packet with the
channel specific key.  Hence the requirement of the padding.  
The padding should be random data.  The packet must be made
multiple by eight (8) or by the block size of the cipher, which
ever is larger.

The SILC header in this packet is encrypted with the session key
of the next receiver of the packet.  Nothing else is encrypted
with that key.  Thus, the actual packet and padding to be
encrypted with the session key is SILC Header plus padding to it
to make it multiple by eight (8) or multiple by the block size
of the cipher, which ever is larger.

Receiver of the the channel message packet is able to determine
the channel the message is destined to by checking the destination
ID from the SILC Packet header which tells the destination channel.
The original sender of the packet is also determined by checking
the source ID from the header which tells the client who sent
the message.

The payload may only be sent with SILC_PACKET_CHANNEL_MESSAGE packet.
It  must not be sent in any other packet type.  The following diagram 
represents the Channel Message Payload.

(*) indicates that the field is not encrypted.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Message Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Message Data                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Padding Length         |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                            Padding                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                              MAC                              ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                       Initial Vector *                        ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 11:  Channel Message Payload


.in 6
o Message Length (2 bytes) - Indicates the length of the
  the Message Data field in the payload, not including any 
  other field.

o Message Data (variable length) - The actual message to
  the channel.

o Padding Length (2 bytes) - Indicates the length of the
  Padding field in the payload, not including any other
  field.

o Padding (variable length) - The padding that must be
  applied because this payload is encrypted separately from
  other parts of the packet.

o MAC (variable legnth) - The MAC computed from the
  Message Length, Message Data, Padding Length and Padding
  fields.  This protects the integrity of the plaintext
  channel message.  The receiver can verify from the MAC
  whether the message decrypted correctly.  Also, if more than
  one private key has been set for the channel, the receiver
  can verify which of the keys decrypted the message 
  correctly.  Note that, this field is encrypted and must
  be added to the padding calculation.

o Initial Vector (variable length) - The initial vector
  that has been used in packet encryption.  It needs to be
  used in the packet decryption as well.  What this field
  includes is implementation issue.  However, it is 
  recommended that it would be random data or, perhaps,
  a timestamp.  It is not recommended to use zero (0) as
  initial vector.  This field is not encrypted.  This field
  is not included into the padding calculation.  Length
  of this field equals the cipher's block size.  This field
  is, however, authenticated.
.in 3


.ti 0
2.3.10 Channel Key Payload

All traffic in channels are protected by channel specific keys.
Channel Key Payload is used to distribute channel keys to all
clients on the particular channel.  Channel keys are sent when
the channel is created, when new user joins to the channel and
whenever a user has left a channel.  Server creates the new
channel key and distributes it to the clients by encrypting this
payload with the session key shared between the server and
the client.  After that, client starts using the key received
in this payload to protect the traffic on the channel.

The client who is joining to the channel receives its key in the
SILC_COMMAND_JOIN command reply message thus it is not necessary to
send this payload to the entity who sent the SILC_COMMAND_JOIN command.

Channel keys are cell specific thus every router in cell have
to create a channel key and distribute it if any client in the
cell has joined to a channel.  Channel traffic between cell's
are not encrypted using channel keys, they are encrypted using
normal session keys between two routers.  Inside a cell, all
channel traffic is encrypted with the specified channel key.
Channel key should expire periodically, say, in one hour, in
which case new channel key is created and distributed.

The payload may only be sent with SILC_PACKET_CHANNEL_KEY packet.
It must not be sent in any other packet type.  The following diagram 
represents the Channel Key Payload.













.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Channel ID Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Channel ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Cipher Name Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Cipher Name                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Channel Key Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Channel Key                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 12:  Channel Key Payload



.in 6
o Channel ID Length (2 bytes) - Indicates the length of the
  Channel ID field in the payload, not including any other
  field.

o Channel ID (variable length) - The Channel ID of the
  channel this key is meant for.

o Cipher Name Length (2 bytes) - Indicates the length of the
  Cipher name field in the payload, not including any other
  field.

o Cipher Name (variable length) - Name of the cipher used
  in the protection of channel traffic.  This name is
  initially decided by the creator of the channel but it
  may change during the life time of the channel as well.

o Channel Key Length (2 bytes) - Indicates the length of the
  Channel Key field in the payload, not including any other
  field.

o Channel Key (variable length) - The actual channel key
  material.  This key is used as such as key material for
  encryption function.
.in 3


.ti 0
2.3.11 Private Message Payload

Private Message Payload is used to send private message between
two clients (or users for that matter).  The messages are sent only
to the specified user and no other user inside SILC network is
able to see the message.  The message is protected by the session 
key established by the SILC Key Exchange Protocol.  However,
it is also possible to agree to use specific keys to protect
just the private messages.  See section 2.3.11 Private Message
Key Payload for detailed description of how to agree to use
specific key.

If normal session key is used to protect the message, every
server between the sender client and the receiving client needs
to decrypt the packet and always re-encrypt it with the session
key of the next receiver of the packet.  See section Client
To Client in [SILC1].

When specific key is used to protect the message, servers between 
the sender and the receiver needs not to decrypt/re-encrypt the 
packet.  Section 4.8.2 Client To Client in [SILC1] gives example of
this scheme as well.

The payload may only be sent with SILC_PACKET_PRIVATE_MESSAGE 
packet.  It must not be sent in any other packet type.  The following 
diagram represents the Private Message Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Nickname Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                            Nickname                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                          Message Data                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 13:  Private Message Payload


.in 6
o Nickname Length (2 bytes) - Indicates the length of the
  Nickname field, not including any other field.

o Nickname (variable length) - Nickname of the sender of the 
  private message.  This should not be trusted as a definite 
  sender of the private message.  The SILC Packet Header in 
  the packet indicates the true sender of the packet and 
  client should verify that the nickname sent here belongs 
  to the Client ID in the SILC Packet Header.  This nickname 
  is merely provided to be displayed by the client.

o Message Data (variable length) - The actual message to
  the client.  Rest of the packet is reserved for the message
  data.
.in 3


.ti 0
2.3.12 Private Message Key Payload

This payload is used to send key from client to another client that
is going to be used to protect the private messages between these
two clients.  If this payload is not sent normal session key 
established by the SILC Key Exchange Protocol is used to protect
the private messages.

This payload may only be sent by client to another client.  Server
must not send this payload at any time.  After sending this payload
the sender of private messages must set the Private Message Key
flag into SILC Packet Header.

The payload may only be sent with SILC_PACKET_PRIVATE_MESSAGE_KEY 
packet.  It must not be sent in any other packet type.  The following 
diagram represents the Private Message Key Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Private Message Key Length   |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                      Private Message Key                      ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Cipher Name Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Cipher Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 14:  Private Message Key Payload




.in 6
o Private Message Key Length (2 bytes) - Indicates the length 
  of the Private Message Key field in the payload, not including 
  any other field.

o Private Message Key (variable length) - The actual private
  message key material.

o Cipher Name Length (2 bytes) - Indicates the length of the
  Cipher Name field in the payload, not including any other
  field.

o Cipher Name (variable length) - Name of the cipher to use
  in the private message encryption.  If this field does not
  exist then the default cipher of the SILC protocol is used.
  See the [SILC1] for defined ciphers.
.in 3



.ti 0
2.3.13 Command Payload

Command Payload is used to send SILC commands from client to server.
Also server may send commands to other servers.  The following diagram
represents the Command Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        | SILC Command  | Arguments Num |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Command Identifier      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 15:  Command Payload


.in 6
o Payload Length (2 bytes) - Length of the entire command 
  payload including any command argument payloads associated 
  with this payload.

o SILC Command (1 byte) - Indicates the SILC command.  This must 
  be set to non-zero value.  If zero (0) value is found in this 
  field the packet must be discarded.

o Arguments Num (1 byte) - Indicates the number of arguments
  associated with the command.  If there are no arguments this
  field is set to zero (0).  The arguments must follow the 
  command payload.  See section 2.3.2.2 for definition of the
  Argument Payload.

o Command Identifier (2 bytes) - Identifies this command at the
  sender's end.  The entity who replies to this command must
  set the value found from this field into the Command Payload
  used to send the reply to the sender.  This way the sender
  can identify which command reply belongs to which originally
  sent command.  What this field includes is implementation
  issue but it is recommended that wrapping counter value is
  used in the field.  Value zero (0) in this field means that
  no specific value is set.
.in 3

See [SILC1] for detailed description of different SILC commands,
their arguments and their reply messages.


.ti 0
2.3.14 Command Reply Payload

Command Reply Payload is used to send replies to the commands.  The
Command Reply Payload is identical to the Command Payload thus see the
upper sections for Command Payload and for Command Argument Payload
specifications.  Command Reply message uses the Command Argument Payload
as well.

The entity who sends the reply packet must set the Command Unifier
field in the reply packet's Command Payload to the value it received
in the original command packet.

See SILC Commands in [SILC1] for detailed description of different
SILC commands, their arguments and their reply messages.


.ti 0
2.3.15 Connection Auth Request Payload

Client may send this payload to server to request the authentication
method that must be used in authentication protocol.  If client knows 
this information beforehand this payload is not necessary to be sent.
Server performing authentication with another server may also send
this payload to request the authentication method.  If the connecting
server already knows this information this payload is not necessary
to be sent.

Server receiving this request must reply with same payload sending
the mandatory authentication method.  Algorithms that may be required
to be used by the authentication method are the ones already 
established by the SILC Key Exchange protocol.  See section Key
Exchange Start Payload in [SILC3] for detailed information.

The payload may only be sent with SILC_PACKET_CONNECTION_AUTH_REQUEST
packet.  It must not be sent in any other packet type.  The following 
diagram represents the Connection Auth Request Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Connection Type        |     Authentication Method     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 16:  Connection Auth Request Payload


.in 6
o Connection Type (2 bytes) - Indicates the type of the ID.
  The following connection types are defined:

     1    Client connection
     2    Server connection
     3    Router connection

  If any other type is found in this field the packet must be
  discarded and the authentication must be failed.

o Authentication Method (2 bytes) - Indicates the authentication
  method to be used in the authentication protocol.  The following
  authentication methods are defined:



     0    NONE        (mandatory)
     1    password    (mandatory)
     2    public key  (mandatory)

  If any other type is found in this field the packet must be
  discarded and the authentication must be failed.  If this
  payload is sent as request to receive the mandatory 
  authentication method this field must be set to zero (0),
  indicating that receiver should send the mandatory 
  authentication method.  The receiver sending this payload
  to the requesting party, may also set this field to zero (0) 
  to indicate that authentication is not required.  In this
  case authentication protocol still must be started but
  server is most likely to respond with SILC_PACKET_SUCCESS
  immediately.
.in 3


.ti 0
2.3.16 New ID Payload

New ID Payload is a multipurpose payload.  It is used to send newly 
created ID's from clients and servers.  When client connects to server
and registers itself to the server by sending SILC_PACKET_NEW_CLIENT
packet, server replies with this packet by sending the created ID for
the client.  Server always creates the ID for the client.

This payload is also used when server tells its router that new client
has registered to the SILC network.  In this case the server sends
the Client ID of the client to the router.  Similary when router
distributes information to other routers about the client in the SILC
network this payload is used.  

Also, when server connects to router, router uses this payload to inform
other routers about new server in the SILC network.  However, every 
server (or router) creates their own ID's thus the ID distributed by 
this payload is not created by the distributor in this case.  Servers
create their own ID's.  Server registers itself to the network by sending
SILC_PACKET_NEW_SERVER to the router it connected to.  The case is same
when router connects to another router.

However, this payload is not and must not be used to send information
about new channels.  New channels are always distributed by sending the
dedicated SILC_PACKET_NEW_CHANNEL packet.

Hence, this payload is very important and used every time when some
new entity is registered to the SILC network.  Client never sends this
payload.  Both client and server (and router) may receive this payload.

The packet uses generic ID Payload as New ID Payload.  See section
2.3.2.1 for generic ID Payload.


.ti 0
2.3.17 New Client Payload

When client is connected to the server, keys has been exchanged and
connection has been authenticated client must register itself to the 
server.  Clients first packet after key exchange and authentication 
protocols must be SILC_PACKET_NEW_CLIENT.  This payload tells server all
the relevant information about the connected user.  Server creates a new
client ID for the client when received this payload and sends it to the
client in New ID Payload.

This payload sends username and real name of the user on the remote host
which is connected to the SILC server with SILC client.  The server 
creates the client ID according the information sent in this payload.
The nickname of the user becomes the username sent in this payload.
However, client should call NICK command after sending this payload to
set the real nickname of the user which is then used to create new 
client ID.

The payload may only be sent with SILC_PACKET_NEW_CLIENT packet.  It
must not be sent in any other packet type.  The following diagram
represents the New Client Payload.



.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Username Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Username                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Real Name Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Real Name                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 17:  New Client Payload


.in 6
o Username Length (2 bytes) - Length of the username.

o Username (variable length) - The username of the user on
  the host where connecting to the SILC server.

o Real Name Length (2 bytes) - Length of the Real Name.

o Real Name (variable length) - The real name of the user
  on the host where connecting to the SILC server.
.in 3


.ti 0
2.3.18 New Server Payload

This payload is sent by server when it has completed successfully both
key exchange and connection authentication protocols.  The server
uses this payload to register itself to the SILC network.  The
first packet after these key exchange and authentication protocols
is SILC_PACKET_NEW_SERVER packet.  The payload includes the Server ID
of the server that it has created by itself.  It also includes a
name of the server that is associated to the Server ID.

The payload may only be sent with SILC_PACKET_NEW_SERVER packet.  It
must not be sent in any other packet type.  The following diagram represents
the New Server Payload.





.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Server ID Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                        Server ID Data                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Server Name Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Server Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 18:  New Server Payload


.in 6
o Server ID Length (2 bytes) - Length of the ID Data area not 
  including the length of any other fields in the payload.

o Server ID Data (variable length) - The actual Server ID
   data.

o Server Name Length (2 bytes) - Length of the server name.

o Server Name (variable length) - The server name.
.in 3


.ti 0
2.3.19 New Channel Payload

Information about newly created channel is broadcasted to all routers
in the SILC network by sending this packet payload.  Channels are
created by router of the cell.  Server never creates channels unless
it is a standalone server and it does not have router connection,
in this case server acts as router.  Normal server send JOIN command
to the router (after it has received JOIN command from client) which
then processes the command and creates the channel.  Client never sends
this packet.

The payload may only be sent with SILC_PACKET_NEW_CHANNEL packet.
It must not be sent in any other packet type.  The following diagram
represents the New Channel Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Channel Name Length      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Channel Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Channel ID Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Channel ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 19:  New Channel Payload



.in 6
o Channel Name Length (2 bytes) - Length of the channel name.

o Channel Name (variable length) - The name of the created
  channel.

o Channel ID Length (2 bytes) - Length of the Channel ID.

o Channel ID (variable length) - The created Channel ID.
.in 3


.ti 0
2.3.20 Key Agreement Payload

This payload is used by clients to request key negotiation between
another client in the SILC Network.  The key agreement protocol used
is the SKE protocol.  The result of the protocol, the secret key
material, can be used for example as private message key between the
two clients.  This significantly adds security as the key agreement
is performed outside the SILC network.  The server and router must not
send this payload.

The sender may tell the receiver of this payload the hostname and the
port where the SKE protocol is running in the sender's end.  The 
receiver may then initiate the SKE negotiation with the sender.  The
sender may also optionally not to include the hostname and the port
of its SKE protocol.  In this case the receiver may reply to the
request by sending the same payload filled with the receiver's hostname
and the port where the SKE protocol is running.  The sender may then
initiate the SKE negotiation with the receiver.

The payload may only be sent with SILC_PACKET_KEY_AGREEMENT packet.
It must not be sent in any other packet type.  The following diagram
represents the Key Agreement Payload.


.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Hostname Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Hostname                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Port                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 20:  Key Agreement Payload


.in 6
o Hostname Length (2 bytes) - Indicates the length of the Hostname
  field.

o Hostname (variable length) - The hostname or IP address where
  the SKE protocol is running.  The sender may fill this field
  when sending the payload.  If the receiver sends this payload
  as reply to the request it must fill this field.

o Port (4 bytes) - The port where the SKE protocol is bound.
  The sender may fill this field when sending the payload.  If
  the receiver sends this payload as reply to the request it 
  must fill this field.  This is a 32 bit MSB first order value.
.in 3


After the key material has been received from the SKE protocol it is
processed as the [SILC3] describes.  If the key material is used as
channel private key then the Sending Encryption Key, as defined in
[SILC3] is used as the channel private key.  Other key material must
be discarded.  The [SILC1] defines the way to use the key material if
it is intended to be used as private message keys.  Any other use for
the key material is undefined.


.ti 0
2.3.21 Cell Routers Payload

Cell Routers payload is used by router to notify its primary router what
other routers exist in the cell.  The other routers are considered to be
backup routers and one of them will come active only in the case of
failure of the primary router.  Normal server can send this packet if it
is acting as backup router.  Client must not send this packet.  To send
more than one backup router set the List flag and assemble the payloads
as list.

The payload may only be sent with SILC_PACKET_CELL_ROUTERS packet.  It
must not be sent in any other packet type.  The Following diagram
represents the Cell Routers Payload.

         
.in 5
.nf
                     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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Hostname Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Hostname                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Port                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Server ID Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Server ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 21:  Cell Routers Payload


.in 6
o Hostname Length (2 bytes) - Indicates the length of the Hostname
  field.

o Hostname (variable length) - The hostname or IP address of
  the backup router.

o Port (4 bytes) - The port of the backup router it currently uses.
  This is a 32 bit MSB first order value.

o Server ID Length (2 bytes) - Indicates the length of the Server
  ID field.

o Server ID (variable length) - Consists of the Server ID of the
  backup router.
.in 3


.ti 0
2.4 SILC ID Types

ID's are extensively used in the SILC network to associate different
entities.  The following ID's has been defined to be used in the SILC
network.

.in 6
0    No ID

   When ever specific ID cannot be used this is used.

1    Server ID

   Server ID to associate servers.  See the format of
   this ID in [SILC1].

2    Client ID

   Client ID to associate clients.  See the format of
   this ID in [SILC1].

3    Channel ID

   Channel ID to associate channels.  See the format of
   this ID in [SILC1].
.in 3


.ti 0
2.5 Packet Encryption And Decryption

SILC packets are encrypted almost entirely.  Only small part of SILC
header is not encrypted as described in section 5.2 SILC Packet Header.
The SILC Packet header is the first part of a packet to be encrypted
and it is always encrypted with the key of the next receiver of the
packet.  The data payload area of the packet is always entirely 
encrypted and it is usually encrypted with the next receiver's key.
However, there are some special packet types and packet payloads
that require special encryption process.  These special cases are
described in the next sections.  First is described the normal packet
encryption process.


.ti 0
2.5.1 Normal Packet Encryption And Decryption

Normal SILC packets are encrypted with the session key of the next
receiver of the packet.  The entire SILC Packet header and the packet
data payload is is also encrypted with the same key.  Padding of the
packet is also encrypted always with the session key, also in special
cases.  Computed MAC of the packet must not be encrypted.

Decryption process in these cases are straightforward.  The receiver
of the packet must first decrypt the SILC Packet header, or some parts
of it, usually first 16 bytes of it.  Then the receiver checks the
packet type from the decrypted part of the header and can determine
how the rest of the packet must be decrypted.  If the packet type is
any of the special cases described in The following sections the packet
decryption is special.  If the packet type is not among those special
packet types rest of the packet may be decrypted with the same key.

Also, note that two bytes of the SILC Packet header are not encrypted
thus it must be noticed in the decryption process by starting the
decryption from the second byte of the header.  This sets some rules
to padding generation as well, see the section 2.7 Packet Padding
Generation.

With out a doubt, this sort of decryption processing causes some
overhead to packet decryption, but never the less, is required.


.ti 0
2.5.2 Channel Message Encryption And Decryption

Channel Messages (Channel Message Payload) are always encrypted with
the channel specific key.  However, the SILC Packet header is not 
encrypted with that key.  As in normal case, the header is encrypted
with the key of the next receiver of the packet, who ever that might
be.  Note that in this case the encrypted data area is not touched
at all; it must not be re-encrypted with the session key.

Receiver of a channel message, who ever that is, is required to decrypt
the SILC Packet header to be able to even recognize the packet to be as
channel message.  This is same procedure as for normal SILC packets.
As the receiver founds the packet to be channel message, rest of the
packet processing is special.  Rest of the SILC Packet header is
decrypted with the same session key along with the padding of the
packet.  After that the packet is protected with the channel specific
key and thus can be decrypted only if the receiver is the client on
the channel.  See section 2.7 Packet Padding Generation for more
information about padding on special packets.

If the receiver of the channel message is router who is routing the
message to another router then it must decrypt the Channel Message
payload.  Between routers (that is, between cells) channel messages
are protected with session keys shared between the routers.  This
causes another special packet processing for channel messages.  If
the channel message is received from another router then the entire
packet, including Channel Message payload, is encrypted with the
session key shared between the routers.  In this case the packet
decryption process is as with normal SILC packets.  Hence, if the
router is sending channel message to another router the Channel
Message payload must have been decrypted and must be re-encrypted
with the session key shared between the another router.  In this
case the packet encryption is as with any normal SILC packet.

It must be noted that this is only when the channel messages are sent
from router to another router.  In all other cases the channel
message encryption and decryption is as described above.  This
different processing of channel messages with router to router
connection is because channel keys are cell specific.  All cells has
their own channel keys thus the channel message traveling from one
cell to another must be protected as it would be any normal SILC
packet.

If the SILC_CMODE_PRIVKEY channel mode has been set for the channel
then the router cannot decrypt the packet as it does not know the
private key.  In this case the entire packet is encrypted with the
session key and sent to the router.  The router receiving the packet
must check the channel mode and decrypt the packet accordingly.


.ti 0
2.5.3 Private Message Encryption And Decryption

By default, private message in SILC are protected by session keys.
In this case the private message encryption and decryption process is
equivalent to normal packet encryption and decryption.

However, private messages can be protected with private message key
which causes the packet to be special packet.  The procedure in this
case is very much alike to channel packets.  The actual private message
is encrypted with the private message key and other parts of the
packet is encrypted with the session key.  See 2.7 Packet Padding
Generation for more information about padding on special packets.

The difference from channel message processing is that server or router
en route never decrypts the actual private message, as it does not
have the key to do that.  Thus, when sending packets between router
the processing is same as in any other case as well; the packet's header
and padding is protected by the session key and the data area is not
touched.

The true receiver of the private message, client, that is, is able
to decrypt the private message as it shares the key with the sender
of the message.


.ti 0
2.6 Packet MAC Generation

Data integrity of a packet is protected by including a message
authentication code (MAC) at the end of the packet.  The MAC is computed
from shared secret MAC key, that is established by the SILC Key Exchange
protocol, and from the original contents of the packet.  The MAC is
always computed before the packet is encrypted, although after it is
compressed if compression is used.

The MAC is computed from entire packet.  Every bit of data in the packet,
including SILC Packet Header is used in the MAC computing.  This way
the entire packet becomes authenticated.

If the packet is special packet MAC is computed from the entire packet
but part of the packet may be encrypted before the MAC is computed.
This is case, for example, with channel messages where the message data
is encrypted with key that server may not now.  In this case the MAC
has been computed from the encrypted data.

See [SILC1] for defined and allowed MAC algorithms.


.ti 0
2.7 Packet Padding Generation

Padding is needed in the packet because the packet is encrypted.  It
must always be multiple by eight (8) or multiple by the size of the
cipher's block size, which ever is larger.  The padding is always
encrypted.

For normal packets the padding is added after the SILC Packet Header
and between the Data Payload area.  The padding for normal packets
are calculated as follows:

.in 6
padding length = 16 - ((packet length - 2) % 16)
.in 3

The 16 is the maximum padding allowed in SILC packet.  Two (2) is 
subtracted from the true length of the packet because two (2) bytes
is not encrypted in SILC Packet Header, see section 2.2 SILC Packet
Header.  Those two bytes that are not encrypted must not be calculated
to the padding length.

For special packets the padding calculation may be different as special
packets may be encrypted differently.  In these cases the encrypted
data area must already be multiple by the block size thus in this case
the padding is calculated only for SILC Packet Header, not for any
other area of the packet.  The same algorithm works in this case as
well, except that the `packet length' is now the SILC Packet Header
length.  In this case, as well, two (2) is subtracted from the
length.

The padding must be random data, preferably, generated by 
cryptographically strong random number generator.


.ti 0
2.8 Packet Compression

SILC Packets may be compressed.  In this case the data payload area
is compressed and all other areas of the packet must remain as they
are.  After compression is performed for the data area, the length
field of Packet Header must be set to the compressed length of the
data.

The compression must always be applied before encryption.  When
the packet is received and decrypted the data area must be decompressed.
Note that the true sender of the packet must apply the compression and
the true receiver of the packet must apply the decompression.  Any
server or router en route must not decompress the packet.



.ti 0
2.9 Packet Sending

The sender of the packet must assemble the SILC Packet Header with
correct values.  It must set the Source ID of the header as its own
ID, unless it is forwarding the packet.  It must also set the Destination
ID of the header to the true destination.  If the destination is client
it will be Client ID, if it is server it will be Server ID and if it is
channel it will be Channel ID.

If the sender wants to compress the packet it must apply the
compression now.  Sender must also compute the padding as described
in above sections.  Then sender must compute the MAC of the packet.

Then sender encrypts the packet as has been described in above
sections according whether the packet is normal packet or special
packet.  The computed MAC must not be encrypted.


.ti 0
2.10 Packet Reception

On packet reception the receiver must check that all fields in the
SILC Packet Header are valid.  It must check the flags of the
header and act accordingly.  It must also check the MAC of the packet
and if it is to be failed the packet must be discarded.  Also if the
header of the packet includes any bad fields the packet must be
discarded.

See above sections on the decryption process of the received packet.
 
The receiver must also check that the ID's in the header are valid
ID's.  Unsupported ID types or malformed ID's must cause packet
rejection.  The padding on the reception is always ignored.

The receiver must also check the packet type and start parsing the
packet according to the type.  However, note the above sections on
special packet types and their parsing.


.ti 0
2.11 Packet Routing

Routers are the primary entities in the SILC network that takes care
of packet routing.  However, normal servers routes packets as well, for
example, when they are routing channel message to the local clients.
Routing is quite simple as every packet tells the true origin and the
true destination of the packet.

It is still recommended for routers that has several routing connections
to create route cache for those destinations that has faster route than
the router's primary route.  This information is available for the router
when other router connects to the router.  The connecting party then
sends all of its locally connected clients, server and channels.  These
informations helps to create the route cache.  Also, when new channels
are created to a cell its information is broadcasted to all routers
in the network.  Channel ID's are based on router's ID thus it is easy
to create route cache based on these informations.  If faster route for
destination does not exist in router's route cache the packet must be
routed to the primary route (default route).

For server who receives a packet to be routed to its locally connected
client the server must check whether the particular packet type is
allowed to be routed to the client.  Not all packets may be sent by
some odd entity to client that is indirectly connected to the sender.
See section 2.3 SILC Packet Types and paragraph about indirectly connected
entities and sending packets to them.  The section mentions the packets
that may be sent to indirectly connected entities.  It is clear that some
server cannot send, for example, disconnect packet to client that is not
directly connected to the server.


.ti 0
2.12 Packet Broadcasting

SILC packets may be broadcasted in SILC network.  However, only router
server may send or receive broadcast packets.  Client and normal server
must not send broadcast packets and they must ignore broadcast packets
if they receive them.  Broadcast packets are sent by setting Broadcast
flag to the SILC packet header.

Broadcasting packets means that the packet is sent to all routers in
the SILC network, except to the router that sent the packet.  The router
receiving broadcast packet must send the packet to its primary route.
The fact that SILC routers may have several router connections may
cause problems, such as race conditions inside the SILC network, if
care is not taken when broadcasting packets.  Router must not send
the broadcast packet to any other route except to its primary route.

If the primary route of the router is the original sender of the packet
the packet must not be sent to the primary route.  This may happen
if router has several router connections and some other router uses
the router as its primary route.

Routers use broadcast packets to broadcast for example information
about newly registered clients, servers, channels etc. so that all the
routers may keep these informations up to date.


.ti 0
2.13 Packet Tunneling

Tunneling is a feature that is available in SILC protocol.  Tunneling
means that extra SILC Packet Header is applied to the original packet
and thus hiding the original packet entirely.  There can be some
interesting applications using tunneling, such as, using ID's based on
private network IP addresses inside in the tunneled packet.  This can
open many interesting features relating to connecting to private network
from the Internet with SILC and many more.  However, this feature is
optional currently in SILC as there does not exist thorough analysis of
this feature.  It is with out a doubt that there will be many more
applications that has not yet been discovered.  Thus, it is left
to Internet Community to investigate the use of tunneling in SILC
protocol.  This document is updated according those investigations
and additional documents on the issue may be written.


.ti 0
3 Security Considerations

Security is central to the design of this protocol, and these security
considerations permeate the specification.  Common security considerations
such as keeping private keys truly private and using adequate lengths for 
symmetric and asymmetric keys must be followed in order to maintain the   
security of this protocol.


.ti 0
4 References

[SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
             Protocol Specification", Internet Draft, June 2000.

[SILC3]      Riikonen, P., "SILC Key Exchange and Authentication 
             Protocols", Internet Draft, June 2000.

[IRC]        Oikarinen, J., and Reed D., "Internet Relay Chat Protocol",
             RFC 1459, May 1993.

[IRC-ARCH]   Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
             April 2000.

[IRC-CHAN]   Kalt, C., "Internet Relay Chat: Channel Management", RFC
             2811, April 2000.

[IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC
             2812, April 2000.

[IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC
             2813, April 2000.

[SSH-TRANS]  Ylonen, T., et al, "SSH Transport Layer Protocol", 
             Internet Draft.

[PGP]        Callas, J., et al, "OpenPGP Message Format", RFC 2440,
             November 1998.

[SPKI]       Ellison C., et al, "SPKI Certificate Theory", RFC 2693,
             September 1999.

[PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key 
             Infrastructure, Certificate and CRL Profile", RFC 2459,
             January 1999.

[Schneier]   Schneier, B., "Applied Cryptography Second Edition",
             John Wiley & Sons, New York, NY, 1996.

[Menezes]    Menezes, A., et al, "Handbook of Applied Cryptography",
             CRC Press 1997.

[OAKLEY]     Orman, H., "The OAKLEY Key Determination Protocol",
             RFC 2412, November 1998.

[ISAKMP]     Maughan D., et al, "Internet Security Association and
             Key Management Protocol (ISAKMP)", RFC 2408, November
             1998.

[IKE]        Harkins D., and Carrel D., "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.

[HMAC]       Krawczyk, H., "HMAC: Keyed-Hashing for Message
             Authentication", RFC 2104, February 1997.

[PKCS1]      Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography
             Specifications, Version 2.0", RFC 2437, October 1998.


.ti 0
5 Author's Address

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Pekka Riikonen
Kasarmikatu 11 A4
70110 Kuopio
Finland

EMail: priikone@poseidon.pspt.fi

This Internet-Draft expires 6 Jun 2001