





Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-pp-09.txt                            15 January 2007
Expires: 15 July 2007


                           SILC Packet Protocol
                      <draft-riikonen-silc-pp-09.txt>

Status of this Draft

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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 [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

   1 Introduction ..................................................  3
     1.1 Requirements Terminology ..................................  4
   2 SILC Packet Protocol ..........................................  4
     2.1 SILC Packet ...............................................  4
     2.2 SILC Packet Header ........................................  5
     2.3 SILC Packet Types .........................................  8
         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 ............................ 17
               2.3.2.3 Argument List Payload ....................... 17
               2.3.2.4 Channel Payload ............................. 18
               2.3.2.5 Public Key Payload .......................... 19
               2.3.2.6 Message Payload ............................. 20
         2.3.3 Disconnect Payload .................................. 23
         2.3.4 Success Payload ..................................... 24
         2.3.5 Failure Payload ..................................... 25
         2.3.6 Reject Payload ...................................... 25
         2.3.7 Notify Payload ...................................... 26
         2.3.8 Error Payload ....................................... 35
         2.3.9 Channel Message Payload ............................. 35
         2.3.10 Channel Key Payload ................................ 36
         2.3.11 Private Message Payload ............................ 38
         2.3.12 Private Message Key Payload ........................ 38
         2.3.13 Command Payload .................................... 40
         2.3.14 Command Reply Payload .............................. 41
         2.3.15 Connection Auth Request Payload .................... 41
         2.3.16 New ID Payload ..................................... 42
         2.3.17 New Client Payload ................................. 43
         2.3.18 New Server Payload ................................. 44
         2.3.19 New Channel Payload ................................ 45
         2.3.20 Key Agreement Payload .............................. 45
         2.3.21 Resume Router Payload .............................. 47
         2.3.22 File Transfer Payload .............................. 47
         2.3.23 Resume Client Payload .............................. 48
         2.3.24 Acknowledgement Payload ............................ 50
     2.4 SILC ID Types ............................................. 50
     2.5 Packet Encryption And Decryption .......................... 51
         2.5.1 Normal Packet Encryption And Decryption ............. 51
         2.5.2 Channel Message Encryption And Decryption ........... 52
         2.5.3 Private Message Encryption And Decryption ........... 53
     2.6 Packet MAC Generation ..................................... 53
     2.7 Packet Padding Generation ................................. 54
     2.8 Packet Compression ........................................ 54
     2.9 Packet Sending ............................................ 55
     2.10 Packet Reception ......................................... 55



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     2.11 Packet Routing ........................................... 55
     2.12 Packet Broadcasting ...................................... 57
   3 Security Considerations ....................................... 57
   4 References .................................................... 57
   5 Author's Address .............................................. 59
   6 Full Copyright Statement ...................................... 59

List of Figures

   Figure 1:   Typical SILC Packet
   Figure 2:   SILC Packet Header
   Figure 3:   ID Payload
   Figure 4:   Argument Payload
   Figure 5:   Argument List Payload
   Figure 6:   Channel Payload
   Figure 7:   Public Key Payload
   Figure 8:   Message Payload
   Figure 9:   Disconnect Payload
   Figure 10:  Success Payload
   Figure 11:  Failure Payload
   Figure 12:  Reject Payload
   Figure 13:  Notify Payload
   Figure 14:  Error Payload
   Figure 15:  Channel Key Payload
   Figure 16:  Private Message Key Payload
   Figure 17:  Command Payload
   Figure 18:  Connection Auth Request Payload
   Figure 19:  New Client Payload
   Figure 20:  New Server Payload
   Figure 21:  Key Agreement Payload
   Figure 22:  Resume Router Payload
   Figure 23:  File Transfer Payload
   Figure 24:  Resume Client Payload


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 [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 packet protocol is designed to be compact to avoid unnecessary
   overhead as much as possible.  This makes the SILC suitable also in
   environment of low bandwidth requirements such as mobile networks.  All
   packet payloads can also be compressed to further reduce the size of
   the packets.



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   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 and the destination of the packet.


1.1 Requirements Terminology

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


2 SILC Packet Protocol

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.

      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     |   n bytes   | 1 - n bytes |      n bytes       |  n bytes
     | SILC Header |   Padding   |    Data Payload    |    MAC
      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

                      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.  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 128 bytes.  The padding is always
   encrypted.  The padding is applied always, even if the packet is
   not encrypted.  See the section 2.7 Padding Generation for more
   detailed information.



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   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.  See the section 2.6 Packet MAC Generation
   for more information.  If compression is used the compression is
   always applied before encryption.

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


2.2 SILC Packet Header

   The 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 SILC packet header.

                          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  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Pad Length  |    RESERVED   | Source ID Len |  Dest ID Len  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Src ID Type  |                                               |
     +-+-+-+-+-+-+-+-+                                               +
     |                                                               |
     ~                           Source ID                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Dst ID Type  |                                               |
     +-+-+-+-+-+-+-+-+                                               +
     |                                                               |
     ~                         Destination ID                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2:  SILC Packet Header

      o Payload Length (2 bytes) - Indicates the length of the
        packet not including the padding of the packet.




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      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 data MUST include private
             message that is encrypted using private key set by
             client.  Servers does not know this key and cannot
             decrypt the payload, but simply passes it 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 and normal
             server cannot send broadcast packets.  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 2.12 Packet Broadcasting for description of
             packet broadcasting.


           Compressed                0x08

             Marks that the payload of the packet is compressed.



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             The sender of the packet marks this flag when it
             compresses the payload, and any server or router
             en route to the recipient MUST NOT unset this flag.
             See section 2.8 Packet Compression for description of
             packet compressing.


           Acknowledgement           0x10

             Marks that the packet needs to be acknowledged by the
             recipient.  The ACK packet MUST NOT have this flag set.
             The acknowledgement packet is SILC_PACKET_ACK packet.
             If the packet is not acknowledged the packet may be
             retransmitted.  This flag is especially useful when
             using UDP/IP and SHOULD NOT be used with TCP/IP.  The
             flag MUST NOT be used with message packets.  The
             SILC_MESSAGE_FLAG_ACK can be used instead.  Broadcast
             packets MUST NOT set this flag.  Retransmission
             may use for example exponential backoff algorithm.


   o Packet Type (1 byte) - Indicates 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 Pad Length (1 byte) - Indicates the length of the padding
     applied after the SILC Packet header.  Maximum length for
     padding is 128 bytes.

   o RESERVED (1 byte) - Reserved field and must include a
     zero (0) value.

   o Source ID Length (1 byte) - Indicates the length of the
     Source ID field in the header, not including this or any
     other fields.

   o Destination ID Length (1 byte) - 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 which is the original sender of the packet.

   o Dst ID Type (1 byte) - Indicates the type of ID in the



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     Destination ID field.  See section 2.4 SILC ID Types for
     defined ID types.

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



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 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 the following packet types.

   The below list of the SILC Packet types includes reference to the packet
   payload as well.  Packet payloads are the actual packet data area.  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 a
   router to send SILC_PACKET_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 does not have
   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.  The following packet
   desription list will define it separately if a packet is allowed to be
   sent to indirectly connected entity.  Other packets MUST NOT be sent or
   accepted, if sent, to indirectly connected entities.

   Some packets MAY be sent as lists by adding the List flag to the Packet
   Header and constructing multiple packet payloads one after the other.
   When this is allowed it is separately defined in the following list.
   Other packets MUST NOT be sent as list and the List flag MUST NOT be set.


   List of SILC Packet types are defined as follows.

      0    SILC_PACKET_NONE

           This type is reserved and it is never sent.


      1    SILC_PACKET_DISCONNECT



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           This packet is sent to disconnect the remote end.  Reason of
           the disconnection is sent inside the packet payload.

           Payload of the packet:  See section 2.3.3 Disconnect Payload


      2    SILC_PACKET_SUCCESS

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

           Payload of the packet:  See section 2.3.4 Success Payload


      3    SILC_PACKET_FAILURE

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

           Payload of the packet:  See section 2.3.5 Failure Payload


      4    SILC_PACKET_REJECT

           This packet MAY be sent upon rejection of a protocol.  The
           status of the rejection is sent in the packet payload.

           Payload of the packet:  See section 2.3.6 Reject Payload


      5    SILC_PACKET_NOTIFY

           This packet is used to send notify message.  The packet is
           usually sent between server and client, but also between
           server and router.  Client MUST NOT send this packet.  Server
           MAY destine this packet to channel as well when the packet is
           distributed to all clients on the channel.  This packet MAY
           be sent as list.

           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 MUST NOT send this packet.  The
           client MAY entirely ignore the packet, however, server is
           most likely to take action anyway.  This packet MAY be sent



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           to entity that is indirectly connected to the sender.

           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.  This packet MAY be sent to entity
           that is indirectly connected to the sender.

           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 when server generates it.  Each channel has their own
           independent keys that is used to protect the traffic on the
           channel.  It is also possible to use channel private keys that
           are not server generated.  In this case this packet is not used.
           Client MUST NOT send this packet.  This packet MAY be sent to
           entity that is indirectly connected to the sender.

           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.  See [SILC1] for private message key generation.
           This packet MAY be sent to entity that is indirectly connected
           to the sender.

           Payload of the packet:  See section 2.3.11 Private Message
                                   Payload


      10   SILC_PACKET_PRIVATE_MESSAGE_KEY

           This packet is OPTIONAL and sender of the packet can indicate
           that a private message key should be used in private message



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           communication.  The actual key material is not sent in this
           packet but must be either static or pre-shared key.  The
           receiver of the packet is considered to be the responder
           when processing the static or pre-shared key material as
           defined in [SILC1] and [SILC3] for private message keys.
           This packet MAY be sent to entity that is indirectly connected
           to the sender.

           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 [SILC4].  The contents of this packet is command specific.
           This packet MAY be sent to entity that is indirectly connected
           to the sender.

           Payload of the packet:  See section 2.3.13 Command Payload


      12   SILC_PACKET_COMMAND_REPLY

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

           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].

           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



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

           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.

           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 an 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 MAY be used to determine it.
           The party receiving this payload SHOULD respond with the same
           packet including the mandatory authentication method.

           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].

           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 IDs from server to
           router and from router to all other routers in SILC network.
           This is used when for example new client is registered to



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           SILC network.  The newly created IDs of these operations are
           distributed by this packet.  Only server may send this packet,
           however, client MUST be able to receive this packet.  This
           packet MAY be sent to entity that is indirectly connected
           to the sender.  This packet MAY be sent as list.

           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 to the server.

           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.  The client
           MUST NOT send or receive this packet.

           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 MAY be sent to entity that is indirectly
           connected to the sender.  This packet MAY be sent as list.

           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



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           [SILC1] for more information.  This packet does not have
           a payload.


      23   SILC_PACKET_REKEY_DONE

           This packet is used to indicate that re-key is performed and
           new keys must be used hereafter.  This packet does not have a
           payload.


      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.


      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_RESUME_ROUTER

           This packet is used during backup router protocol when the
           original primary router of the cell comes back online and wishes
           to resume the position as being the primary router of the cell.

           Payload of the packet:  See section 2.3.21 Resume Router Payload


      27   SILC_PACKET_FTP

           This packet is used to perform an file transfer protocol in the
           SILC session with some entity in the network.  The packet is
           multi purpose.  The packet is used to tell other entity in the
           network that the sender wishes to perform an file transfer
           protocol.  The packet is also used to actually tunnel the
           file transfer protocol stream.  The file transfer protocol



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           stream is always protected with the SILC binary packet protocol.

           Payload of the packet:  See section 2.3.22 File Transfer Payload


      28   SILC_PACKET_RESUME_CLIENT

           This packet is used to resume a client back to the network
           after it has been detached.  A client is able to detach from
           the network but the client is still valid client in the network.
           The client may then later resume its session back by sending
           this packet to a server.  Routers also use this packet to notify
           other routers in the network that the detached client has resumed.

           Payload of the packet:  See section 2.3.23 Resume Client Payload


      29   SILC_PACKET_ACK

           This packet is used to acknowledge a packet that had the
           Acknowledgement packet flag set.

           Payload of the packet:  See section 2.3.24 Acknowledgement
           Payload


      30 - 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
           MUST NOT be sent.


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 SILC
   packet header and padding.  All fields in the packet payload are always



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   encrypted, as they reside in the data area of the packet which is
   always encrypted.  Most of the payloads may only be sent with specific
   packet type which is defined in the description of the payload.

   There are some other payloads in 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],
   [SILC3] and [SILC4].


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 payload.


2.3.2.1 ID Payload

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

   The following diagram represents the ID Payload.

                          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                             ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 3:  ID Payload


      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.  The encoding
        of the ID data is defined in section 2.4 SILC ID Types.





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2.3.2.2 Argument Payload

   Argument Payload is used to set arguments for any packet payload that
   need and support arguments, such as commands.  Number of arguments
   associated with a packet MUST be indicated by the packet payload which
   need 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.

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Data Length          | Argument Type |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
     |                                                               |
     ~                        Argument Data                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 4:  Argument Payload


      o Data Length (2 bytes) - Length of the Argument Data field
        not including the length of any other field in the payload.

      o Argument Type (1 byte) - Indicates the type of the argument.
        Every argument can have a specific type that are defined
        by the packet payload needing the argument.  For example
        every command specify a number for each argument that may be
        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) value.

      o Argument Data (variable length) - Argument data.


2.3.2.3 Argument List Payload

   Argument List Payload is a list of Argument Payloads appended one
   after the other.  The number of arguments is indicated in the
   payload.

   The following diagram represents the Argument List Payload.

                          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



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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Argument Nums         |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                        Argument Payloads                      ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5:  Argument List Payload


      o Argument Nums (2 bytes) - Indicates the number of Argument
        Payloads.  If zero (0) value is found in this field no
        arguments are present.

      o Argument Payloads (variable length) - The Argument Payloads
        appended one after the other.  The payloads can be decoded
        since the length of the payload is indicated in each of
        the Argument Payload.





2.3.2.4 Channel Payload

   Generic Channel Payload may be used to send information about a channel,
   its name, the Channel ID and a mode.

   The following diagram represents the Channel Payload.


                          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                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Mode Mask                           |



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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 6:  New Channel Payload


      o Channel Name Length (2 bytes) - Length of the Channel Name
        field.

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

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

      o Channel ID (variable length) - The encoded Channel ID.

      o Mode Mask (4 bytes) - A mode.  This can be the mode of the
        channel but it can also be the mode of a client on the
        channel.  The contents of this field is dependent of the
        usage of this payload.  The usage is defined separately
        when this payload is used.  This is a 32 bit MSB first value.






2.3.2.5 Public Key Payload

   Generic Public Key Payload may be used to send different type of
   public keys and certificates.

   The following diagram represents the Public Key Payload.

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Public Key Length       |        Public Key Type        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                  Public Key (or certificate)                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7:  Public Key Payload


      o Public Key Length (2 bytes) - The length of the Public Key
        (or certificate) field, not including any other field.




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      o Public Key Type (2 bytes) - The public key (or certificate)
        type.  This field indicates the type of the public key in
        the packet.  See the [SILC3] for defined public key types.

      o Public Key (or certificate) (variable length) - The
        encoded public key or certificate data.


2.3.2.6 Message Payload

   Generic Message Payload can be used to send messages in SILC.  It
   is used to send channel messages and private messages.

   The following diagram represents the Message Payload.

   (*) indicates that the field is not encrypted.









                          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  Flags         |         Message Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         Message Data                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Padding Length         |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                            Padding                            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                    Initialization Vector *                    ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                              MAC *                            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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                        Figure 8:  Message Payload


      o Message Flags (2 bytes) - Includes the Message Flags of the
        message.  The flags can indicate a reason or a purpose for
        the message.  The following Message Flags are defined:

        0x0000  SILC_MESSAGE_FLAG_NONE

                No specific flags set.

        0x0001  SILC_MESSAGE_FLAG_AUTOREPLY

                This message is an automatic reply to an earlier
                received message.

        0x0002  SILC_MESSAGE_FLAG_NOREPLY

                There should not be reply messages to this
                message.

        0x0004  SILC_MESSAGE_FLAG_ACTION

                The sender is performing an action and the message
                is the indication of the action.

        0x0008  SILC_MESSAGE_FLAG_NOTICE

                The message is for example an informational notice
                type message.

        0x0010  SILC_MESSAGE_FLAG_REQUEST

                This is a generic request flag to send request
                messages.  A separate document should define any
                payloads associated to this flag.

        0x0020  SILC_MESSAGE_FLAG_SIGNED

                This flag indicates that the message is signed
                with sender's private key and thus can be verified
                by the receiver using the sender's public key.  A
                separate document should define the detailed procedure
                of the signing process and any associated payloads
                for this flag.

        0x0040  SILC_MESSAGE_FLAG_REPLY




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                This is a generic reply flag to send a reply to
                previously received request.  A separate document
                should define any payloads associated to this flag.

        0x0080  SILC_MESSAGE_FLAG_DATA

                This is a generic data flag, indicating that the
                message includes some data which can be interpreted
                in a specific way.  Using this flag any kind of data
                can be delivered inside message payload.  A separate
                document should define how this flag is interpreted
                and define any associated payloads.

        0x0100  SILC_MESSAGE_FLAG_UTF8

                This flag indicates that the message is UTF-8 encoded
                textual message.  When sending text messages in SILC
                this flag SHOULD be used.  When this flag is used the
                text sent as message MUST be UTF-8 encoded.

        0x0200  SILC_MESSAGE_FLAG_ACK

                This flag indicates the sender requires the recpipient
                to acknowledge the received message.  This same flag
                is used in the acknowledgement.  A separate document
                should define how the acknowledgement is performed.

        0x0400 - 0x1000 RESERVED

                Reserved for future flags.

        0x2000 - 0x8000 PRIVATE RANGE

                Private range for free use.

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

      o Message Data (variable length) - The actual message data.

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

      o Padding (variable length) - If this payload is used as
        channel messages, the padding MUST be applied because
        this payload is encrypted separately from other parts



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        of the packet.  If this payload is used as private
        messages, the padding is present only when the payload
        is encrypted with private message key.  If encrypted
        with session keys this field MUST NOT be present and the
        Padding Length field includes a zero (0) value.  The
        padding SHOULD be random data.

      o Initialization Vector (variable length) - This field MUST
        be present when this payload is used as channel messages.
        The IV SHOULD be random data for each channel message.

        When encrypting private messages with session keys this
        field MUST NOT be present.  For private messages this field
        is present only when encrypting with a static private
        message key (pre-shared key).  If randomly generated key
        material is used this field MUST NOT be present.  Also,
        If Key Agreement (SKE) was used to negotiate fresh key
        material for private message key this field MUST NOT be
        present.  See the section 4.6 in [SILC1] for more
        information about IVs when encrypting private messages.

        This field includes the initialization vector used in message
        encryption.  It need to be used in the packet decryption
        as well.  Contents of this field depends on the encryption
        algorithm and encryption mode.  This field is not encrypted,
        is not included in padding calculation and its length
        equals to cipher's block size.  This field is authenticated
        by the message MAC.

      o MAC (variable length) - The MAC computed from the
        Message Flags, Message Length, Message Data, Padding Length,
        Padding and Initialization Vector fields in that order.
        The MAC is computed after the payload is encrypted.  This
        is so called Encrypt-Then-MAC order; first encrypt, then
        compute MAC from ciphertext.  The MAC protects the integrity
        of the Message Payload.  Also, when used as channel messages
        it is possible to have multiple private channel keys set,
        and receiver can use the MAC to verify which of the keys
        must be used in decryption.  This field is not present
        when encrypting private messages with session key.  This
        field is not encrypted.  This field is authenticated by
        the SILC packet MAC.


2.3.3 Disconnect Payload

   Disconnect payload is sent upon disconnection.  Reason of the
   disconnection is sent to the disconnected party in the payload.



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   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.


                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Status     |                                               |
     +-+-+-+-+-+-+-+-+                                               +
     |                                                               |
     ~                      Disconnect Message                       ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 9:  Disconnect Payload

      o Status (1 byte) - Indicates the Status Type, defined in [SILC3]
        for the reason of disconnection.

      o Disconnect Message (variable length) - Human readable UTF-8
        encoded string indicating reason of the disconnection.  This
        field MAY be omitted.


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 may be any data, including binary or human readable data, and
   it is protocol dependent.

                          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                       ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 10:  Success Payload


      o Success Indication (variable length) - Indication of
        the success.  This may be 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



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        Packet Header.


2.3.5 Failure Payload

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

                          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                       ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 11:  Failure Payload


      o Failure Indication (variable length) - Indication of
        the failure.  This may be 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.


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 and is protocol dependent.


                          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                       ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 12:  Reject Payload


      o Reject Indication (variable length) - Indication of
        the rejection.  This maybe for example some flag that



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        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.



2.3.7 Notify Payload

   Notify payload is used to send notify messages.  The payload is usually
   sent from server to client and from server to router.  It is also used
   by routers to notify other routers in the network.  This payload MAY also
   be sent to a channel.  Client MUST NOT send this payload.  When this
   packet is received by client it SHOULD process it.  Servers and routers
   MUST process notify packets.

   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.



                          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 |
     +-+-+-+-+-+-+-+-+

                        Figure 13:  Notify Payload


      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 (1 byte) - Indicates the number of Argument
        Payloads associated to this payload.  Notify types may define
        arguments to be sent along the notify message.

   Following the list of currently defined notify types.  The format for
   notify arguments is same as in SILC commands described in [SILC4].
   Note that all IDs sent in arguments are sent inside ID Payload.  Also
   note that all strings sent as arguments MUST be UTF-8 [RFC3629] encoded,
   unless otherwise defined.  Also note that all public keys or



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   certificates sent inside arguments are actually Public Key Payloads.


      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 an client is invited to a channel.  This is also sent
            when the invite list of the channel is changed.  This notify type
            is sent to local servers on the channel, but MUST NOT be sent
            to clients on the channel.  Router MUST broadcast this to its
            primary router and to local servers on the channel.  When a client
            was directly invited to the channel this is also sent to that
            client.  In this case the packet is destined to the client.

            Max Arguments:  5
                Arguments:  (1) <Channel ID>          (2) <channel name>
                            (3) [<sender Client ID>]  (4) [<add | del>]
                            (5) [<invite list>]

            The <Channel ID> is the channel.  The <channel name> is the name
            of the channel and is provided because the client which receives
            this notify packet may not have a way to resolve the name of the
            channel from the <Channel ID>.  The <sender Client ID> is the
            Client ID which invited the client to the channel.  The
            <add | del> is an argument of size of 1 byte where 0x00 means
            adding a client to invite list, and 0x01 means deleting a client
            from invite list.  The <invite list>, if present, indicates the
            information to be added to or removed from the invite list.
            The <invite list> format is defined in [SILC4] with
            SILC_COMMAND_INVITE command.  When this notify is destined to
            a client the <add | del> and <invite list> MUST NOT be sent.
            When <add | del> is used  to announce information during server
            connecting phase the argument type MUST be 0x03.  See section
            4.2.1 in [SILC1] for more information.


      2     SILC_NOTIFY_TYPE_JOIN



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            Sent when client has joined to a channel.  The server MUST
            distribute this type to the local clients on the channel and then
            send it to its primary router.  Note that, when router is joining
            the client on behalf of normal server then router MUST send this
            notify type locally and globally.  The router or server receiving
            the packet distributes this type to the local clients on the
            channel and broadcast it to the network.  This notify is sent
            also to the client that joined the channel.

            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 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.  This notify MUST NOT be sent to
            the leaving client.

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

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


      4     SILC_NOTIFY_TYPE_SIGNOFF

            Sent when client signoff from SILC network.  The server MUST
            distribute this type 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.  This notify
            MUST NOT be sent to the quitting client.  The Destination ID
            in the packet may be any ID depending to who it is destined.

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

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




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      5     SILC_NOTIFY_TYPE_TOPIC_SET

            Sent when topic is set/changed on a channel.  This type may be
            sent only to the clients which are joined on the channel which
            topic was just set or changed.  The packet is destined to the
            channel.

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

            The <ID Payload> is the ID of the entity who set the topic.
            It usually is Client ID but it can be Server ID and Channel ID
            as well.


      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.  This packet is
            destined directly to the sent entity.  This MUST be sent to those
            clients that are joined on same channels as the client that
            changed the nickname.  This notify MUST NOT be sent multiple
            times to the same recipient.  This notify MUST be sent also to
            the client that changed the nickname.

            Max Arguments:  3
                Arguments:  (1) <Old Client ID>  (2) <New Client ID>
                            (3) <nickname>

            The <Old Client ID> is the old ID of the client which changed
            the nickname.  The <New Client ID> is the new ID generated by
            the change of the nickname.  The <nickname> is the new nickname.
            Note that it is possible to send this notify even if the
            nickname has not changed, but client ID was changed.


      7     SILC_NOTIFY_TYPE_CMODE_CHANGE

            Sent when channel mode has changed.  This type MUST be sent only
            to the clients which are joined on the channel which mode was
            changed.  This packet is destined to the channel.

            Max Arguments:  8
                Arguments:  (1) <ID Payload>       (2) <mode mask>
                            (3) [<cipher>]         (4) <[hmac>]



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                            (5) [<passphrase>]     (6) [<founder public key>]
                            (7) [<channel pubkey>] (8) [<user limit>]

            The <ID Payload> is the ID (usually Client ID but it can be
            Server ID as well when the router is enforcing channel mode
            change) of the entity which changed the mode.  The <mode mask>
            is the new mode mask of the channel.  The client can safely
            ignore the <cipher> argument since the SILC_PACKET_CHANNEL_KEY
            packet will force the new channel key change anyway.  The <hmac>
            argument is important since the client is responsible of setting
            the new HMAC and the hmac key into use.  The <passphrase> is
            the passphrase of the channel, if it was now set.  The <founder
            public key> argument is sent when the founder mode on the
            channel was set.  All routers and servers that receive the packet
            MUST save the founder's public key so that the founder can
            reclaim the channel founder rights back for the channel on any
            server in the network.  The <user limit> argument is present when
            the user limit was set or changed on the channel.

            The <channel pubkey> is an Argument List Payload and it is used
            to add and/or remove channel public keys from the channel.  Also,
            when announcing channel information between servers and routers
            during connecting phase this argument includes the list of channel
            public keys.  To add a public key to channel public key list the
            SILC_CMODE_CHANNEL_AUTH mode is set and the argument type is 0x00,
            and the argument is the public key.  To remove a public key from
            the channel public key list the argument type is 0x01, and the
            argument is the public key to be removed.  If the mode
            SILC_CMODE_CHANNEL_AUTH is unset (and was set earlier) all public
            keys are removed at once.  Implementation MAY add and remove
            multiple public keys at the same time by including multiple
            arguments to the <channel pubkey> Argument List Payload where each
            argument is one Public Key Payload.  When <channel pubkey> is used
            to announce information during server connecting phase the
            argument type MUST be 0x03.  See section 4.2.1 in [SILC1] for
            more information.


      8     SILC_NOTIFY_TYPE_CUMODE_CHANGE

            Sent when user mode on channel has changed.  This type MUST be
            sent only to the clients which are joined on the channel where
            the target client is on.  This packet is destined to the channel.

            Max Arguments:  4
                Arguments:  (1) <ID Payload>        (2) <mode mask>
                            (3) <Target Client ID>  (4) [<founder pubkey>]




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            The <ID Payload> is the ID (usually Client ID but it can be
            Server ID as well when the router is enforcing user's mode
            change) of the entity which 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.  The <founder pubkey>
            is the public key of the channel founder and may be sent only
            when first time setting the channel founder mode using the
            SILC_COMMAND_CUMODE command, and when sending this notify.


      9     SILC_NOTIFY_TYPE_MOTD

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

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

            The <motd> is the Message of the Day.  This notify MAY be
            ignored and is OPTIONAL.


      10    SILC_NOTIFY_TYPE_CHANNEL_CHANGE

            Sent when channel's ID has changed for a reason or another.
            This is sent by normal server to the client.  This can also be
            sent by router to other server to force the Channel ID change.
            The Channel ID MUST be changed to use the new one.  When sent
            to clients, this type MUST be sent only to the clients which are
            joined on the channel.  This packet is destined to the sent
            entity.

            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.
            Server which receives this from router MUST re-announce the
            channel to the router by sending SILC_PACKET_NEW_CHANNEL packet
            with the new Channel ID.


      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.
            This packet is destined to the sent entity.

            Max Arguments:  256



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                Arguments:  (1) <Server ID>   (n) [<Client ID>]   [...]

            The <Server ID> is the server's ID.  The rest of the arguments
            are the Client IDs of the clients which are coming from this
            server and are thus quitting the SILC network also.  If the
            maximum number of arguments are reached another
            SILC_NOTIFY_TYPE_SERVER_SIGNOFF notify packet MUST be sent.
            When this notify packet is sent between routers the Client ID's
            MAY be omitted.  Server receiving the Client ID's in the payload
            may use them directly to remove the client.


      12    SILC_NOTIFY_TYPE_KICKED

            Sent when a client has been kicked from a channel.  This MUST
            also be sent to the client which was kicked from the channel.
            The client which was kicked from the channel MUST be removed
            from the channel.  The client MUST also be removed from channel's
            invite list if it is explicitly added in the list.  This packet
            is 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:  3
                Arguments:  (1) <Client ID>           (2) [<comment>]
                            (3) <Kicker's Client ID>

            The <Client ID> is the client which was kicked from the channel.
            The kicker may have set the <comment> string to indicate the
            reason for the kicking.  The <Kicker's Client ID> is the kicker.


      13    SILC_NOTIFY_TYPE_KILLED

            Sent when a client has been killed from the network.  This MUST
            also be sent to the client which was killed from the network.
            This notify MUST be sent to those clients which are joined on
            same channels as the killed client.  The client which was killed
            MUST be removed from the network.  This packet is destined
            directly to the sent entity.  The router or server receiving
            the packet distributes this type to the local clients on the
            channel and broadcast it to the network.  The client MUST also
            be removed from joined channels invite list if it is explicitly
            added in the lists.  This notify MUST NOT be sent multiple
            times to same recipient.

            Max Arguments:  3
                Arguments:  (1) <Client ID>           (2) [<comment>]



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                            (3) <Killer's ID>

            The <Client ID> is the client which was killed from the network.
            The killer may have set the  <comment> string to indicate the
            reason for the killing.  The <Killer's ID> is the killer, which
            may be client but also router server.


      14    SILC_NOTIFY_TYPE_UMODE_CHANGE

            Sent when user's mode in the SILC changes.  This type is sent
            only between routers as broadcast packet.

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

            The <Client ID> is the client which mode was changed.  The
            <mode mask> is the new mode mask.


      15    SILC_NOTIFY_TYPE_BAN

            Sent when the ban list of the channel is changed.  This notify
            type is sent to local servers on the channel, but MUST NOT be
            sent to clients on the channel.  Router MUST broadcast this to
            its primary router and to local servers on the channel.

            Max Arguments:  3
                Arguments:  (1) <Channel ID>         (2) [<add | del>]
                            (3) [<ban list>]

            The <Channel ID> is the channel which ban list was changed.
            The <add | del> is an argument of size of 1 byte where 0x00 means
            adding a client to ban list, and 0x01 means deleting a client
            from ban list.  The <ban list> indicates the information to be
            added to or removed from the ban list.  The <ban list> format
            format is defined in [SILC4] with SILC_COMMAND_BAN command.
            When <add | del> is used  to announce information during server
            connecting phase the argument type MUST be 0x03.  See section
            4.2.1 in [SILC1] for more information.


      16    SILC_NOTIFY_TYPE_ERROR

            Sent when an error occurs during processing some SILC procedure.
            This is not used when error occurs during command processing, see
            [SILC4] for more information about commands and command replies.
            This type is sent directly to the sender of the packet whose



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            packet caused the error.  See [SILC1] for definition when this
            type can be sent.

            Max Arguments:  256
                Arguments:  (1) <Status Type>        (n) [...]

            The <Status Type> is the error type defined in [SILC4].  Note
            that same types are also used with command replies to indicate
            the status of a command.  Both commands and this notify type
            share same status types.  Rest of the arguments are status type
            dependent and are specified with those status types that can be
            sent currently inside this notify type in [SILC4].  The <Status
            Type> is size of 1 byte.


      17    SILC_NOTIFY_TYPE_WATCH

            Sent to indicate change in a watched user.  Client can set
            nicknames to be watched with SILC_COMMAND_WATCH command, and
            receive notifications when they login to network, signoff from
            the network or their user mode is changed.  This notify type
            is used to deliver these notifications.  The notify type is
            sent directly to the watching client.

            Max Arguments:  5
                Arguments:  (1) <Client ID>        (2) [<nickname>]
                            (3) <user mode>        (4) [<Notify Type>]
                            (5) [<public key>]

            The <Client ID> is the user's Client ID which is being watched,
            and the <nickname> is its nickname.  If the client just
            changed the nickname, then <nickname> is the new nickname, but
            the <Client ID> is the old client ID.  The <user mode> is the
            user's current user mode.  The <Notify Type> can be same as the
            Notify Payload's Notify Type, and is 16 bit MSB first order
            value.  If provided it may indicate the notify that occurred
            for the client.  If client logged in to the network the
            <Notify Type> MUST NOT be present.  The <public key> MAY be
            present, and it is the public key of the client being watched.

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

   Router server which receives SILC_NOTIFY_TYPE_SIGNOFF,
   SILC_NOTIFY_TYPE_SERVER_SIGNOFF, SILC_NOTIFY_TYPE_KILLED,
   SILC_NOTIFY_TYPE_NICK_CHANGE and SILC_NOTIFY_TYPE_UMODE_CHANGE
   MUST check whether someone in the local cell is watching the nickname
   the client has, and send the SILC_NOTIFY_TYPE_WATCH notify to the



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   watcher, unless the watched client in case has the user mode
   SILC_UMODE_REJECT_WATCHING set.  If the watcher client and the client
   that was watched is same the notify SHOULD NOT be sent.


2.3.8 Error Payload

   Error payload is sent upon error in protocol.  Error may occur in
   various conditions when server sends this packet.  Client MUST NOT
   send this payload but MUST be able to accept it.  However, client
   MAY 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.


                          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                         ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 14:  Error Payload


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


2.3.9 Channel Message Payload

   Channel Message Payload is used to send message to channels, a group
   of users.  These messages can only be sent if client has joined to
   some channel.  Even though this packet is very 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 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.



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   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 which sent
   the message.  The Destination ID MUST be Channel ID in the SILC
   Packet header.

   This packet use generic Message Payload as Channel Message Payload.
   See section 2.3.2.6 for generic Message Payload.


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 MUST start using the key received
   in this payload to protect the traffic on the channel.

   The client which 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 which sent the SILC_COMMAND_JOIN
   command.

   Channel keys are cell specific thus every router in the 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.

   Note that, this packet is not used if SILC_CMODE_PRIVKEY mode is set
   on channel.  This means that channel uses channel private keys which
   are not server generated.  For this reason server cannot send this
   packet as it does not know the key.

   The destination ID in the packet SHOULD be the entity to whom the
   packet is sent.  Using Channel ID as destination ID is not
   necessary as the Channel ID is included in the Channel Key Payload.

   The payload may only be sent with SILC_PACKET_CHANNEL_KEY packet.



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   It MUST NOT be sent in any other packet type.  The following diagram
   represents the Channel Key Payload.



                          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                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 15:  Channel Key Payload



      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.

      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



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        Channel Key field in the payload, not including any other
        field.

      o Channel Key (variable length) - The actual channel key
        material.  See [SILC1] on how to start using the key.


2.3.11 Private Message Payload

   Private Message Payload is used to send private message between
   two clients.  The messages are sent only to the specified user
   and no other user inside SILC network is able to see the message.

   The message can be protected by the session key established by the
   SILC Key Exchange Protocol.  However, it is also possible to agree
   to use a private message key to protect just the private messages.
   It is for example possible to perform Key Agreement between two
   clients.  See section 2.3.20 Key Agreement Payload how to perform
   key agreement.  It is also possible to use static or pre-shared keys
   to protect private messages.  See the 2.3.12 Private Message Key
   Payload and [SILC1] section 4.6 for detailed description for private
   message key generation.

   If normal session key is used to protect the message, every server
   between the sender client and the receiving client MUST 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 the private message key is used, and the Private Message Key
   flag was set in the SILC Packet header no server or router en route
   is able to decrypt or re-encrypt the packet.  In this case only the
   SILC Packet header is processed by the servers and routers en route.
   Section Client To Client in [SILC1] gives example of this scheme.

   This packet use generic Message Payload as Private Message Payload.
   See section 2.3.2.6 for generic Message Payload.


2.3.12 Private Message Key Payload

   This payload is OPTIONAL and can be used to indicate that a static
   or pre-shared key should be used in the private message communication
   to protect the messages.  The actual key material has to be sent
   outside the SILC network, or it has to be a static or pre-shared key.
   The sender of this packet is considered to be the initiator and the
   receiver the responder when processing the raw key material as
   described in the section 4.6 in [SILC1] and in the section 2.3 in
   [SILC3].



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   Note that it is also possible to use static or pre-shared keys in
   client implementations without sending this packet.  Clients may
   naturally agree to use a key without sending any kind of indication
   to each other.  The key may be for example a long-living static key
   that the clients has agreed to use at all times.  Note that it is
   also possible to agree to use private message key by performing
   a Key Agreement.  See the section 2.3.20 Key Agreement Payload.

   This payload may only be sent by client to another client.  Server
   MUST NOT send this payload.  After sending this payload and setting the
   key into use this payload the sender of private messages MUST set the
   Private Message Key flag into the 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.


                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Cipher Name Length       |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                          Cipher Name                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       HMAC Name Length        |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                           HMAC Name                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 16:  Private Message Key Payload



      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.

      o HMAC Name Length (2 bytes) - Indicates the length of the



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        HMAC Name field in the payload, not including any other
        field.

      o HMAC Name (variable length) - Name of the HMAC to use
        in the private message MAC computation.  If this field does
        not exist then the default HMAC of the SILC protocol is used.
        See the [SILC1] for defined HMACs.


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.


                          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      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 17:  Command Payload


      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 which 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



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        used in the field.

   See [SILC4] for detailed description of different SILC commands,
   their arguments and their reply messages.


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 2.3.13 section for the payload specification.

   The entity which sends the reply packet MUST set the Command Identifier
   field in the reply packet's Command Payload to the value it received
   in the original command packet.

   See SILC Commands in [SILC4] for detailed description of different
   SILC commands, their arguments and their reply messages.


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 SHOULD 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.


                          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     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 18:  Connection Auth Request Payload



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      o Connection Type (2 bytes) - Indicates the type of the
        connection.  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.


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.  Similarly 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 use this payload to inform
   other routers about new server in the SILC network.  However, every



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   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.

   This payload MUST NOT be used to send information about new channels.
   New channels are always distributed by sending the dedicated
   SILC_PACKET_NEW_CHANNEL packet.  Client MUST NOT send this payload.
   Both client and server (and router) MAY receive this payload.

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


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.  Client's 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 nickname sent in this payload.

   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.



                          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        |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |



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     ~                           Real Name                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 19:  New Client Payload


      o Username Length (2 bytes) - Length of the Username field.

      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 field.

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


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
   MUST register itself to the SILC Network by sending this payload.
   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.





                          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        |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |



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     ~                          Server Name                          ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 20:  New Server Payload


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

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

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

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


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 MUST NOT
   send this packet.  Server MAY send this packet to a router when it is
   announcing its existing channels to the router after it has connected
   to the router.

   The packet use generic Channel Payload as New Channel Payload.  See
   section 2.3.2.3 for generic Channel Payload.  The Mode Mask field in the
   Channel Payload is the mode of the channel.


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 clients agree
   about the key without any server interaction.  The protocol is executed
   peer to peer.  The server and router MUST NOT send this payload.

   The sender MAY tell the receiver of this payload the hostname and the



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   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.

   This payload may be sent with SILC_PACKET_KEY_AGREEMENT and
   SILC_PACKET_FTP packet types.  It MUST NOT be sent in any other packet
   types.  The following diagram represents the Key Agreement Payload.


                          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                            ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Protocol           |             Port              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 21:  Key Agreement Payload


      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, as UTF-8 encoded string.  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 Protocol (2 bytes) - The internet protocol used for the key
        agreement connection.  Possible values are 0 for TCP and 1 for
        UDP.  Other values are unsupported.  This is a 16 bit MSB first
        order value.  If Hostname field is not present, the value in
        this field is ignored.

      o Port (2 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 16 bit MSB first order value.





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   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] in section 4.6 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.


2.3.21 Resume Router Payload

   See the [SILC1] for Resume Router protocol where this payload is
   used.  The payload may only be sent with SILC_PACKET_RESUME_ROUTER
   packet.  It MUST NOT be sent in any other packet type.  The following
   diagram represents the Resume Router Payload.


                                          1
                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |      Type     |  Session ID   |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 22:  Resume Router Payload


      o Type (1 byte) - Indicates the type of the backup resume
        protocol packet.  The type values are defined in [SILC1].

      o Session ID (1 bytes) - Indicates the session ID for the
        backup resume protocol.  The sender of the packet sets this
        value and the receiver MUST set the same value in subsequent
        reply packet.




2.3.22 File Transfer Payload

   File Transfer Payload is used to perform file transfer protocol between
   two entities in the network.  The actual file transfer protocol is always
   encapsulated inside the SILC Packet.  The actual data stream is also sent
   peer to peer outside SILC network.

   When an entity, usually a client wishes to perform file transfer protocol
   with another client in the network, they perform Key Agreement protocol
   as described in the section 2.3.20 Key Agreement Payload and in [SILC3],
   inside File Transfer Payload.  After the Key Agreement protocol has been



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   performed the subsequent packets in the data stream will be protected
   using the new key material.  The actual file transfer protocol is also
   initialized in this stage.  All file transfer protocol packets are always
   encapsulated in the File Transfer Payload and protected with the
   negotiated key material.

   The payload may only be sent with SILC_PACKET_FTP packet.  It MUST NOT
   be sent in any other packet type.  The following diagram represents the
   File Transfer Payload.


                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |                                               |
     +-+-+-+-+-+-+-+-+                                               +
     |                                                               |
     ~                             Data                              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 23:  File Transfer Payload


      o Type (1 byte) - Indicates the type of the file transfer
        protocol.  The following file transfer protocols has been
        defined:

          1    Secure File Transfer Protocol (SFTP)  (mandatory)

        If zero (0) value or any unsupported file transfer protocol
        type is found in this field the packet MUST be discarded.
        The currently mandatory file transfer protocol is SFTP.
        The SFTP protocol is defined in [SFTP].

      o Data (variable length) - Arbitrary file transfer data.  The
        contents and encoding of this field is dependent of the usage
        of this payload and the type of the file transfer protocol.
        When this payload is used to perform the Key Agreement
        protocol, this field include the Key Agreement Payload,
        as defined in the section 2.3.20 Key Agreement Payload.
        When this payload is used to send the actual file transfer
        protocol data, the encoding is defined in the corresponding
        file transfer protocol.


2.3.23 Resume Client Payload




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   This payload is used by client to resume its detached session in the
   SILC Network.  A client is able to detach itself from the network by
   sending SILC_COMMAND_DETACH command to its server.  The network
   connection to the client is lost but the client remains as valid
   client in the network.  The client is able to resume the session back
   by sending this packet and including the old Client ID, and an
   Authentication Payload [SILC1] which the server use to verify with
   the detached client's public key.  This also implies that the
   mandatory authentication method is public key authentication.

   Server or router that receives this from the client also sends this,
   without the Authentication Payload, to routers in the network so that
   they know the detached client has resumed.  Refer to the [SILC1] for
   detailed description how the detaching and resuming procedure is
   performed.

   The payload may only be sent with SILC_PACKET_RESUME_CLIENT packet.  It
   MUST NOT be sent in any other packet type.  The following diagram
   represents the Resume Client Payload.

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Client ID Length        |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
     |                                                               |
     ~                           Client ID                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                     Authentication Payload                    ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 24:  Resume Client Payload


      o Client ID Length (1 byte) - The length of the Client ID
        field not including any other field.

      o Client ID (variable length) - The detached client's Client
        ID.  The client that sends this payload must know the Client
        ID.

      o Authentication Payload (variable length) - The authentication
        payload that the server will verify with the detached client's
        public key.  If the server doesn't know the public key, it must
        retrieve it for example with SILC_COMMAND_GETKEY command.



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2.3.24 Acknowledgement Payload

   This payload is used to acknowledge a packet that had the Acknowledgement
   packet flag set.  The payload includes the sequence number of the packet
   that had the flag set, which the recipient can use to identify that the
   packet was acknowledged.

   The payload may only be sent with SILC_PACKET_ACK packet.  It
   MUST NOT be sent in any other packet type.  The following diagram
   represents the Acknowledgement Payload.

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Packet Sequence Number                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 24:  Resume Client Payload


      o Packet Sequence Number (4 bytes) - The packet sequence number
        of the packet that had the Acknowledgement flag set.


2.4 SILC ID Types

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

      0    No ID

           This is used when other ID type is available at the time.

      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].




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   When encoding different IDs into the ID Payload, all fields are always
   in MSB first order.  The IP address, port, and/or the random number
   are encoded in the MSB first order.


2.5 Packet Encryption And Decryption

   SILC packets are encrypted almost entirely.  Only the MAC at the end
   of the packet is never encrypted.  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.



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 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 can be decrypted with the same key.
   At this point the receiver is also able to determine the length of the
   packet.

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

   The MAC of the packet is also verified at this point.  The MAC is
   computed from the ciphertext of the packet so it can be verified
   at this stage.  The length of the packet need to be known to be able
   to verify the MAC from the ciphertext so the first 16 bytes need to
   be decrypted to determine the packet length.  However, the MAC MUST
   be verified from the entire ciphertext.




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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.  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 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 which is routing the
   message to another router then it MUST decrypt the Channel Message
   payload too.  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, MUST be 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 before.  This
   different processing of channel messages with router to router
   connection is because channel keys are cell specific.  All cells have
   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 MUST be 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.



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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 MAY 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 and is not re-encrypted.

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


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, from packet sequence number, and from the encrypted packet
   data.  The MAC is always computed after packet is encrypted.  This is
   so called Encrypt-Then-MAC order; packet is first encrypted, then MAC
   is computed from the encrypted data.

   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.

   Hence, packet's MAC generation is as follows:

     mac = MAC(key, sequence number | Encrypted SILC packet)

   The MAC key is negotiated during the SKE protocol.  The sequence number
   is a 32 bit MSB first value starting from zero for first packet and
   increasing for subsequent packets, finally wrapping after 2^32 packets.
   The value is never reset, not even after rekey has been performed.
   However, rekey MUST be performed before the sequence number wraps
   and repeats from zero.  Note that the sequence number is incremented only



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   when MAC is computed for a packet.  If packet is not encrypted and MAC is
   not computed then the sequence number is not incremented.  Hence, the
   sequence number is zero for the very first encrypted packet.

   See [SILC1] for defined and allowed MAC algorithms.


2.7 Packet Padding Generation

   Padding is needed in the packet because the packet is encrypted.  It
   always MUST be multiple by eight (8) or multiple by the block size
   of the cipher, 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
   may be calculated as follows:

      padding_length = 16 - (packet_length mod block_size)
      if (padding_length < 8)
        padding_length += block_size

   The `block_size' is the block size of the cipher.  The maximum padding
   length is 128 bytes, and minimum is 8 bytes.  For example, packets that
   include a passphrase or a password for authentication purposes SHOULD
   pad the packet up to the maximum padding length.  The maximum padding
   is calculated as follows:

      padding_length = 128 - (packet_length mod block_size)

   For special packets the padding calculation is 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.

   The padding MUST be random data, preferably, generated by
   cryptographically strong random number generator for each packet
   separately.


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



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   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 SHOULD NOT decompress the packet.


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 encrypt the packet as has been
   described in above sections according whether the packet is normal
   packet or special packet.  Then sender MUST compute the MAC of the
   packet.  The computed MAC MUST NOT be encrypted.


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.


2.11 Packet Routing




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   Routers are the primary entities in the SILC network that takes care
   of packet routing.  Normal servers performs packet forwarding, for
   example, when they are forwarding 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, servers 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).

   However, there are some issues when routing channel messages to group
   of users.  Routers are responsible of routing the channel message to
   other routers, local servers and local clients as well.  Routers MUST
   send the channel message to only one router in the network, preferably
   to the shortest route to reach the channel users.  The message can be
   routed into either upstream or downstream.  After the message is sent
   to a router in the network it MUST NOT be sent to any other router in
   either same route or other route.  The message MUST NOT be routed to
   the router it came from.

   When routing for example private messages they should be routed to the
   shortest route always to reach the destination client as fast as possible.

   For server which receives a packet to be forwarded to an entity that is
   indirectly connected to the sender, the server MUST check whether that
   particular packet type is allowed to be sent to that destination.  Not
   all packets may be sent by some odd entity to for example a local client,
   or to some remote server or router, 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.  That section defines the
   packets that may be sent to indirectly connected entities.  When a server
   or a router receives a packet that may be sent to indirectly connected
   entity and it is destined to other entity except that server, it MUST
   route it further either to shortest route or to the primary route to reach
   that destination.

   Routers form a ring in the SILC network.  However, routers may have other
   direct connections to other routers in the network too.  This can cause
   interesting routing problems in the network.  Since the network is a ring,
   the packets usually should be routed into clock-wise direction, or if it



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   cannot be used then always counter clock-wise (primary route) direction.
   Problems may arise when a faster direct route exists and router is routing
   a channel message.  Currently channel messages must be routed either
   in upstream or downstream, they cannot be routed to other direct routes.
   The SILC protocol should have a shortest path discovery protocol, and some
   existing routing protocol, that can handle a ring network with other
   direct routes inside the ring (so called hybrid ring-mesh topology),
   MAY be defined to be used with the SILC protocol.  Additional
   specifications MAY be written on the subject to permeate this
   specification.


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 can
   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.


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.


4 References



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   [SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
                Protocol Specification", Internet Draft, January 2007.

   [SILC3]      Riikonen, P., "SILC Key Exchange and Authentication
                Protocols", Internet Draft, January 2007.

   [SILC4]      Riikonen, P., "SILC Commands", Internet Draft, January 2007.

   [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.



Riikonen                                                       [Page 58]

Internet Draft                                           15 January 2007


   [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.

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

   [SFTP]       Ylonen T., and Lehtinen S., "Secure Shell File Transfer
                Protocol", Internet Draft, March 2001.

   [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3629, November 2003.


5 Author's Address

   Pekka Riikonen
   Helsinki
   Finland

   EMail: priikone@iki.fi


6 Full Copyright Statement

   Copyright (C) The Internet Society (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.








Riikonen                                                       [Page 59]

