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[silc.git] / doc / draft-riikonen-silc-pp-09.nroff

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

.in 3

.ce 2
SILC Packet Protocol
<draft-riikonen-silc-pp-09.txt>

.ti 0
Status of this Draft

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts. Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress".

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.



.ti 0
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

.nf
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
  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

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List of Figures

.nf
Figure 1:   Typical SILC Packet
Figure 2:   SILC Packet Header
Figure 3:   ID Payload
Figure 4:   Argument Payload
Figure 5:   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


.ti 0
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.

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.


.ti 0
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].


.ti 0
2 SILC Packet Protocol

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2.1 SILC Packet

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

The following diagram illustrates typical SILC packet.

.in 5
.nf
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|   n bytes   | 1 - n bytes |      n bytes       |  n bytes
| SILC Header |   Padding   |    Data Payload    |    MAC
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.in 3

.ce
Figure 1:  Typical SILC Packet


SILC Header is always the first part of the packet and its purpose
is to provide information about the packet.  It provides for example
the packet type, origin of the packet and the destination of the packet.
The header is variable in length.  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.

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.


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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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        |     Flags     |  Packet Type  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Pad Length  |    RESERVED   | Source ID Len |  Dest ID Len  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Src ID Type  |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                           Source ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Dst ID Type  |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                         Destination ID                        ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 2:  SILC Packet Header

.in 6
o Payload Length (2 bytes) - Indicates the length of the
  packet not including the padding of the packet.

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.
       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.

.in 3

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
  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.



.ti 0
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.

.in 1
     0    SILC_PACKET_NONE

          This type is reserved and it is never sent.


     1    SILC_PACKET_DISCONNECT

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

          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
          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
          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

          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
          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
          [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
          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.
.in 3


.ti 0
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
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].


.ti 0
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.


.ti 0
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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             ID Type           |           ID Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                           ID Data                             ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 3:  ID Payload


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

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

o ID Data (variable length) - The actual ID data.  The encoding
  of the ID data is defined in section 2.4 SILC ID Types.
.in 3


.ti 0
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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Data Length          | Argument Type |               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
|                                                               |
~                        Argument Data                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 4:  Argument Payload


.in 6
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.
.in 3


.ti 0
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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Argument Nums         |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                        Argument Payloads                      ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 5:  Argument List Payload


.in 6
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.
.in 3





.ti 0
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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Channel Name Length      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Channel Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Channel ID Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Channel ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Mode Mask                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 6:  New Channel Payload


.in 6
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.
.in 3






.ti 0
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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Public Key Length       |        Public Key Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                  Public Key (or certificate)                  ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 7:  Public Key Payload


.in 6
o Public Key Length (2 bytes) - The length of the Public Key
  (or certificate) field, not including any other field.

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.
.in 3


.ti 0
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.









.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Message  Flags         |         Message Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                         Message Data                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Padding Length         |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                            Padding                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                    Initialization Vector *                    ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                              MAC *                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 8:  Message Payload


.in 6
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

          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
  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.
.in 3


.ti 0
2.3.3 Disconnect Payload

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

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


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Status     |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                      Disconnect Message                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 9:  Disconnect Payload

.in 6
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.
.in 3


.ti 0
2.3.4 Success Payload

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

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                      Success Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 10:  Success Payload


.in 6
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
  Packet Header.
.in 3


.ti 0
2.3.5 Failure Payload

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

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                      Failure Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 11:  Failure Payload


.in 6
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.
.in 3


.ti 0
2.3.6 Reject Payload

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


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                       Reject Indication                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 12:  Reject Payload


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



.ti 0
2.3.7 Notify Payload

Notify payload is used to send notify messages.  The payload is usually
sent from server to client 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.



.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Notify Type          |        Payload Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Argument Nums |
+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 13:  Notify Payload


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

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

o Argument Nums (1 byte) - Indicates the number of Argument
  Payloads associated to this payload.  Notify types may define
  arguments to be sent along the notify message.
.in 3

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
certificates sent inside arguments are actually Public Key Payloads.


.in 6
0     SILC_NOTIFY_TYPE_NONE

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

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

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


1     SILC_NOTIFY_TYPE_INVITE

      Sent when 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

      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.


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>]
                      (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>]

      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
          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>]
                      (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
      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.
.in 3

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
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.


.ti 0
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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                         Error Message                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 14:  Error Payload


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


.ti 0
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.

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.


.ti 0
2.3.10 Channel Key Payload

All traffic in channels are protected by channel specific keys.
Channel Key Payload is used to distribute channel keys to all
clients on the particular channel.  Channel keys are sent when
the channel is created, when new user joins to the channel and
whenever a user has left a channel.  Server creates the new
channel key and distributes it to the clients by encrypting this
payload with the session key shared between the server and
the client.  After that, client 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.
It MUST NOT be sent in any other packet type.  The following diagram
represents the Channel Key Payload.



.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Channel ID Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Channel ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Cipher Name Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Cipher Name                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Channel Key Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Channel Key                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 15:  Channel Key Payload



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

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

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

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

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

o Channel Key (variable length) - The actual channel key
  material.  See [SILC1] on how to start using the key.
.in 3


.ti 0
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.


.ti 0
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].

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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Cipher Name Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Cipher Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       HMAC Name Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           HMAC Name                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 16:  Private Message Key Payload



.in 6
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
  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.
.in 3


.ti 0
2.3.13 Command Payload

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


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        | SILC Command  | Arguments Num |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Command Identifier      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 17:  Command Payload


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

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

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

o Command Identifier (2 bytes) - Identifies this command at the
  sender's end.  The entity 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
  used in the field.
.in 3

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


.ti 0
2.3.14 Command Reply Payload

Command Reply Payload is used to send replies to the commands.  The
Command Reply Payload is identical to the Command Payload thus see
the 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.


.ti 0
2.3.15 Connection Auth Request Payload

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

Server receiving this request 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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Connection Type        |     Authentication Method     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 18:  Connection Auth Request Payload


.in 6
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.
.in 3


.ti 0
2.3.16 New ID Payload

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

This payload is also used when server tells its router that new client
has registered to the SILC network.  In this case the server sends
the Client ID of the client to the router.  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
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.


.ti 0
2.3.17 New Client Payload

When client is connected to the server, keys has been exchanged and
connection has been authenticated, client MUST register itself to the
server.  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.



.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Username Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Username                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Real Name Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Real Name                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 19:  New Client Payload


.in 6
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.
.in 3


.ti 0
2.3.18 New Server Payload

This payload is sent by server when it has completed successfully both
key exchange and connection authentication protocols.  The server
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.





.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Server ID Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                        Server ID Data                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Server Name Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Server Name                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 20:  New Server Payload


.in 6
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.
.in 3


.ti 0
2.3.19 New Channel Payload

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


.ti 0
2.3.20 Key Agreement Payload

This payload is used by clients to request key negotiation between
another client in the SILC Network.  The key agreement protocol used
is the SKE protocol.  The result of the protocol, the secret key
material, can be used for example as private message key between the
two clients.  This significantly adds security as the 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
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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Hostname Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Hostname                            ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Protocol           |             Port              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 21:  Key Agreement Payload


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

o Hostname (variable length) - The hostname or IP address where
  the SKE protocol is running, 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.
.in 3


After the key material has been received from the SKE protocol it is
processed as the [SILC3] describes.  If the key material is used as
channel private key then the Sending Encryption Key, as defined in
[SILC3] is used as the channel private key.  Other key material must
be discarded.  The [SILC1] 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.


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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.


.in 21
.nf
                     1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |  Session ID   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 22:  Resume Router Payload


.in 6
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.
.in 3




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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
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.


.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                                                               |
~                             Data                              ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 23:  File Transfer Payload


.in 6
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.
.in 3


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2.3.23 Resume Client Payload

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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Client ID Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                           Client ID                           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                     Authentication Payload                    ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 24:  Resume Client Payload


.in 6
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.
.in 3


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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.

.in 5
.nf
                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Packet Sequence Number                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 24:  Resume Client Payload


.in 6
o Packet Sequence Number (4 bytes) - The packet sequence number
  of the packet that had the Acknowledgement flag set.
.in 3


.ti 0
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.

.in 6
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].
.in 3

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.


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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.



.ti 0
2.5.1 Normal Packet Encryption And Decryption

Normal SILC packets are encrypted with the session key of the next
receiver of the packet.  The entire SILC Packet header and the packet
data payload is is 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.


.ti 0
2.5.2 Channel Message Encryption And Decryption

Channel Messages (Channel Message Payload) are always encrypted with
the channel specific key.  However, the SILC Packet header is not
encrypted with that key.  As in normal case, the header is encrypted
with the key of the next receiver of the packet.  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.


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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
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.


.ti 0
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:

.in 6
padding_length = 16 - (packet_length mod block_size)
if (padding_length < 8)
  padding_length += block_size
.in 3

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:

.in 6
padding_length = 128 - (packet_length mod block_size)
.in 3

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.


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2.8 Packet Compression

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

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


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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.


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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.


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2.11 Packet Routing

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
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.


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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.


.ti 0
3 Security Considerations

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


.ti 0
4 References

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

[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.


.ti 0
5 Author's Address

.nf
Pekka Riikonen
Helsinki
Finland

EMail: priikone@iki.fi


.ti 0
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.