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Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-ke-auth-06.txt                      25 November 2002
Expires: 25 April 2003

.in 3

.ce 2
SILC Key Exchange and Authentication Protocols
<draft-riikonen-silc-ke-auth-06.txt>

.ti 0
Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.  Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups.  Note that other groups may also
distribute working documents as Internet-Drafts.

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

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

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

The distribution of this memo is unlimited.


.ti 0
Abstract

This memo describes two protocols used in the Secure Internet Live
Conferencing (SILC) protocol, specified in the Secure Internet Live
Conferencing, Protocol Specification [SILC1].  The SILC Key Exchange
(SKE) protocol provides secure key exchange between two parties
resulting into shared secret key material.  The protocol is based
on Diffie-Hellman key exchange algorithm and its functionality is
derived from several key exchange protocols.  SKE use best parts
of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol
and the OAKLEY Key Determination protocol [OAKLEY].

The second protocol, SILC Connection Authentication protocol provides
user level authentication used when creating connections in SILC
network.  The protocol is transparent to the authentication data
which means that it can be used to authenticate the user with, for
example, passphrase (pre-shared secret) or public key (and certificate)
based on digital signatures.



.ti 0
Table of Contents

.nf
1 Introduction ..................................................  2
  1.1 Requirements Terminology ..................................  3
2 SILC Key Exchange Protocol ....................................  3
  2.1 Key Exchange Payloads .....................................  4
      2.1.1 Key Exchange Start Payload ..........................  4
      2.1.2 Key Exchange Payload ................................  8
  2.2 Key Exchange Procedure .................................... 11
  2.3 Processing the Key Material ............................... 12
  2.4 SILC Key Exchange Groups .................................. 14
      2.4.1 diffie-hellman-group1 ............................... 14
      2.4.2 diffie-hellman-group2 ............................... 15
      2.4.3 diffie-hellman-group3 ............................... 15
  2.5 Key Exchange Status Types ................................. 16
3 SILC Connection Authentication Protocol ....................... 17
  3.1 Connection Auth Payload ................................... 18
  3.2 Connection Authentication Types ........................... 19
      3.2.1 Passphrase Authentication ........................... 19
      3.2.2 Public Key Authentication ........................... 20
  3.3 Connection Authentication Status Types .................... 21
4 Security Considerations ....................................... 21
5 References .................................................... 21
6 Author's Address .............................................. 23


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List of Figures

.nf
Figure 1:  Key Exchange Start Payload
Figure 2:  Key Exchange Payload
Figure 3:  Connection Auth Payload


.ti 0
1 Introduction

This memo describes two protocols used in the Secure Internet Live
Conferencing (SILC) protocol specified in the Secure Internet Live
Conferencing, Protocol Specification [SILC1].  The SILC Key Exchange
(SKE) protocol provides secure key exchange between two parties
resulting into shared secret key material.  The protocol is based on
Diffie-Hellman key exchange algorithm and its functionality is derived
from several key exchange protocols.  SKE use best parts of the SSH2
Key Exchange protocol, Station-To-Station (STS) protocol and the
OAKLEY Key Determination protocol [OAKLEY].

The second protocol, SILC Connection Authentication protocol provides
user level authentication used when creating connections in SILC
network.  The protocol is transparent to the authentication data which
means that it can be used to authenticate the user with, for example,
passphrase (pre-shared secret) or public key (and certificate) based
on digital signatures.

The basis of secure SILC session requires strong and secure key exchange
protocol and authentication.  The authentication protocol is secured and
no authentication data is ever sent in the network without encrypting
and authenticating it first.  Thus, authentication protocol may be used
only after the key exchange protocol has been successfully completed.

This document constantly refers to other SILC protocol specifications
that should be read to be able to fully understand the functionality
and purpose of these protocols.  The most important references are
the Secure Internet Live Conferencing, Protocol Specification [SILC1]
and the SILC Packet Protocol [SILC2].

The protocol is intended to be used with the SILC protocol thus it
does not define own framework that could be used.  The framework is
provided by the SILC protocol.


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1.1 Requirements Terminology

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


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2 SILC Key Exchange Protocol

SILC Key Exchange Protocol (SKE) is used to exchange shared secret
between connecting entities.  The result of this protocol is a key
material used to secure the communication channel.  The protocol use
Diffie-Hellman key exchange algorithm and its functionality is derived
from several key exchange protocols.  SKE use best parts of the SSH2
Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY
Key Determination protocol.  The protocol does not claim any conformance
to any of these protocols, they were only used as a reference when
designing this protocol.

The purpose of SILC Key Exchange protocol is to create session keys to
be used in current SILC session.  The keys are valid only for some period
of time (usually an hour) or at most until the session ends.  These keys
are used to protect packets traveling between the two entities.
Usually all traffic is secured with the key material derived from this
protocol.

The Diffie-Hellman implementation used in the SILC  SHOULD be compliant
to the PKCS #3.


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2.1 Key Exchange Payloads

During the key exchange procedure public data is sent between initiator
and responder.  This data is later used in the key exchange procedure.
There are several payloads used in the key exchange.  As for all SILC
packets, SILC Packet Header, described in [SILC2], is at the start of
all packets sent in during this protocol.  All the fields in the
following payloads are in MSB (most significant byte first) order.
Following descriptions of these payloads.


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2.1.1 Key Exchange Start Payload

The key exchange between two entities MUST be started by sending the
SILC_PACKET_KEY_EXCHANGE packet containing Key Exchange Start Payload.
Initiator sends the Key Exchange Start Payload to the responder filled
with all security properties it supports.  The responder then checks
whether it supports the security properties.

It then sends a Key Exchange Start Payload to the initiator filled with
security properties it selected from the original payload.  The payload
sent by responder MUST include only one chosen property per list.  The
character encoding for the security property values as defined in [SILC1]
SHOULD be UTF-8 [RFC2279] in Key Exchange Start Payload.

The Key Exchange Start Payload is used to tell connecting entities what
security properties and algorithms should be used in the communication.
The Key Exchange Start Payload is sent only once per session.  Even if
the PFS (Perfect Forward Secrecy) flag is set the Key Exchange Start
Payload is not re-sent.  When PFS is desired the Key Exchange Payloads
are sent to negotiate new key material.  The procedure is equivalent to
the very first negotiation except that the Key Exchange Start Payload
is not sent.

As this payload is used only with the very first key exchange the payload
is never encrypted, as there are no keys to encrypt it with.

A cookie is also sent in this payload.  A cookie is used to randomize the
payload so that none of the key exchange parties can determine this
payload before the key exchange procedure starts.  The cookie MUST be
returned to the original sender unmodified by the responder.

Following diagram represents the Key Exchange Start Payload.  The lists
mentioned below are always comma (`,') separated and the list MUST NOT
include white spaces (` ').


.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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   RESERVED    |     Flags     |         Payload Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +  
|                                                               |
+                            Cookie                             +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Version String Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Version String                        ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Key Exchange Grp Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                      Key Exchange Groups                      ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        PKCS Alg Length        |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         PKCS Algorithms                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Encryption Alg Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                      Encryption Algorithms                    ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Hash Alg Length         |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                         Hash Algorithms                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         HMAC Length           |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                             HMACs                             ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Compression Alg Length     |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                     Compression Algorithms                    ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 1:  Key Exchange Start Payload



.in 6
o RESERVED (1 byte) - Reserved field.  Sender fills this with
  zero (0) value.

o Flags (1 byte) - Indicates flags to be used in the key
  exchange.  Several flags can be set at once by ORing the
  flags together.  The following flags are reserved for this
  field:

     No flags                 0x00

       In this case the field is ignored.

     IV Included              0x01

       This flag is used to indicate that Initial Vector (IV)
       in encryption will be included in the ciphertext
       which the recipient must use in decryption.  The IV
       MUST be set after the last ciphertext block.  With
       this flag it is possible to use SILC protocol on
       unreliable transport such as UDP/IP which may cause
       packet reordering and packet losses.  By default,
       this flag is not set and thus IV is not included
       in the ciphertext.  Setting this flag increases the
       ciphertext size by one ciphertext block.  Responder
       MAY override this flag for the initiator.

     PFS                      0x02

       Perfect Forward Secrecy (PFS) to be used in the
       key exchange protocol.  If not set, re-keying
       is performed using the old key.  See the [SILC1]
       for more information on this issue.  When PFS is
       used, re-keying and creating new keys for any
       particular purpose MUST cause new key exchange.
       In this key exchange only the Key Exchange Payload
       is sent and the Key Exchange Start Payload MUST
       NOT be sent.  When doing PFS the Key Exchange
       Payloads are encrypted with the old keys.

     Mutual Authentication    0x04

       Both of the parties will perform authentication
       by providing signed data for the other party to
       verify.  By default, only responder will provide
       the signature data.  If this is set then the
       initiator must also provide it.  Initiator MAY
       set this but also responder MAY set this even if
       initiator did not set it.

     Rest of the flags are reserved for the future and
     MUST NOT be set.

o Payload Length (2 bytes) - Length of the entire Key Exchange
  Start payload, not including any other field.

o Cookie (16 bytes) - Cookie that randomize this payload so
  that each of the party cannot determine the payload before
  hand.  This field MUST be present.

o Version String Length (2 bytes) - The length of the Version
  String field, not including any other field.

o Version String (variable length) - Indicates the version of
  the sender of this payload.  Initiator sets this when sending
  the payload and responder sets this when it replies by sending
  this payload.  See [SILC1] for definition for the version
  string format.  This field MUST be present and include valid
  version string.

o Key Exchange Grp Length (2 bytes) - The length of the
  key exchange group list, not including any other field.

o Key Exchange Group (variable length) - The list of
  key exchange groups.  See the section 2.4 SILC Key Exchange
  Groups for definitions of these groups.  This field MUST
  be present.

o PKCS Alg Length (2 bytes) - The length of the PKCS algorithms
  list, not including any other field.

o PKCS Algorithms (variable length) - The list of PKCS 
  algorithms.  This field MUST be present.

o Encryption Alg Length (2 bytes) - The length of the encryption
  algorithms list, not including any other field.

o Encryption Algorithms (variable length) - The list of
  encryption algorithms.  This field MUST be present.

o Hash Alg Length (2 bytes) - The length of the Hash algorithm
  list, not including any other field.

o Hash Algorithms (variable length) - The list of Hash
  algorithms.  The hash algorithms are mainly used in the
  SKE protocol.  This field MUST be present.

o HMAC Length (2 bytes) - The length of the HMAC list, not
  including any other field.

o HMACs (variable length) - The list of HMACs.  The HMAC's
  are used to compute the Message Authentication Code (MAC)
  of the SILC packets.  This field MUST be present.

o Compression Alg Length (2 bytes) - The length of the
  compression algorithms list, not including any other field.

o Compression Algorithms (variable length) - The list of 
  compression algorithms.  This field MAY be omitted.
.in 3


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2.1.2 Key Exchange Payload

Key Exchange payload is used to deliver the public key (or certificate),
the computed Diffie-Hellman public value and possibly signature data
from one party to the other.  When initiator is using this payload
and the Mutual Authentication flag is not set then the initiator MUST
NOT provide the signature data.  If the flag is set then the initiator
MUST provide the signature data so that the responder can verify it.

The Mutual Authentication flag is usually used when a separate 
authentication protocol will not be executed for the initiator of the
protocol.  This is case for example when the SKE is performed between
two SILC clients.  In normal case, where client is connecting to a
server, or server is connecting to a router the Mutual Authentication
flag MAY be omitted.  However, if the connection authentication protocol 
for the connecting entity is not based on digital signatures (it is
based on pre-shared key) then the Mutual Authentication flag SHOULD be 
enabled.  This way the connecting entity has to provide proof of
possession of the private key for the public key it will provide in
this protocol.

When performing re-key with PFS selected this is the only payload that
is sent in the SKE protocol.  The Key Exchange Start Payload MUST NOT
be sent at all.  However, this payload does not have all the fields
present.  In the re-key with PFS the public key and a possible signature
data SHOULD NOT be present.  If they are present they MUST be ignored.
The only field that is present is the Public Data that is used to create
the new key material.  In the re-key the Mutual Authentication flag, that
may be set in the initial negotiation, MUST also be ignored.

This payload is sent inside SILC_PACKET_KEY_EXCHANGE_1 and inside
SILC_PACKET_KEY_EXCHANGE_2 packet types.  The initiator uses the 
SILC_PACKET_KEY_EXCHANGE_1 and the responder the latter.

The following diagram represent the Key Exchange 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 of the party (or certificate)           ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Public Data Length      |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                          Public Data                          ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Signature Length       |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
~                        Signature Data                         ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3

.ce
Figure 2:  Key Exchange 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.  Following types are defined:

     1    SILC style public key (mandatory)
     2    SSH2 style public key (optional)
     3    X.509 Version 3 certificate (optional)
     4    OpenPGP certificate (optional)
     5    SPKI certificate (optional)

  The only required type to support is type number 1.  See 
  [SILC1] for the SILC public key specification.  See
  SSH2 public key specification in [SSH-TRANS].  See X.509v3
  certificate specification in [PKIX-Part1].  See OpenPGP
  certificate specification in [PGP].  See SPKI certificate
  specification in [SPKI].  If this field includes zero (0)
  or unsupported type number the protocol MUST be aborted
  sending SILC_PACKET_FAILURE message and the connection SHOULD
  be closed immediately.

o Public Key (or certificate) (variable length) - The
  public key or certificate of the party.  This public key
  is used to verify the digital signature.  The public key
  or certificate in this field is encoded in the manner as
  defined in their respective definitions; see previous field.

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

o Public Data (variable length) - The public data to be
  sent to the receiver (Diffie-Hellman public values).  See
  section 2.2 Key Exchange Procedure for detailed description
  how this field is computed.  This value is binary encoded.

o Signature Length (2 bytes) - The length of the signature,
  not including any other field.

o Signature Data (variable length) - The signature signed
  by the sender.  The receiver of this signature MUST
  verify it.  The verification is done using the sender's
  public key.  See section 2.2 Key Exchange Procedure for
  detailed description how to produce the signature.  If
  the Mutual Authentication flag is not set then initiator
  MUST NOT provide this field and the Signature Length field
  MUST be set to zero (0) value.  If the flag is set then
  also the initiator MUST provide this field.  The responder
  always MUST provide this field.
.in 3


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2.2 Key Exchange Procedure

The key exchange begins by sending SILC_PACKET_KEY_EXCHANGE packet with
Key Exchange Start Payload to select the security properties to be used
in the key exchange and later in the communication.

After Key Exchange Start Payload has been processed by both of the
parties the protocol proceeds as follows:


Setup:  p is a large and public safe prime.  This is one of the
        Diffie Hellman groups.  q is order of subgroup (largest
        prime factor of p).  g is a generator and is defined
        along with the Diffie Hellman group.

    1.  Initiator generates a random number x, where 1 < x < q, 
        and computes e = g ^ x mod p.  The result e is then 
        encoded into Key Exchange Payload, with the public key
        (or certificate) and sent to the responder.

        If the Mutual Authentication flag is set then initiator
        MUST also produce signature data SIGN_i which the responder
        will verify.  The initiator MUST compute a hash value
        HASH_i = hash(Initiator's Key Exchange Start Payload |
        public key (or certificate) | e).  The '|' stands for
        concatenation.  It then signs the HASH_i value with its
        private key resulting a signature SIGN_i.

    2.  Responder generates a random number y, where 1 < y < q,
        and computes f = g ^ y mod p.  It then computes the
        shared secret KEY = e ^ y mod p, and, a hash value 
        HASH = hash(Initiator's Key Exchange Start Payload |
        public key (or certificate) | Initiator's public key
        (or certificate) | e | f | KEY).  It then signs
        the HASH value with its private key resulting a signature
        SIGN.  

        It then encodes its public key (or certificate), f and 
        SIGN into Key Exchange Payload and sends it to the 
        initiator.

        If the Mutual Authentication flag is set then the responder
        SHOULD verify that the public key provided in the payload
        is authentic, or if certificates are used it verifies the
        certificate.  The responder MAY accept the public key without
        verifying it, however, doing so may result to insecure key
        exchange (accepting the public key without verifying may be
        desirable for practical reasons on many environments.  For
        long term use this is never desirable, in which case
        certificates would be the preferred method to use).  It then
        computes the HASH_i value the same way initiator did in the
        phase 1.  It then verifies the signature SIGN_i from the
        payload with the hash value HASH_i using the received public
        key.

    3.  Initiator verifies that the public key provided in
        the payload is authentic, or if certificates are used
        it verifies the certificate.  The initiator MAY accept
        the public key without verifying it, however, doing
        so may result to insecure key exchange (accepting the
        public key without verifying may be desirable for 
        practical reasons on many environments.  For long term
        use this is never desirable, in which case certificates
        would be the preferred method to use).

        Initiator then computes the shared secret KEY = 
        f ^ x mod p, and, a hash value HASH in the same way as
        responder did in phase 2.  It then verifies the 
        signature SIGN from the payload with the hash value
        HASH using the received public key.


If any of these phases is to fail the SILC_PACKET_FAILURE MUST be sent
to indicate that the key exchange protocol has failed, and the connection
SHOULD be closed immediately.  Any other packets MUST NOT be sent or
accepted during the key exchange except the SILC_PACKET_KEY_EXCHANGE_*,
SILC_PACKET_FAILURE and SILC_PACKET_SUCCESS packets.

The result of this protocol is a shared secret key material KEY and
a hash value HASH.  The key material itself is not fit to be used as 
a key, it needs to be processed further to derive the actual keys to be
used.  The key material is also used to produce other security parameters
later used in the communication.  See section 2.3 Processing the Key
Material for detailed description how to process the key material.

If the Mutual Authentication flag was set the protocol produces also
a hash value HASH_i.  This value, however, must be discarded.

After the keys are processed the protocol is ended by sending the
SILC_PACKET_SUCCESS packet.  Both entities send this packet to 
each other.  After this both parties will start using the new keys.


.ti 0
2.3 Processing the Key Material

Key Exchange protocol produces secret shared key material KEY.  This
key material is used to derive the actual keys used in the encryption
of the communication channel.  The key material is also used to derive
other security parameters used in the communication.  Key Exchange
protocol produces a hash value HASH as well.

The keys MUST be derived from the key material as follows:

.in 6
Sending Initial Vector (IV)     = hash(0x0 | KEY | HASH)
Receiving Initial Vector (IV)   = hash(0x1 | KEY | HASH)
Sending Encryption Key          = hash(0x2 | KEY | HASH)
Receiving Encryption Key        = hash(0x3 | KEY | HASH)
Sending HMAC Key                = hash(0x4 | KEY | HASH)
Receiving HMAC Key              = hash(0x5 | KEY | HASH)
.in 3


The Initial Vector (IV) is used in the encryption when doing for
example CBC mode.  As many bytes as needed are taken from the start of
the hash output for IV.  Sending IV is for sending key and receiving IV
is for receiving key.  For receiving party, the receiving IV is actually
sender's sending IV, and, the sending IV is actually sender's receiving
IV.  Initiator uses IV's as they are (sending IV for sending and
receiving IV for receiving).

The Encryption Keys are derived as well from the hash().  If the hash()
output is too short for the encryption algorithm more key material MUST
be produced in the following manner:

.in 6
K1 = hash(0x2 | KEY | HASH)
K2 = hash(KEY | HASH | K1)
K3 = hash(KEY | HASH | K1 | K2)  ...

Sending Encryption Key = K1 | K2 | K3 ...


K1 = hash(0x3 | KEY | HASH)
K2 = hash(KEY | HASH | K1)
K3 = hash(KEY | HASH | K1 | K2)  ...

Receiving Encryption Key = K1 | K2 | K3 ...
.in 3


The key is distributed by hashing the previous hash with the original
key material.  The final key is a concatenation of the hash values.
For Receiving Encryption Key the procedure is equivalent.  Sending key
is used only for encrypting data to be sent.  The receiving key is used
only to decrypt received data.  For receiving party, the receive key is
actually sender's sending key, and, the sending key is actually sender's
receiving key.  Initiator uses generated keys as they are (sending key
for sending and receiving key for receiving).

The HMAC keys are used to create MAC values to packets in the
communication channel.  As many bytes as needed are taken from the start
of the hash output to generate the MAC keys.

These procedures are performed by all parties of the key exchange
protocol.  This MUST be done before the protocol has been ended by
sending the SILC_PACKET_SUCCESS packet, to assure that parties can
successfully process the key material.

This same key processing procedure MAY be used in the SILC in some
other circumstances as well.  Any changes to this procedure is defined
separately when this procedure is needed.  See the [SILC1] and the
[SILC2] for these circumstances.


.ti 0
2.4 SILC Key Exchange Groups

The Following groups may be used in the SILC Key Exchange protocol.
The first group diffie-hellman-group1 is REQUIRED, other groups MAY be 
negotiated to be used in the connection with Key Exchange Start Payload
and SILC_PACKET_KEY_EXCHANGE packet.  However, the first group MUST be
proposed in the Key Exchange Start Payload regardless of any other
requested group (however, it does not have to be the first in the list).


.ti 0
2.4.1 diffie-hellman-group1

The length of this group is 1024 bits.  This is REQUIRED group.
The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.

Its hexadecimal value is

.in 6
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
FFFFFFFF FFFFFFFF
.in 3


The generator used with this prime is g = 2.  The group order q is
(p - 1) / 2.

This group was taken from the OAKLEY specification.


.ti 0
2.4.2 diffie-hellman-group2

The length of this group is 1536 bits.  This is OPTIONAL group.
The prime is 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }.

Its hexadecimal value is

.in 6
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
.in 3

The generator used with this prime is g = 2.  The group order q is
(p - 1) / 2.

This group was taken from the OAKLEY specification.


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2.4.3 diffie-hellman-group3

The length of this group is 2048 bits.  This is OPTIONAL group.
This prime is: 2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }.

Its hexadecimal value is

.in 6
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
15728E5A 8AACAA68 FFFFFFFF FFFFFFFF
.in 3

The generator used with this prime is g = 2.  The group order q is
(p - 1) / 2.





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2.5 Key Exchange Status Types

This section defines all key exchange protocol status types that may
be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets
to indicate the status of the protocol.  Implementations may map the
status types to human readable error message.  All types except the
SILC_SKE_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet.
The length of status is 32 bits (4 bytes).  The following status types
are defined:

.in 6
0   SILC_SKE_STATUS_OK

    Protocol were executed successfully.


1   SILC_SKE_STATUS_ERROR

    Unknown error occurred.  No specific error type is defined.


2   SILC_SKE_STATUS_BAD_PAYLOAD

    Provided KE payload were malformed or included bad fields.


3   SILC_SKE_STATUS_UNSUPPORTED_GROUP

    None of the provided groups were supported.


4   SILC_SKE_STATUS_UNSUPPORTED_CIPHER

    None of the provided ciphers were supported.


5   SILC_SKE_STATUS_UNSUPPORTED_PKCS

    None of the provided public key algorithms were supported.


6   SILC_SKE_STATUS_UNSUPPORTED_HASH_FUNCTION

    None of the provided hash functions were supported.


7   SILC_SKE_STATUS_UNSUPPORTED_HMAC

    None of the provided HMACs were supported.


8   SILC_SKE_STATUS_UNSUPPORTED_PUBLIC_KEY

    Provided public key type is not supported.


9   SILC_SKE_STATUS_INCORRECT_SIGNATURE

    Provided signature was incorrect.


10  SILC_SKE_STATUS_BAD_VERSION

    Provided version string was not acceptable.


11  SILC_SKE_STATUS_INVALID_COOKIE

    The cookie in the Key Exchange Start Payload was malformed,
    because responder modified the cookie.
.in 3


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3 SILC Connection Authentication Protocol

Purpose of Connection Authentication protocol is to authenticate the
connecting party with server.  Usually connecting party is client but
server may connect to router server as well.  Its other purpose is to
provide information for the server about which type of connection this
is.  The type defines whether this is client, server or router
connection.  Server use this information to create the ID for the
connection.

Server MUST verify the authentication data received and if it is to fail
the authentication MUST be failed by sending SILC_PACKET_FAILURE packet.
If authentication is successful the protocol is ended by server by sending
SILC_PACKET_SUCCESS packet.

The protocol is executed after the SILC Key Exchange protocol.  It MUST
NOT be executed in any other time.  As it is performed after key exchange
protocol all traffic in the connection authentication protocol is
encrypted with the exchanged keys.

The protocol MUST be started by the connecting party by sending the
SILC_PACKET_CONNECTION_AUTH packet with Connection Auth Payload,
described in the next section.  This payload MUST include the
authentication data.  The authentication data is set according
authentication method that MUST be known by both parties.  If connecting
party does not know what is the mandatory authentication method it MAY
request it from the server by sending SILC_PACKET_CONNECTION_AUTH_REQUEST
packet.  This packet is not part of this protocol and is described in
section Connection Auth Request Payload in [SILC2].  However, if
connecting party already knows the mandatory authentication method
sending the request is not necessary.

See [SILC1] and section Connection Auth Request Payload in [SILC2] also
for the list of different authentication methods.  Authentication method
MAY also be NONE, in which case the server does not require
authentication.  However, in this case the protocol still MUST be
executed; the authentication data is empty indicating no authentication
is required.

If authentication method is passphrase the authentication data is
plaintext passphrase.  As the payload is encrypted it is safe to have
plaintext passphrase.  It is also provided as plaintext passphrase
because the receiver may need to pass the entire passphrase into a
passphrase verifier, and a message digest of the passphrase would
prevent this.  See the section 3.2.1 Passphrase Authentication for
more information.

If authentication method is public key authentication the authentication
data is a digital signature of the hash value of hash HASH and Key 
Exchange Start Payload, established by the SILC Key Exchange protocol.
This signature MUST then be verified by the server.  See the section
3.2.2 Public Key Authentication for more information.

See the section 4 SILC Procedures in [SILC1] for more information about
client creating connection to server, and server creating connection
to router, and how to register the session in the SILC Network after
successful Connection Authentication protocol.


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3.1 Connection Auth Payload

Client sends this payload to authenticate itself to the server.  Server
connecting to another server also sends this payload.  Server receiving
this payload MUST verify all the data in it and if something is to fail
the authentication MUST be failed by sending SILC_PACKET_FAILURE packet.

The payload may only be sent with SILC_PACKET_CONNECTION_AUTH packet.
It MUST NOT be sent in any other packet type.  The following diagram 
represent the Connection Auth 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         |        Connection Type        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                     Authentication Data                       ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.in 3
 
.ce
Figure 3:  Connection Auth Payload


.in 6
o Payload Length (2 bytes) - Length of the entire Connection 
  Auth Payload.

o Connection Type (2 bytes) - Indicates the type of the 
  connection.  See section Connection Auth Request Payload
  in [SILC2] for the list of connection types.  This field MUST
  include valid connection type or the packet MUST be discarded
  and authentication MUST be failed. 

o Authentication Data (variable length) - The actual 
  authentication data.  Contents of this depends on the 
  authentication method known by both parties.  If no
  authentication is required this field does not exist.
.in 3


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3.2 Connection Authentication Types

SILC supports two authentication types to be used in the connection
authentication protocol; passphrase authentication or public key
authentication based on digital signatures.  The following sections
defines the authentication methods.  See [SILC2] for defined numerical
authentication method types.


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3.2.1 Passphrase Authentication

Passphrase authentication or pre-shared key based authentication is 
simply an authentication where the party that wants to authenticate 
itself to the other end sends the passphrase that is required by
the other end, for example server.  The plaintext passphrase is put
to the payload, that is then encrypted.  The plaintext passphrase
MUST be in UTF-8 [RFC2279] encoding.  If the passphrase is in the
sender's system in some other encoding it MUST be UTF-8 encoded 
before transmitted.  The receiver MAY change the encoding of the
passphrase to its system's default character encoding before verifying
the passphrase.

If the passphrase matches with the one in the server's end the
authentication is successful.  Otherwise SILC_PACKET_FAILURE MUST be
sent to the sender and the protocol execution fails.

This is REQUIRED authentication method to be supported by all SILC
implementations.

When password authentication is used it is RECOMMENDED that maximum
amount of padding is applied to the SILC packet.  This way it is not
possible to approximate the length of the password from the encrypted
packet.



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3.2.2 Public Key Authentication

Public key authentication may be used if passphrase based authentication
is not desired.  The public key authentication works by sending a
digital signature as authentication data to the other end, say, server.
The server MUST then verify the signature by the public key of the sender,
which the server has received earlier in SKE protocol.

The signature is computed using the private key of the sender by signing
the HASH value provided by the SKE protocol previously, and the Key
Exchange Start Payload from SKE protocol that was sent to the server.
These are concatenated and hash function is used to compute a hash value
which is then signed.

  auth_hash = hash(HASH | Key Exchange Start Payload);
  signature = sign(auth_hash);

The hash() function used to compute the value is the hash function
negotiated in the SKE protocol.  The server MUST verify the data, thus
it must keep the HASH and the Key Exchange Start Payload saved during
SKE and authentication protocols.  These values can be discarded after
Connection Authentication protocol is completed.

If the verified signature matches the sent signature, the authentication
were successful and SILC_PACKET_SUCCESS is sent.  If it failed the
protocol execution is stopped and SILC_PACKET_FAILURE is sent.

This is REQUIRED authentication method to be supported by all SILC
implementations.



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3.3 Connection Authentication Status Types

This section defines all connection authentication status types that
may be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets
to indicate the status of the protocol.  Implementations may map the
status types to human readable error message.  All types except the
SILC_AUTH_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet.
The length of status is 32 bits (4 bytes).  The following status types
are defined:

0   SILC_AUTH_OK

    Protocol was executed successfully.


1   SILC_AUTH_FAILED

    Authentication failed.


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4 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.


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5 References

[SILC1]      Riikonen, P., "Secure Internet Live Conferencing (SILC),
             Protocol Specification", Internet Draft, May 2002.

[SILC2]      Riikonen, P., "SILC Packet Protocol", Internet Draft,
             May 2002.

[SILC4]      Riikonen, P., "SILC Commands", Internet Draft, May 2002.

[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.

[RFC2279]    Yergeau, F., "UTF-8, a transformation format of ISO
             10646", RFC 2279, January 1998.


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6 Author's Address

.nf
Pekka Riikonen
Snellmaninkatu 34 A 15
70100 Kuopio
Finland

EMail: priikone@iki.fi

This Internet-Draft expires 25 April 2003