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[silc.git] / doc / draft-riikonen-silc-ke-auth-05.nroff

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Network Working Group                                        P. Riikonen
Internet-Draft
draft-riikonen-silc-ke-auth-05.txt                           15 May 2002
Expires: 15 November 2002

.in 3

.ce 2
SILC Key Exchange and Authentication Protocols
<draft-riikonen-silc-ke-auth-05.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 internet-draft [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 uses best parts
of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol
and the OAKLEY Key Determination protocol [OAKLEY].

The 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).



.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 .................................... 10
  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 ............................... 14
  2.5 Key Exchange Status Types ................................. 15
3 SILC Connection Authentication Protocol ....................... 16
  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 .................... 20
4 Security Considerations ....................................... 21
5 References .................................................... 21
6 Author's Address .............................................. 22


.ti 0
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 Internet-Draft [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 uses best parts
of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol
and the OAKLEY Key Determination protocol.

The 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).

The basis of secure SILC session requires strong and secure key exchange
protocol and authentication.  The authentication protocol is entirely
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 refers constantly to other SILC protocol specification
Internet Drafts that are a must read for those who wants to understand
the function of these protocols.  The most important references are
the Secure Internet Live Conferencing, Protocol Specification [SILC1]
and the SILC Packet Protocol [SILC2] Internet Drafts.

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].


.ti 0
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 uses
Diffie-Hellman key exchange algorithm and its functionality is derived
from several key exchange protocols.  SKE uses 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 merely 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 like commands, command replies and other
communication between two entities.  If connection is server to router
connection, the keys are used to protect all traffic between those
servers.  In client connections usually all the packets are protected
with this key except channel messages; channels has their own keys and 
they are not exchanged with this protocol.

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


.ti 0
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. The same is done with these payloads as well.  All the
fields in the payloads are always in MSB (most significant byte first)
order.  Following descriptions of these payloads.


.ti 0
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].

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 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.

     No Reply                 0x01

       If set the receiver of the payload does not reply to 
       the packet.

     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.

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 of the version
  string format.

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.

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.

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.

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.

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 Codes (MAC)
  of the SILC packets.

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.
.in 3


.ti 0
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 public key authentication (it
is based on passphrase) 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
SILC Key Exchange 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
  SSH 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.  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.  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
  MUST always provide this field.
.in 3


.ti 0
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(Key Exchange Start Payload | public key
        (or certificate) | e).  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(Key Exchange Start Payload data | 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(0 | KEY | HASH)
Receiving Initial Vector (IV)   = hash(1 | KEY | HASH)
Sending Encryption Key          = hash(2 | KEY | HASH)
Receiving Encryption Key        = hash(3 | KEY | HASH)
Sending HMAC Key                = hash(4 | KEY | HASH)
Receiving HMAC Key              = hash(5 | 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(2 | KEY | HASH)
K2 = hash(KEY | HASH | K1)
K3 = hash(KEY | HASH | K1 | K2)  ...

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


K1 = hash(3 | 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.

This same procedure is used in the SILC in some other circumstances
as well.  Any changes to this procedure is mentioned 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 decimal value is

.in 6
179769313486231590770839156793787453197860296048756011706444
423684197180216158519368947833795864925541502180565485980503
646440548199239100050792877003355816639229553136239076508735
759914822574862575007425302077447712589550957937778424442426
617334727629299387668709205606050270810842907692932019128194
467627007
.in 3

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 decimal value is

.in 6
241031242692103258855207602219756607485695054850245994265411
694195810883168261222889009385826134161467322714147790401219
650364895705058263194273070680500922306273474534107340669624
601458936165977404102716924945320037872943417032584377865919
814376319377685986952408894019557734611984354530154704374720
774996976375008430892633929555996888245787241299381012913029
459299994792636526405928464720973038494721168143446471443848
8520940127459844288859336526896320919633919
.in 3

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.


.ti 0
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


.ti 0
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 uses this information to create the ID for the
connection.

After the authentication protocol has been successfully completed
SILC_PACKET_NEW_ID must be sent to the connecting client by the server.
See the [SILC1] for the details of the connecting procedure.

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 everything checks out fine 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 at all.  However, in this case the protocol still MUST be
executed; the authentication data just is empty indicating no
authentication is required.

If authentication method is passphrase the authentication data is
plaintext passphrase.  As the payload is entirely 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 checker, and hash 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 signature of the hash value of hash HASH plus 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.

The connecting client of this protocol MUST wait after successful execution
of this protocol for the SILC_PACKET_NEW_ID packet where it will receive
the ID it will be using in the SILC network.  The connecting client cannot
start normal SILC session (sending messages or commands) until it has
received its ID.  The ID's are always created by the server except
for server to router connection where servers create their own ID's.


.ti 0
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




.ti 0
3.2 Connection Authentication Types

SILC supports two authentication types to be used in the connection
authentication protocol; passphrase or public key based authentication.
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
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.

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.



.ti 0
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.


.ti 0
6 Author's Address

.nf
Pekka Riikonen
Snellmaninkatu 34 A 15
70100 Kuopio
Finland

EMail: priikone@iki.fi

This Internet-Draft expires 15 November 2002