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Versions: 00 01

Network Working Group                                          D. Miller
Internet-Draft                                                P. Valchev
Intended status: Standards Track                                 OpenSSH
Expires: March 6, 2008                                 September 3, 2007


          The use of UMAC in the SSH Transport Layer Protocol
                     draft-miller-secsh-umac-01.txt

Status of this Memo

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   This Internet-Draft will expire on March 6, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo describes the use of the UMAC Message Authentication Code
   in the SSH transport protocol.









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Table of Contents

   1.  Requirements notation . . . . . . . . . . . . . . . . . . . . . 3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  MAC calculation . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  New MAC methods . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  umac-32 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.2.  umac-64 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.3.  umac-96 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.4.  umac-128  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6




































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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2.  Overview

   SSH [RFC4251] is a popular protocol for secure remote login and data
   transfer on the Internet.  Among the security properties that SSH
   offers is integrity of communication against active adversaries.
   This integrity is provided in the SSH transport protocol [RFC4253]
   through the use of a per-packet Message Authentication Code (MAC).

   This memo describes the use of the UMAC Message Authentication Code
   [RFC4418] in the SSH transport protocol.  UMAC offers improved
   performance over the current HMAC-based MACs supported by SSH.
   Furthermore, UMAC represents a different cryptographic approach to
   message authentication to that of HMAC, and its use in SSH would
   provide a diversity that may be of benefit if HMAC or one of its
   underlying hash algorithms is found to be vulnerable to a new attack.

   UMAC offers a choice of four authentication tag sizes: 32, 64, 96 and
   128 bits.  This allows users to trade between fast, compact tags and
   longer tags that are slower to generate but offer better security.
   These four tag sizes are represented in the new MAC methods
   introduced in this memo: "umac-32", "umac-64", "umac-96" and "umac-
   128".


3.  MAC calculation

   Message authentication tags in the SSH transport protocol are
   specified to be calculated over the packet sequence number followed
   by the entire unencrypted packet:

           mac = MAC(key, sequence_number || unencrypted_packet)

   However, UMAC accepts a nonce as an explicit input to MAC
   calculation.  In order for the security proofs that accompany UMAC to
   apply, this nonce must never be repeated under a given key.  In the
   context of the SSH protocol, this may be achieved by using the packet
   sequence number as the nonce.  The sequence number is guaranteed to
   be unique over the life of the MAC key by the requirement that
   implementations perform a key re-exchange before the sequence number
   wraps (section 9.3.3 of [RFC4251]).




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   To use the sequence number as the MAC nonce, it is encoded into a SSH
   protocol uint64 (as described in section 5 of [RFC4251]) and it is
   supplied to the UMAC algorithm instead of prepending it to the packet
   to be authenticated:

           mac = UMAC(key, sequence_number, unencrypted_packet)

   Where UMAC is one of the UMAC-32, UMAC-64, UMAC-96 or UMAC-128
   algorithms specified in [RFC4418], section 4.2, using the default
   underlying block cipher of AES-128.


4.  New MAC methods

   This memo introduces four new MAC methods, one for each UMAC
   authentication tag length specified in [RFC4418]: "umac-32",
   "umac-64", "umac-96" and "umac-128".

4.1.  umac-32

   Calculate message authentication tags using the UMAC-32 algorithm
   specified in [RFC4418], section 4.2.

4.2.  umac-64

   Calculate message authentication tags using the UMAC-64 algorithm
   specified in [RFC4418], section 4.2.

4.3.  umac-96

   Calculate message authentication tags using the UMAC-96 algorithm
   specified in [RFC4418], section 4.2.

4.4.  umac-128

   Calculate message authentication tags using the UMAC-128 algorithm
   specified in [RFC4418], section 4.2.


5.  Security Considerations

   As mentioned above, it is imperative that implementations perform a
   re-keying operation before reusing a packet sequence number as a UMAC
   nonce.  Implementations conforming to [RFC4251] are already required
   to do this.






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6.  IANA Considerations

   IANA must assign the MAC Algorithm Names "umac-32", "umac-64",
   "umac-96" and "umac-128".


7.  Normative References

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

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4253]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

   [RFC4418]  Krovetz, T., "UMAC: Message Authentication Code using
              Universal Hashing", RFC 4418, March 2006.


Authors' Addresses

   Damien Miller
   OpenSSH

   Email: djm@openssh.com


   Peter Valchev
   OpenSSH

   Email: pvalchev@openssh.com


















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   Administrative Support Activity (IASA).





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