Internet DRAFT - draft-weber-secsh-pkalg-none

draft-weber-secsh-pkalg-none










INTERNET DRAFT                                         Joel N. Weber II
                                                          June 20, 2003
                                              Expires December 20, 2003

               Secure Shell ``none'' Public Key Algorithm
                  draft-weber-secsh-pkalg-none-00.txt

   This document is an Internet-Draft and is subject to all the
   provisions of Section 10 of RFC2026.

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   The distribution of this memo is unlimited.  It is filed as draft-
   weber-secsh-pkalg-none-00.txt, and expires 20 December 2003.  Please
   send comments to the author or to ietf-ssh@netbsd.org


ABSTRACT

   This document describes the ``none'' public key encryption algorithm
   for the Secure Shell protocol, which is useful for rekeying when the
   server has no keys to support for non-GSSAPI key exchange and when
   GSSAPI credentials are expired, or for use in embedded systems where
   there is a desire to minimize the overhead of rekeying to prevent
   sequence number rollover.

INTRODUCTION

   The mechanisms currently defined for key exchange in the Secure Shell
   protocol all have the property of requiring host key verification.

   This is valuable at the begining of an ssh session, but key exchange
   is also used to rekey in the middle of a session to prevent problems
   with sequence number rollover.  Given that [SECSH] recommends



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   rekeying based upon the need to prevent sequence number rollover, and
   does not describe any other expected threats, it may be reasonable to
   assume that PFS within a session is of limited importance.

   [SECSHGSS] defines a family of key exchange methods which can be used
   to do key exchange in the Secure Shell protocol using GSSAPI.  Some
   GSSAPI mechanisms, such as Kerberos 5, use credentials which expire,
   and will fail if the credentials expire.  The convention used is
   often to allow connections to remain active indefinitely as long as
   the credentials were valid at the time the connection was initiated.
   It is possible for a host which uses a GSSAPI mechanism to not have a
   host key which is usable with the diffie-hellman-group1-sha1 key
   exchange method.  In such a case, one may want to be able to rekey
   the session without having any host key available.

   There is also some concern about rekeying potentially being
   computationally intensive for some embedded devices, and removing the
   need to make a public key signature may be worthwhile.

   This document defines the use of the ``none'' host key algorithm with
   the diffie-hellman-group1-sha1 and similar key exchange algorithms
   that support the same set of host key algorithms.

GENERATING AND VERIFYING SIGNATURES

   When using the ``none'' algorithm, the server public key data and
   signature data sent from the server to the client is zero length, and
   the client verifies these things by ensuring that they are zero
   length.

APPLICABILITY

   The ``none'' algorithm MUST NOT be used for initial key exchange.  It
   MAY be used when rekeying.

SECURITY CONSIDERATIONS

   When using ``none'' for rekeying, an attacker who has compromised the
   symetric encryption keys can perform a man in the middle attack
   against the key exchange, in which the attacker does key exchange
   with the client, and does key exchange with the server, rather than
   allowing the client to do key exchange with the server.  If an
   algorithm other than ``none'' is used, the client will notice that
   such an attack has happened.  However, such an attack is believed to
   be essentially impossible to carry out, due to the use of strong
   symmetric encryption algorithms.

AUTHOR'S ADDRESS



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   Joel N. Weber II
   185 Lowell St #2
   Somerville MA 02144-2629
   Email: weber@joelweber.com

NORMATIVE REFERENCES

   [SECSH] RFC-Editor: To be replaced by RFC number for draft-ietf-
   secsh-architecture

   [RFC2026] S. Bradner, RFC2026, BCP 9:  "The Internet Standard Process
   - Revision 3," October 1996, Obsoletes - RFC 1602, Status: Best
   Current Practice.

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

INFORMATIVE REFERENCES

   [SECSHGSS] RFC-Editor: To be replaced by RFC number for draft-ietf-
   secsh-gsskeyex

IPR NOTICES

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COPYRIGHT NOTICE

   Copyright (C) The Internet Society 2003. All Rights Reserved.




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