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Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: October 21, 2005                                   O. Saarenmaa
                                                    F-Secure Corporation
                                                          April 19, 2005


                      X.509 authentication in SSH2
                      draft-ietf-secsh-x509-01.txt

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The X.509 extension specifies how X.509 keys and signatures are used
   within the SSH2 protocol.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Certificate validation . . . . . . . . . . . . . . . . . . . .  3
     2.1   Host Authentication  . . . . . . . . . . . . . . . . . . .  3
     2.2   User Authentication  . . . . . . . . . . . . . . . . . . .  3
   3.  Use in SSH2 Protocol . . . . . . . . . . . . . . . . . . . . .  3
     3.1   x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . . . . . .  4
     3.2   x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . .  4
     3.3   x509v3-sign  . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Implementation Considerations  . . . . . . . . . . . . . . . .  5
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     7.1   Normative References . . . . . . . . . . . . . . . . . . .  6
     7.2   Informative References . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

































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1.  Introduction

   The SSH protocol can use public keys for both host and user
   authentication.  However, particularly for host authentication, plain
   public keys lack a good method of verifying that the the key provided
   really does belong to the host asserting ownership.  X.509v3
   certificates can address this problem in environments where a PKI
   infrastructure is available.

2.  Certificate validation

   Implementations are expected to follow the basic certificate and
   certificate path validation guidelines described in [RFC3280].  No
   SSH specific X.509 certificate extensions are defined in this
   document.

2.1  Host Authentication

   The client MAY verify that the serverAuth option, as specified in
   [RFC3280], is present in the host certificate's extendedKeyUsage
   field.

   Implementations SHOULD validate the host certificates by matching the
   host's fully qualified domain name [RFC1034] against the host
   certificate's subjectAltName extensions's dNSName entries.  If the
   certificate does not contain dNSName subjectAltName extensions, the
   (most specific) Common Name field in the certificate Subject is to be
   used.  This is similiar to host validation in [RFC2818].

2.2  User Authentication

   The server MAY verify that the clientAuth option, as specified in
   [RFC3280], is present in the user certificate's extendedKeyUsage
   field.

   No constraints are placed on the presence of user accounts in the
   certificates used for user authentication.  Their validation is left
   as an implementation and configuration detail for the implementors
   and deployers.

3.  Use in SSH2 Protocol

   Key type names are of the form "x509v3-sign*".  Keys are encoded as
   follows:

       string    key-type-name
       string    DER encoded x.509v3 certificate data




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3.1  x509v3-sign-rsa-sha1

   Certificates that use the RSA public key algorithm SHOULD use the
   "x509v3-sign-rsa-sha1" key-type-name.

   Signing and verifying using this key format, uses the certificate's
   private key, in exactly the same manner specified for "ssh-rsa"
   public keys in [I-D.ietf-secsh-transport].  That is to say, signing
   and verifying using this key format is performed according to the
   RSASSA-PKCS1-v1_5 scheme in [RFC3447] using the SHA-1 hash.

   The signature format for x509v3-sign-rsa-sha1 certificates is the
   "ssh-rsa" signing format specified in [I-D.ietf-secsh-transport].
   This format is as follows:

       string    "ssh-rsa"
       string    rsa_signature_blob

   The value for 'rsa_signature_blob' is encoded as a string containing
   s (which is an integer, without lengths or padding, unsigned and in
   network byte order).

3.2  x509v3-sign-dss-sha1

   Certificates that use the DSA public key algorithm SHOULD use the
   "x509v3-sign-dss-sha1" key-type-name.

   Signing and verifying using this key format, uses the certificate's
   private key, in exactly the same manner specified for "ssh-dss"
   public keys in [I-D.ietf-secsh-transport].  That is to say, signing
   and verifying using this key format is done according to the Digital
   Signature Standard [FIPS-186-2] using the SHA-1 hash [FIPS-180-2].

   The signature format for x509v3-sign-dss-sha1 certificates is the
   "ssh-dss" signing format specified in [I-D.ietf-secsh-transport].
   This format is as follows:

       string    "ssh-dss"
       string    dss_signature_blob

   The value for 'dss_signature_blob' is encoded as a string containing
   r followed by s (which are 160-bit integers, without lengths or
   padding, unsigned and in network byte order).

3.3  x509v3-sign

   Certificates that use another algorithm other than the two specified
   above, MUST use the "x509v3-sign" key-type-name.



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   Signing and verifying is done according to the specification
   associated with the public-key algorithm oid encoded in the
   certificate.

   The signature, and description of the signature algorithms is encoded
   as specificied in [PKCS.7.1993].  The signature MUST be detached (the
   signed data MUST NOT be includeded in the pkcs7 data).

   The pkcs7 data is encoded in the SSH protocol as follows:

       string    "pkcs7"
       string    DER encoded PKCS7 data


4.  Implementation Considerations

   Implemenations should be careful when using x.509v3 certificates as
   hostkeys.  If the peer does not implement the required algorithms to
   validate both the x.509v3 certificate and all certificates in the
   chain, it MUST disconnect.  There is no way to renegotiate the key
   during key exchange.

   This is especially true when using the "x509v3-sign" key type, since
   in this case the peer has no knowledge whatsoever of required
   algorithms.

5.  IANA Considerations

   This document reserves all key types beginning with "x509v3-sign" in
   the SSH publickey type registery.

   This document specifically adds "x509v3-sign-rsa-sha1", "x509v3-sign-
   dss-sha1", and "x509v3-sign" to the SSH publickey type registry.

   This document adds "x509v3-sign-rsa" and "x509v3-sign-dss" to the SSH
   publickey type registry as "poisoned" by historical use.

6.  Security Considerations

   PKI is an extremely complex topic, and care must be taken by both
   implementors and deployers to understand the complex interactions
   involved.

   Implementations should carefully validate the certificate, including,
   but not limitted to, certificate expiration, certificate signature,
   certificate revokation lists, etc.

   For more information, implementors should refer to [ITU.X509.2000]



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   and [RFC3280].

7.  References

7.1  Normative References

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

   [PKCS.7.1993]
              RSA Laboratories, "Cryptographic Message Syntax Standard.
              Version 1.5", PKCS 7, November 1993.

   [FIPS-180-2]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", Federal Information Processing
              Standards Publication 180-2, August 2002.

   [FIPS-186-2]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", Federal Information Processing
              Standards Publication 186-2, January 2000.

   [ITU.X509.2000]
              International Telecommunications Union, "Information
              technology - Open Systems Interconnection - The Directory:
              Public-key and attribute certificate frameworks", ITU-
              T Recommendation X.509, ISO Standard 9594-8, March 2000.

7.2  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.





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Authors' Addresses

   Joseph Galbraith
   VanDyke Software
   4848 Tramway Ridge Blvd
   Suite 101
   Albuquerque, NM  87111
   US

   Phone: +1 505 332 5700
   Email: galb-list@vandyke.com


   Oskari Saarenmaa
   F-Secure Corporation
   Tammasaarenkatu 7
   Helsinki  00180
   FI

   Email: oskari.saarenmaa@f-secure.com

Trademark notice

   "ssh" is a registered trademark in the United States and/or other
   countries.


























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