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Secure Shell Working Group                                  O. Saarenmaa
Internet-Draft                                                  F-Secure
Expires: September 1, 2006                                  J. Galbraith
                                                        VanDyke Software
                                                       February 28, 2006


                      X.509 authentication in SSH
                      draft-ietf-secsh-x509-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies how X.509 certificates and signatures are
   used within the Secure Shell protocol for user and server
   authentication.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Certificate validation . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Certificate Extensions . . . . . . . . . . . . . . . . . .  3
       3.1.1.  ExtendedKeyUsage . . . . . . . . . . . . . . . . . . .  3
     3.2.  Server Authentication  . . . . . . . . . . . . . . . . . .  4
     3.3.  User Authentication  . . . . . . . . . . . . . . . . . . .  4
   4.  Use in SSH Protocol  . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  x509v3-sign  . . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . . . . . .  5
     4.3.  x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . .  6
   5.  Implementation Considerations  . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10






























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1.  Introduction

   The Secure Shell protocol can use public keys for both server and
   user authentication.  However, particularly for server
   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.  Conventions Used in This Document

   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].


3.  Certificate validation

   Implementations are expected to follow the basic certificate and
   certificate path validation guidelines defined in [RFC3280].  This
   document does not define any new X.509 certificate extensions.

   Users deploying certificates have often had little control over the
   capabilities of CAs available to them.  Implementations of this
   specification MAY include configuration knobs to disable checks
   required by this specification in order to permit use with inflexible
   and/or noncompliant CAs.  Before disabling any checks the
   administrators and users need to understand the purposes of those
   checks as well as the security implications that may raise when they
   are disabled.

3.1.  Certificate Extensions

   Implementations MUST recognize the following extensions:
   BasicConstraints, KeyUsage, and SubjectAltName.  Implementations also
   MUST be able to handle all other extensions that have been marked
   critical or reject the certificate.

3.1.1.  ExtendedKeyUsage

   Certificates meant for use within the SSH protocol SHOULD NOT include
   the ExtendedKeyUsage extension.  If the certificates require an EKU
   extension because of use in another protocol or application, it is
   RECOMMENDED to also specify the anyExtendedKeyUsage keyPurposeID
   [RFC3280].

   Nevertheless, this document defines several ExtendedKeyUsage



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   keyPurposeID that MAY be used to limit a certificate's use.  These
   are id-kp-ssh-server, for use in server certificates, id-kp-ssh-
   client for use in client (user) certificates, and id-kp-ssh-
   clientHostbased for use in server certificates that can be used with
   hostbased authentication [RFC4252].  The object identifiers are
   listed below:

       id-kp-ssh-server OBJECT IDENTIFIER
         ::= { 1.3.6.1.4.1.2213.15.1.1 }
       id-kp-ssh-client OBJECT IDENTIFIER
         ::= { 1.3.6.1.4.1.2213.15.1.2 }
       id-kp-ssh-clientHostbased OBJECT IDENTIFIER
         ::= { 1.3.6.1.4.1.2213.15.1.3 }

3.2.  Server Authentication

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

3.3.  User Authentication

   No constraints are placed on the presence of user account information
   in the certificates used for user authentication.  The mapping of
   user certificates to user accounts is left as an implementation
   choice and configuration issue for the implementors and deployers.


4.  Use in SSH Protocol

   This document defines three new key formats which are in the form
   "x509v3-sign*".  Each of the formats encodes the key type name in the
   beginning of the key blob.

4.1.  x509v3-sign

   This is the most flexible key and signature format defined by the
   document.  It is RECOMMENDED that implementations prefer this
   algorithm over the two other x509v3-sign* algorithms that this
   document defines and may be supported.  This format supports multiple
   certificates in a chain as well as including OCSP-responses [RFC2560]
   along with the certificate data.  It also supports multiple different
   hash algorithms for signatures.  Keys using this format are encoded
   as follows:




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       string "x509v3-sign"
       uint32  number of certificates
       string[1..] DER encoded X.509v3 certificate data
       uint32  number of ocsp responses
       string[0..] OCSP response data

   The first certificate in the list MUST be the end-entity one, and any
   other certificates MUST be part of the end-entity certificate's path.

   Signatures are encoded as follows:

       string "x509v3-sign"
       string hash algorithm OID
       string signature data

   Possible hash algorithms include, but are not limited to, SHA1
   (1.3.14.3.2.26) [FIPS-180-2], SHA256 (2.16.840.1.101.3.4.2.1) [FIPS-
   180-2], MD5 (1.2.840.113549.2.5) [RFC1321] and RIPEMD160
   (1.3.36.3.2.1) [RIPEMD-160].

4.2.  x509v3-sign-rsa-sha1

   Certificates that use the RSA public key algorithm MAY use the
   "x509v3-sign-rsa-sha1" key format.  This key type uses the following
   format:

       string   "x509v3-sign-rsa-sha1"
       string   DER encoded X.509v3 certificate data

   Signing using this key format, uses the certificate's private key, in
   exactly the same manner specified for "ssh-rsa" public keys in
   [RFC4253].  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 [FIPS-180-2].

   The signature format for x509v3-sign-rsa-sha1 certificates is the
   "ssh-rsa" signing format specified in [RFC4253].  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).






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4.3.  x509v3-sign-dss-sha1

   Certificates that use the DSA public key algorithm MAY use the
   "x509v3-sign-rsa-sha1" key format.  This key type uses the following
   format:

       string   "x509v3-sign-dss-sha1"
       string   DER encoded X.509v3 certificate data

   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 [RFC4253].  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 [RFC4253].  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).


5.  Implementation Considerations

   Implementations should be careful when using X.509v3 certificates as
   hostkeys.  If the peer does not implement the required algorithms to
   validate both the end-entity 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.  The peer might also refuse a "x509v3-sign" key if the
   required intermediate certificates and OCSP responses are not
   included.


6.  IANA Considerations

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

   This document specifically adds "x509v3-sign-rsa-sha1", "x509v3-sign-



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   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.


7.  Security Considerations

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

   This document suggests that validation of the ExtendedKeyUsage
   extension MAY be disabled by configuration in the implementations.
   Disabling validation of other extensions such as KeyUsage or
   BasicConstraints MUST NOT be done, as that might lead into invalid
   trust paths being established.

   Implementations should carefully validate the certificate, including
   but not limited to, certificate expiration, certificate signature,
   certification revocation status etcetera.  Implementations must also
   be careful to validate all these properties of all certificates in
   the path leading to a trust anchor.  For more information
   implementors should refer to [ITU.X509.2000] and [RFC3280].


8.  References

8.1.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

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

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [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.



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   [RFC4252]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

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

   [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.

   [RIPEMD-160]
              Dobbertin, H., Bosselaers, A., and B. Preneel, "RIPEMD-
              160: A Strengthened Version of RIPEMD", April 1996.

8.2.  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.

Trademark notice

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














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Authors' Addresses

   Oskari Saarenmaa
   F-Secure
   Tammasaarenkatu 7
   PL 24
   Helsinki  00181
   FI

   Email: oskari.saarenmaa@f-secure.com


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

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






























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