Secure Shell Working Group                                  O. Saarenmaa
Internet-Draft                                                  F-Secure
Expires: September 1, 2006                                  J. Galbraith
                                                        VanDyke Software
Expires: January 16,
                                                       February 28, 2006                                   O. Saarenmaa
                                                    F-Secure Corporation
                                                           July 15, 2005

                      X.509 authentication in SSH2
                      draft-ietf-secsh-x509-02.txt SSH

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Copyright Notice

   Copyright (C) The Internet Society (2005). (2006).


   The X.509 extension

   This document specifies how X.509 keys certificates and signatures are
   used within the SSH2 protocol. Secure Shell protocol for user and server

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Certificate validation . . . . . . . . . . . . . . . . . . . .  3
     3.1   Host Authentication
     3.1.  Certificate Extensions . . . . . . . . . . . . . . . . . .  3
       3.1.1.  ExtendedKeyUsage . . . . . . . . . . . . . . . . . . .  3
     3.2.  Server Authentication  . . . . . . . . . . . . . . . . . .  4
     3.3.  User Authentication  . . . . . . . . . . . . . . . . . . .  3  4
   4.  Use in SSH2 SSH Protocol  . . . . . . . . . . . . . . . . . . . . .  4
     4.1   x509v3-sign-rsa-sha1
     4.1.  x509v3-sign  . . . . . . . . . . . . . . . . . . .  4
     4.2   x509v3-sign-dss-sha1 . . . .  4
     4.2.  x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . .  4
     4.3   x509v3-sign . . . .  5
     4.3.  x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . .  5  6
   5.  Implementation Considerations  . . . . . . . . . . . . . . . .  5  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5  6
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     8.1  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  6
     8.2  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  7  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  7 . .  9
   Intellectual Property and Copyright Statements . . . . . . . .  8 . . 10

1.  Introduction

   The SSH Secure Shell protocol can use public keys for both host server and
   user authentication.  However, particularly for host 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",
   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 described defined in [RFC3280].  No
   SSH specific  This
   document does not define any new X.509 certificate extensions are defined in extensions.

   Users deploying certificates have often had little control over the
   capabilities of CAs available to them.  Implementations of this

3.1  Host Authentication

   The client
   specification MAY verify that 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 serverAuth option, purposes of those
   checks as specified 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
   [RFC3280], another protocol or application, it is present in
   RECOMMENDED to also specify the host anyExtendedKeyUsage keyPurposeID

   Nevertheless, this document defines several ExtendedKeyUsage
   keyPurposeID that MAY be used to limit a certificate's extendedKeyUsage
   field. 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
         ::= { }
       id-kp-ssh-client OBJECT IDENTIFIER
         ::= { }
       id-kp-ssh-clientHostbased OBJECT IDENTIFIER
         ::= { }

3.2.  Server Authentication

   Implementations SHOULD MUST validate the server host certificates by
   matching the
   host's server's fully qualified domain name [RFC1034] against
   the host 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 is to MUST be
   used.  This is similar to host validation in HTTP Over TLS [RFC2818].


3.3.  User Authentication

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

   No constraints are placed on

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

4.  Use in SSH2 SSH Protocol

   Key type names

   This document defines three new key formats which are of 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:

       string    key-type-name
       string "x509v3-sign"
       uint32  number of certificates
       string[1..] DER encoded x.509v3 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
   ( [FIPS-180-2], SHA256 (2.16.840. [FIPS-
   180-2], MD5 (1.2.840.113549.2.5) [RFC1321] and RIPEMD160
   ( [RIPEMD-160].

4.2.  x509v3-sign-rsa-sha1

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

       string   "x509v3-sign-rsa-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-rsa" public keys in [I-D.ietf-secsh-transport].
   [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. hash [FIPS-180-2].

   The signature format for x509v3-sign-rsa-sha1 certificates is the
   "ssh-rsa" signing format specified in [I-D.ietf-secsh-transport]. [RFC4253].  This format is as

       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).


4.3.  x509v3-sign-dss-sha1

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

       string   "x509v3-sign-dss-sha1" key-type-name.
       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 [I-D.ietf-secsh-transport]. [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 [I-D.ietf-secsh-transport]. [RFC4253].  This format is as

       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).

4.3  x509v3-sign

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

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

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

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

       string    "pkcs7"
       string    DER encoded PKCS7 data

5.  Implementation Considerations

   Implementations should be careful when using x.509v3 X.509v3 certificates as
   hostkeys.  If the peer does not implement the required algorithms to
   validate both the x.509v3 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

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-
   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

   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, including
   but not limited to, certificate expiration, certificate signature,
   certification revocation lists, etc. 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, 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.


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

   [RFC4253]  Ylonen, T. and C. Lonvick, C., "SSH "The Secure Shell (SSH)
              Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

              RSA Laboratories, "Cryptographic Message Syntax Standard.
              Version 1.5", PKCS 7, November 1993. RFC 4253, January 2006.

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

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


              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

Authors' Addresses

   Oskari Saarenmaa
   Tammasaarenkatu 7
   PL 24
   Helsinki  00181


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

   Phone: +1 505 332 5700

   Oskari Saarenmaa
   F-Secure Corporation
   Tammasaarenkatu 7
   Helsinki  00180


Trademark notice

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

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