Secure Shell Working Group J. Galbraith
Internet-Draft VanDyke Software
Expires: September 23, 2005 O. Saarenmaa
F-Secure
March 22, 2005
X.509 authentication in SSH2
draft-ietf-secsh-x509-00.txt
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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. x509v3 keys . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 x509v3-sign-rsa2 . . . . . . . . . . . . . . . . . . . . . 4
2.2 x509v3-sign-dss2 . . . . . . . . . . . . . . . . . . . . . 4
2.3 x509v3-sign . . . . . . . . . . . . . . . . . . . . . . . 5
3. Implementation Considerations . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1 Normative References . . . . . . . . . . . . . . . . . . . 9
6.2 Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 11
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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.
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2. x509v3 keys
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
2.1 x509v3-sign-rsa2
Certificates that use the RSA public key algorithm SHOULD use the
"x509v3-sign-rsa2" 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-rsa2 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).
2.2 x509v3-sign-dss2
Certificates that use the DSA public key algorithm SHOULD use the
"x509v3-sign-dss2" 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-rsa2 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
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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).
2.3 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
certificate.
The signature, and description of the signature algorithms is encoded
as specificied in pkcs7 [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
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3. 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.
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4. IANA Considerations
This document reserves all key types beginning with "x509v3-sign" in
the SSH publickey type registery.
This document specifically adds "x509v3-sign-rsa2",
"x509v3-sign-dss2", 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.
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5. 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]
and [RFC3820].
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6. References
6.1 Normative References
[I-D.ietf-secsh-transport]
Lonvick, C., "SSH Transport Layer Protocol",
Internet-Draft draft-ietf-secsh-transport-24, March 2005.
[PKCS.7.1993]
RSA Laboratories, "Cryptographic Message Syntax Standard.
Version 1.5", PKCS 7, November 1993.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3820] Tuecke, S., Welch, V., Engert, D., Pearlman, L. and M.
Thompson, "Internet X.509 Public Key Infrastructure (PKI)
Proxy Certificate Profile", RFC 3820, June 2004.
[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.
[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.
6.2 Informative References
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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
A Street Address
A City, State or Region A Postal Code
FI
Email: oskari.saarenmaa@f-secure.com
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