Secure Shell Working Group J. Galbraith
Internet-Draft VanDyke Software
Expires: January 16, 2006 O. Saarenmaa
F-Secure Corporation
July 15, 2005
X.509 authentication in SSH2
draft-ietf-secsh-x509-02.txt
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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. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Certificate validation . . . . . . . . . . . . . . . . . . . . 3
3.1 Host Authentication . . . . . . . . . . . . . . . . . . . 3
3.2 User Authentication . . . . . . . . . . . . . . . . . . . 3
4. Use in SSH2 Protocol . . . . . . . . . . . . . . . . . . . . . 4
4.1 x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . . . . . . 4
4.2 x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . . 4
4.3 x509v3-sign . . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation Considerations . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1 Normative References . . . . . . . . . . . . . . . . . . . 6
8.2 Informative References . . . . . . . . . . . . . . . . . . 7
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. 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 described in [RFC3280]. No
SSH specific X.509 certificate extensions are defined in this
document.
3.1 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 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 be
used. This is similar to host validation in [RFC2818].
3.2 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 account information
in the certificates used for user authentication. Their validation
and mapping is left as an implementation and configuration detail for
the implementors and deployers.
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4. 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
4.1 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).
4.2 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
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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).
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
certificate.
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 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.
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.
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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.
Implementations should carefully validate the certificate, including,
but not limited to, certificate expiration, certificate signature,
certificate revocation lists, etc.
For more information, implementors should refer to [ITU.X509.2000]
and [RFC3280].
8. References
8.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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[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.
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.
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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