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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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