<- RFC Index (6101..6200)
RFC 6187
Internet Engineering Task Force (IETF) K. Igoe
Request for Comments: 6187 National Security Agency
Category: Standards Track D. Stebila
ISSN: 2070-1721 Queensland University of Technology
March 2011
X.509v3 Certificates for Secure Shell Authentication
Abstract
X.509 public key certificates use a signature by a trusted
certification authority to bind a given public key to a given digital
identity. This document specifies how to use X.509 version 3 public
key certificates in public key algorithms in the Secure Shell
protocol.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6187.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Igoe & Stebila Standards Track [Page 1]
RFC 6187 X.509v3 Certificates for SSH March 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Public Key Algorithms Using X.509 Version 3 Certificates . . . 4
2.1. Public Key Format . . . . . . . . . . . . . . . . . . . . 4
2.2. Certificate Extensions . . . . . . . . . . . . . . . . . . 6
2.2.1. KeyUsage . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2. ExtendedKeyUsage . . . . . . . . . . . . . . . . . . . 7
3. Signature Encoding . . . . . . . . . . . . . . . . . . . . . . 8
3.1. x509v3-ssh-dss . . . . . . . . . . . . . . . . . . . . . . 8
3.2. x509v3-ssh-rsa . . . . . . . . . . . . . . . . . . . . . . 8
3.3. x509v3-rsa2048-sha256 . . . . . . . . . . . . . . . . . . 9
3.4. x509v3-ecdsa-sha2-* . . . . . . . . . . . . . . . . . . . 9
4. Use in Public Key Algorithms . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15
1. Introduction
There are two Secure Shell (SSH) protocols that use public key
cryptography for authentication. The Transport Layer Protocol,
described in [RFC4253], requires that a digital signature algorithm
(called the "public key algorithm") MUST be used to authenticate the
server to the client. Additionally, the User Authentication Protocol
described in [RFC4252] allows for the use of a digital signature to
authenticate the client to the server ("publickey" authentication).
In both cases, the validity of the authentication depends upon the
strength of the linkage between the public signing key and the
identity of the signer. Digital certificates, such as those in X.509
version 3 (X.509v3) format [RFC5280], are used in many corporate and
government environments to provide identity management. They use a
chain of signatures by a trusted root certification authority and its
intermediate certificate authorities to bind a given public signing
key to a given digital identity.
Igoe & Stebila Standards Track [Page 2]
RFC 6187 X.509v3 Certificates for SSH March 2011
The following public key authentication algorithms are currently
available for use in SSH:
+--------------+-----------+
| Algorithm | Reference |
+--------------+-----------+
| ssh-dss | [RFC4253] |
| | |
| ssh-rsa | [RFC4253] |
| | |
| pgp-sign-dss | [RFC4253] |
| | |
| pgp-sign-rsa | [RFC4253] |
| | |
| ecdsa-sha2-* | [RFC5656] |
+--------------+-----------+
Since Pretty Good Privacy (PGP) has its own method for binding a
public key to a digital identity, this document focuses solely upon
the non-PGP methods. In particular, this document defines the
following public key algorithms, which differ from the above solely
in their use of X.509v3 certificates to convey the signer's public
key.
+-----------------------+
| Algorithm |
+-----------------------+
| x509v3-ssh-dss |
| |
| x509v3-ssh-rsa |
| |
| x509v3-rsa2048-sha256 |
| |
| x509v3-ecdsa-sha2-* |
+-----------------------+
Public keys conveyed using the x509v3-ecdsa-sha2-* public key
algorithms can be used with the ecmqv-sha2 key exchange method.
Implementation of this specification requires familiarity with the
Secure Shell protocol [RFC4251] [RFC4253] and X.509v3 certificates
[RFC5280]. Data types used in describing protocol messages are
defined in Section 5 of [RFC4251].
This document is concerned with SSH implementation details;
specification of the underlying cryptographic algorithms and the
handling and structure of X.509v3 certificates is left to other
Igoe & Stebila Standards Track [Page 3]
RFC 6187 X.509v3 Certificates for SSH March 2011
standards documents, particularly [RFC3447], [FIPS-186-3],
[FIPS-180-2], [FIPS-180-3], [SEC1], and [RFC5280].
An earlier proposal for the use of X.509v3 certificates in the Secure
Shell protocol was introduced by O. Saarenmaa and J. Galbraith; while
this document is informed in part by that earlier proposal, it does
not maintain strict compatibility.
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 RFC 2119 [RFC2119].
2. Public Key Algorithms Using X.509 Version 3 Certificates
This document defines the following new public key algorithms for use
in the Secure Shell protocol: x509v3-ssh-dss, x509v3-ssh-rsa,
x509v3-rsa2048-sha256, and the family of algorithms given by
x509v3-ecdsa-sha2-*. In these algorithms, a public key is stored in
an X.509v3 certificate. This certificate, a chain of certificates
leading to a trusted certificate authority, and optional messages
giving the revocation status of the certificates are sent as the
public key data in the Secure Shell protocol according to the format
in this section.
2.1. Public Key Format
The reader is referred to [RFC5280] for a general description of
X.509 version 3 certificates. For the purposes of this document, it
suffices to know that in X.509 a chain or sequence of certificates
(possibly of length one) allows a trusted root certificate authority
and its intermediate certificate authorities to cryptographically
bind a given public key to a given digital identity using public key
signatures.
For all of the public key algorithms specified in this document, the
key format consists of a sequence of one or more X.509v3 certificates
followed by a sequence of 0 or more Online Certificate Status
Protocol (OCSP) responses as in Section 4.2 of [RFC2560]. Providing
OCSP responses directly in this data structure can reduce the number
of communication rounds required (saving the implementation from
needing to perform OCSP checking out-of-band) and can also allow a
client outside of a private network to receive OCSP responses from a
server behind a firewall. As with any use of OCSP data,
implementations SHOULD check that the production time of the OCSP
response is acceptable. It is RECOMMENDED, but not REQUIRED, that
implementations reject certificates for which the certificate status
is revoked.
Igoe & Stebila Standards Track [Page 4]
RFC 6187 X.509v3 Certificates for SSH March 2011
The key format has the following specific encoding:
string "x509v3-ssh-dss" / "x509v3-ssh-rsa" /
"x509v3-rsa2048-sha256" / "x509v3-ecdsa-sha2-[identifier]"
uint32 certificate-count
string certificate[1..certificate-count]
uint32 ocsp-response-count
string ocsp-response[0..ocsp-response-count]
In the figure above, the string [identifier] is the identifier of the
elliptic curve domain parameters. The format of this string is
specified in Section 6.1 of [RFC5656]. Information on the REQUIRED
and RECOMMENDED sets of elliptic curve domain parameters for use with
this algorithm can be found in Section 10 of [RFC5656].
Each certificate and ocsp-response MUST be encoded as a string of
octets using the Distinguished Encoding Rules (DER) encoding of
Abstract Syntax Notation One (ASN.1) [ASN1]. An example of an SSH
key exchange involving one of these public key algorithms is given in
Appendix A.
Additionally, the following constraints apply:
o The sender's certificate MUST be the first certificate and the
public key conveyed by this certificate MUST be consistent with
the public key algorithm being employed to authenticate the
sender.
o Each following certificate MUST certify the one preceding it.
o The self-signed certificate specifying the root authority MAY be
omitted. All other intermediate certificates in the chain leading
to a root authority MUST be included.
o To improve the chances that a peer can verify certificate chains
and OCSP responses, individual certificates and OCSP responses
SHOULD be signed using only signature algorithms corresponding to
public key algorithms supported by the peer, as indicated in the
server_host_key_algorithms field of the SSH_MSG_KEXINIT packet
(see Section 7.1 of [RFC4253]). However, other algorithms MAY be
used. The choice of signature algorithm used by any given
certificate or OCSP response is independent of the signature
algorithms chosen by other elements in the chain.
o Verifiers MUST be prepared to receive certificate chains and OCSP
responses that use algorithms not listed in the
server_host_key_algorithms field of the SSH_MSG_KEXINIT packet,
including algorithms that potentially have no Secure Shell
Igoe & Stebila Standards Track [Page 5]
RFC 6187 X.509v3 Certificates for SSH March 2011
equivalent. However, peers sending such chains should recognize
that such chains are more likely to be unverifiable than chains
that use only algorithms listed in the server_host_key_algorithms
field.
o There is no requirement on the ordering of OCSP responses. The
number of OCSP responses MUST NOT exceed the number of
certificates.
Upon receipt of a certificate chain, implementations MUST verify the
certificate chain according to Section 6.1 of [RFC5280] based on a
root of trust configured by the system administrator or user.
Issues associated with the use of certificates (such as expiration of
certificates and revocation of compromised certificates) are
addressed in [RFC5280] and are outside the scope of this document.
However, compliant implementations MUST comply with [RFC5280].
Implementations providing and processing OCSP responses MUST comply
with [RFC2560].
When no OCSP responses are provided, it is up to the implementation
and system administrator to decide whether or not to accept the
certificate. It may be possible for the implementation to retrieve
OCSP responses based on the id-ad-ocsp access description in the
certificate's Authority Information Access data (Section 4.2.2.1 of
[RFC5280]). However, if the id-ad-ocsp access description indicates
that the certificate authority employs OCSP, and no OCSP response
information is available, it is RECOMMENDED that the certificate be
rejected.
[RFC5480] and [RFC5758] describe the structure of X.509v3
certificates to be used with Elliptic Curve Digital Signature
Algorithm (ECDSA) public keys. [RFC3279] and [RFC5280] describe the
structure of X.509v3 certificates to be used with RSA and Digital
Signature Algorithm (DSA) public keys. [RFC5759] provides additional
guidance for ECDSA keys in Suite B X.509v3 certificate and
certificate revocation list profiles.
2.2. Certificate Extensions
Certificate extensions allow for the specification of additional
attributes associated with a public key in an X.509v3 certificate
(see Section 4.2 of [RFC5280]). The KeyUsage and ExtendedKeyUsage
extensions may be used to restrict the use of X.509v3 certificates in
the context of the Secure Shell protocol as specified in the
following sections.
Igoe & Stebila Standards Track [Page 6]
RFC 6187 X.509v3 Certificates for SSH March 2011
2.2.1. KeyUsage
The KeyUsage extension MAY be used to restrict a certificate's use.
In accordance with Section 4.2.1.3 of [RFC5280], if the KeyUsage
extension is present, then the certificate MUST be used only for one
of the purposes indicated. There are two relevant keyUsage
identifiers for the certificate corresponding to the public key
algorithm in use:
o If the KeyUsage extension is present in a certificate for the
x509v3-ssh-dss, x509v3-ssh-rsa, x509v3-rsa2048-sha256, or x509v3-
ecdsa-sha2-* public key algorithms, then the digitalSignature bit
MUST be set.
o If the KeyUsage extension is present in a certificate for the
ecmqv-sha2 key exchange method, then the keyAgreement bit MUST be
set.
For the remaining certificates in the certificate chain,
implementations MUST comply with existing conventions on KeyUsage
identifiers and certificates as in Section 4.2.1.3 of [RFC5280].
2.2.2. ExtendedKeyUsage
This document defines two ExtendedKeyUsage key purpose IDs that MAY
be used to restrict a certificate's use: id-kp-secureShellClient,
which indicates that the key can be used for a Secure Shell client,
and id-kp-secureShellServer, which indicates that the key can be used
for a Secure Shell server. In accordance with Section 4.2.1.12 of
[RFC5280], if the ExtendedKeyUsage extension is present, then the
certificate MUST be used only for one of the purposes indicated. The
object identifiers of the two key purpose IDs defined in this
document are as follows:
o id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
o id-kp OBJECT IDENTIFIER ::= { id-pkix 3 } -- extended key purpose
identifiers
o id-kp-secureShellClient OBJECT IDENTIFIER ::= { id-kp 21 }
o id-kp-secureShellServer OBJECT IDENTIFIER ::= { id-kp 22 }
Igoe & Stebila Standards Track [Page 7]
RFC 6187 X.509v3 Certificates for SSH March 2011
3. Signature Encoding
Signing and verifying using the X.509v3-based public key algorithms
specified in this document (x509v3-ssh-dss, x509v3-ssh-rsa,
x509v3-ecdsa-sha2-*) is done in the analogous way for the
corresponding non-X.509v3-based public key algorithms (ssh-dss,
ssh-rsa, ecdsa-sha2-*, respectively); the x509v3-rsa2048-sha256
public key algorithm provides a new mechanism, similar to ssh-rsa,
but has a different hash function and additional key size
constraints. For concreteness, we specify this explicitly below.
3.1. x509v3-ssh-dss
Signing and verifying using the x509v3-ssh-dss key format is done
according to the Digital Signature Standard [FIPS-186-3] using the
SHA-1 hash [FIPS-180-2].
The resulting signature is encoded 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 fixed-length 160-bit integers, without
lengths or padding, unsigned, and in network byte order).
This format is the same as for ssh-dss signatures in Section 6.6 of
[RFC4253].
3.2. x509v3-ssh-rsa
Signing and verifying using the x509v3-ssh-rsa key format is
performed according to the RSASSA-PKCS1-v1_5 scheme in [RFC3447]
using the SHA-1 hash [FIPS-180-2].
The resulting signature is encoded 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).
This format is the same as for ssh-rsa signatures in Section 6.6 of
[RFC4253].
Igoe & Stebila Standards Track [Page 8]
RFC 6187 X.509v3 Certificates for SSH March 2011
3.3. x509v3-rsa2048-sha256
Signing and verifying using the x509v3-rsa2048-sha256 key format is
performed according to the RSASSA-PKCS1-v1_5 scheme in [RFC3447]
using the SHA-256 hash [FIPS-180-3]; RSA keys conveyed using this
format MUST have a modulus of at least 2048 bits.
The resulting signature is encoded as follows:
string "rsa2048-sha256"
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).
Unlike the other public key formats specified in this document, the
x509v3-rsa2048-sha256 public key format does not correspond to any
previously existing SSH non-certificate public key format. The main
purpose of introducing this public key format is to provide an RSA-
based public key format that is compatible with current
recommendations on key size and hash functions. For example,
National Institute of Standards and Technology's (NIST's) draft
recommendations on cryptographic algorithms and key lengths
[SP-800-131] specify that digital signature generation using an RSA
key with modulus less than 2048 bits or with the SHA-1 hash function
is acceptable through 2010 and deprecated from 2011 through 2013,
whereas an RSA key with modulus at least 2048 bits and SHA-256 is
acceptable for the indefinite future. The introduction of other non-
certificate-based SSH public key formats compatible with the above
recommendations is outside the scope of this document.
3.4. x509v3-ecdsa-sha2-*
Signing and verifying using the x509v3-ecdsa-sha2-* key formats is
performed according to the ECDSA algorithm in [FIPS-186-3] using the
SHA2 hash function family [FIPS-180-3]. The choice of hash function
from the SHA2 hash function family is based on the key size of the
ECDSA key as specified in Section 6.2.1 of [RFC5656].
The resulting signature is encoded as follows:
string "ecdsa-sha2-[identifier]"
string ecdsa_signature_blob
The string [identifier] is the identifier of the elliptic curve
domain parameters. The format of this string is specified in Section
6.1 of [RFC5656].
Igoe & Stebila Standards Track [Page 9]
RFC 6187 X.509v3 Certificates for SSH March 2011
The ecdsa_signature_blob value has the following specific encoding:
mpint r
mpint s
The integers r and s are the output of the ECDSA algorithm.
This format is the same as for ecdsa-sha2-* signatures in Section
3.1.2 of [RFC5656].
4. Use in Public Key Algorithms
The public key algorithms and encodings defined in this document
SHOULD be accepted any place in the Secure Shell protocol suite where
public keys are used, including, but not limited to, the following
protocol messages for server authentication and user authentication:
o in the SSH_MSG_USERAUTH_REQUEST message when "publickey"
authentication is used [RFC4252]
o in the SSH_MSG_USERAUTH_REQUEST message when "hostbased"
authentication is used [RFC4252]
o in the SSH_MSG_KEXDH_REPLY message [RFC4253]
o in the SSH_MSG_KEXRSA_PUBKEY message [RFC4432]
o in the SSH_MSG_KEXGSS_HOSTKEY message [RFC4462]
o in the SSH_MSG_KEX_ECDH_REPLY message [RFC5656]
o in the SSH_MSG_KEX_ECMQV_REPLY message [RFC5656]
When a public key from this specification is included in the input to
a hash algorithm, the exact bytes that are transmitted on the wire
must be used as input to the hash functions. In particular,
implementations MUST NOT omit any of the chain certificates or OCSP
responses that were included on the wire, nor change encoding of the
certificate or OCSP data. Otherwise, hashes that are meant to be
computed in parallel by both peers will have differing values.
For the purposes of user authentication, the mapping between
certificates and user names is left as an implementation and
configuration issue for implementers and system administrators.
For the purposes of server authentication, it is RECOMMENDED that
implementations support the following mechanism mapping host names to
certificates. However, local policy MAY disable the mechanism or MAY
Igoe & Stebila Standards Track [Page 10]
RFC 6187 X.509v3 Certificates for SSH March 2011
impose additional constraints before considering a matching
successful. Furthermore, additional mechanisms mapping host names to
certificates MAY be used and are left as implementation and
configuration issues for implementers and system administrators.
The RECOMMENDED server authentication mechanism is as follows. The
subjectAlternativeName X.509v3 extension, as described in Section
4.2.1.6 of [RFC5280], SHOULD be used to convey the server host name,
using either dNSName entries or iPAddress entries to convey domain
names or IP addresses as appropriate. Multiple entries MAY be
specified. The following rules apply:
o If the client's reference identifier (e.g., the host name typed by
the client) is a DNS domain name, the server's identity SHOULD be
checked using the rules specified in [RFC6125]. Support for the
DNS-ID identifier type is RECOMMENDED in client and server
software implementations. Certification authorities that issue
certificates for use by Secure Shell servers SHOULD support the
DNS-ID identifier type. Service providers SHOULD include the
DNS-ID identifier type in certificate requests. The DNS-ID MAY
contain the wildcard character '*' as the complete left-most label
within the identifier.
o If the client's reference identifier is an IP address as defined
by [RFC791] or [RFC2460], the client SHOULD convert that address
to the "network byte order" octet string representation and
compare it against a subjectAltName entry of type iPAddress. A
match occurs if the octet strings are identical for the reference
identifier and any presented identifier.
5. Security Considerations
This document provides new public key algorithms for the Secure Shell
protocol that convey public keys using X.509v3 certificates. For the
most part, the security considerations involved in using the Secure
Shell protocol apply, since all of the public key algorithms
introduced in this document are based on existing algorithms in the
Secure Shell protocol. However, implementers should be aware of
security considerations specific to the use of X.509v3 certificates
in a public key infrastructure, including considerations related to
expired certificates and certificate revocation lists.
The reader is directed to the security considerations sections of
[RFC5280] for the use of X.509v3 certificates, [RFC2560] for the use
of OCSP response, [RFC4253] for server authentication, and [RFC4252]
for user authentication. Implementations SHOULD NOT use revoked
certificates because many causes of certificate revocation mean that
the critical authentication properties needed are no longer true.
Igoe & Stebila Standards Track [Page 11]
RFC 6187 X.509v3 Certificates for SSH March 2011
For example, compromise of a certificate's private key or issuance of
a certificate to the wrong party are common reasons to revoke a
certificate.
If a party to the SSH exchange attempts to use a revoked X.509v3
certificate, this attempt along with the date, time, certificate
identity, and apparent origin IP address of the attempt SHOULD be
logged as a security event in the system's audit logs or the system's
general event logs. Similarly, if a certificate indicates that OCSP
is used and there is no response to the OCSP query, the absence of a
response along with the details of the attempted certificate use (as
before) SHOULD be logged.
As with all specifications involving cryptographic algorithms, the
quality of security provided by this specification depends on the
strength of the cryptographic algorithms in use, the security of the
keys, the correctness of the implementation, and the security of the
public key infrastructure and the certificate authorities.
Accordingly, implementers are encouraged to use high-assurance
methods when implementing this specification and other parts of the
Secure Shell protocol suite.
6. IANA Considerations
Consistent with Section 8 of [RFC4251] and Section 4.6 of [RFC4250],
this document makes the following registrations:
In the Public Key Algorithm Names registry:
o The SSH public key algorithm "x509v3-ssh-dss".
o The SSH public key algorithm "x509v3-ssh-rsa".
o The SSH public key algorithm "x509v3-rsa2048-sha256".
o The family of SSH public key algorithm names beginning with
"x509v3-ecdsa-sha2-" and not containing the at-sign ('@').
The two object identifiers used in Section 2.2.2 were assigned from
an arc delegated by IANA to the PKIX Working Group.
7. References
7.1. Normative References
[ASN1] International Telecommunications Union, "Abstract
Syntax Notation One (ASN.1): Specification of basic
notation", X.680, July 2002.
Igoe & Stebila Standards Track [Page 12]
RFC 6187 X.509v3 Certificates for SSH March 2011
[FIPS-180-2] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
[FIPS-180-3] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-3, October 2008.
[FIPS-186-3] National Institute of Standards and Technology,
"Digital Signature Standard (DSS)", FIPS 186-3,
June 2009.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[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.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3279, April 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[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.
Igoe & Stebila Standards Track [Page 13]
RFC 6187 X.509v3 Certificates for SSH March 2011
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T.
Polk, "Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, March 2009.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, December 2009.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and
T. Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and
ECDSA", RFC 5758, January 2010.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service
Identity within Internet Public Key Infrastructure
Using X.509 (PKIX) Certificates in the Context of
Transport Layer Security (TLS)", RFC 6125, March 2011.
[SEC1] Standards for Efficient Cryptography Group, "Elliptic
Curve Cryptography", SEC 1, September 2000,
<http://www.secg.org/download/aid-780/sec1-v2.pdf>.
7.2. Informative References
[RFC4432] Harris, B., "RSA Key Exchange for the Secure Shell
(SSH) Transport Layer Protocol", RFC 4432, March 2006.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V.
Welch, "Generic Security Service Application Program
Interface (GSS-API) Authentication and Key Exchange for
the Secure Shell (SSH) Protocol", RFC 4462, May 2006.
[RFC5759] Solinas, J. and L. Zieglar, "Suite B Certificate and
Certificate Revocation List (CRL) Profile", RFC 5759,
January 2010.
[SP-800-131] Barker, E. and A. Roginsky, "DRAFT Recommendation for
the Transitioning of Cryptographic Algorithms and Key
Lengths", NIST Special Publication 800-131, June 2010.
Igoe & Stebila Standards Track [Page 14]
RFC 6187 X.509v3 Certificates for SSH March 2011
Appendix A. Example
The following example illustrates the use of an X.509v3 certificate
for a public key for the Digital Signature Algorithm when used in a
Diffie-Hellman key exchange method. In the example, there is a chain
of certificates of length 2, and a single OCSP response is provided.
byte SSH_MSG_KEXDH_REPLY
string 0x00 0x00 0xXX 0xXX -- length of the remaining data in
this string
0x00 0x00 0x00 0x0D -- length of string "x509v3-ssh-dss"
"x509v3-ssh-dss"
0x00 0x00 0x00 0x02 -- there are 2 certificates
0x00 0x00 0xXX 0xXX -- length of sender certificate
DER-encoded sender certificate
0x00 0x00 0xXX 0xXX -- length of issuer certificate
DER-encoded issuer certificate
0x00 0x00 0x00 0x01 -- there is 1 OCSP response
0x00 0x00 0xXX 0xXX -- length of OCSP response
DER-encoded OCSP response
mpint f
string signature of H
Appendix B. Acknowledgements
The authors gratefully acknowledge helpful comments from Ran
Atkinson, Samuel Edoho-Eket, Joseph Galbraith, Russ Housley, Jeffrey
Hutzelman, Jan Pechanec, Peter Saint-Andre, Sean Turner, and Nicolas
Williams.
O. Saarenmaa and J. Galbraith previously drafted a document on a
similar topic.
Igoe & Stebila Standards Track [Page 15]
RFC 6187 X.509v3 Certificates for SSH March 2011
Authors' Addresses
Kevin M. Igoe
National Security Agency
NSA/CSS Commercial Solutions Center
United States of America
EMail: kmigoe@nsa.gov
Douglas Stebila
Queensland University of Technology
Information Security Institute
Level 7, 126 Margaret St
Brisbane, Queensland 4000
Australia
EMail: douglas@stebila.ca
Igoe & Stebila Standards Track [Page 16]