draft-ietf-secsh-x509-02.txt   draft-ietf-secsh-x509-03.txt 
Secure Shell Working Group J. Galbraith Secure Shell Working Group O. Saarenmaa
Internet-Draft VanDyke Software Internet-Draft F-Secure
Expires: January 16, 2006 O. Saarenmaa Expires: September 1, 2006 J. Galbraith
F-Secure Corporation VanDyke Software
July 15, 2005 February 28, 2006
X.509 authentication in SSH2 X.509 authentication in SSH
draft-ietf-secsh-x509-02.txt draft-ietf-secsh-x509-03.txt
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
The X.509 extension specifies how X.509 keys and signatures are used This document specifies how X.509 certificates and signatures are
within the SSH2 protocol. used within the Secure Shell protocol for user and server
authentication.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Certificate validation . . . . . . . . . . . . . . . . . . . . 3 3. Certificate validation . . . . . . . . . . . . . . . . . . . . 3
3.1 Host Authentication . . . . . . . . . . . . . . . . . . . 3 3.1. Certificate Extensions . . . . . . . . . . . . . . . . . . 3
3.2 User Authentication . . . . . . . . . . . . . . . . . . . 3 3.1.1. ExtendedKeyUsage . . . . . . . . . . . . . . . . . . . 3
4. Use in SSH2 Protocol . . . . . . . . . . . . . . . . . . . . . 4 3.2. Server Authentication . . . . . . . . . . . . . . . . . . 4
4.1 x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . . . . . . 4 3.3. User Authentication . . . . . . . . . . . . . . . . . . . 4
4.2 x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . . 4 4. Use in SSH Protocol . . . . . . . . . . . . . . . . . . . . . 4
4.3 x509v3-sign . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. x509v3-sign . . . . . . . . . . . . . . . . . . . . . . . 4
5. Implementation Considerations . . . . . . . . . . . . . . . . 5 4.2. x509v3-sign-rsa-sha1 . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4.3. x509v3-sign-dss-sha1 . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5. Implementation Considerations . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8.1 Normative References . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8.2 Informative References . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
1. Introduction 1. Introduction
The SSH protocol can use public keys for both host and user The Secure Shell protocol can use public keys for both server and
authentication. However, particularly for host authentication, plain user authentication. However, particularly for server
public keys lack a good method of verifying that the the key provided authentication, plain public keys lack a good method of verifying
really does belong to the host asserting ownership. X.509v3 that the the key provided really does belong to the host asserting
certificates can address this problem in environments where a PKI ownership. X.509v3 certificates can address this problem in
infrastructure is available. environments where a PKI infrastructure is available.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Certificate validation 3. Certificate validation
Implementations are expected to follow the basic certificate and Implementations are expected to follow the basic certificate and
certificate path validation guidelines described in [RFC3280]. No certificate path validation guidelines defined in [RFC3280]. This
SSH specific X.509 certificate extensions are defined in this document does not define any new X.509 certificate extensions.
document.
3.1 Host Authentication 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.
The client MAY verify that the serverAuth option, as specified in 3.1. Certificate Extensions
[RFC3280], is present in the host certificate's extendedKeyUsage
field.
Implementations SHOULD validate the host certificates by matching the Implementations MUST recognize the following extensions:
host's fully qualified domain name [RFC1034] against the host BasicConstraints, KeyUsage, and SubjectAltName. Implementations also
certificate's subjectAltName extension's dNSName entries. If the MUST be able to handle all other extensions that have been marked
certificate does not contain dNSName subjectAltName extensions, the critical or reject the certificate.
(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 3.1.1. ExtendedKeyUsage
The server MAY verify that the clientAuth option, as specified in Certificates meant for use within the SSH protocol SHOULD NOT include
[RFC3280], is present in the user certificate's extendedKeyUsage the ExtendedKeyUsage extension. If the certificates require an EKU
field. 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
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 No constraints are placed on the presence of user account information
in the certificates used for user authentication. Their validation in the certificates used for user authentication. The mapping of
and mapping is left as an implementation and configuration detail for user certificates to user accounts is left as an implementation
the implementors and deployers. choice and configuration issue for the implementors and deployers.
4. Use in SSH2 Protocol 4. Use in SSH Protocol
Key type names are of the form "x509v3-sign*". Keys are encoded as This document defines three new key formats which are in the form
follows: "x509v3-sign*". Each of the formats encodes the key type name in the
beginning of the key blob.
string key-type-name 4.1. x509v3-sign
string DER encoded x.509v3 certificate data
4.1 x509v3-sign-rsa-sha1 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:
Certificates that use the RSA public key algorithm SHOULD use the string "x509v3-sign"
"x509v3-sign-rsa-sha1" key-type-name. uint32 number of certificates
string[1..] DER encoded X.509v3 certificate data
uint32 number of ocsp responses
string[0..] OCSP response data
Signing and verifying using this key format, uses the certificate's The first certificate in the list MUST be the end-entity one, and any
private key, in exactly the same manner specified for "ssh-rsa" other certificates MUST be part of the end-entity certificate's path.
public keys in [I-D.ietf-secsh-transport]. That is to say, signing
and verifying using this key format is performed according to the Signatures are encoded as follows:
RSASSA-PKCS1-v1_5 scheme in [RFC3447] using the SHA-1 hash.
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 The signature format for x509v3-sign-rsa-sha1 certificates is the
"ssh-rsa" signing format specified in [I-D.ietf-secsh-transport]. "ssh-rsa" signing format specified in [RFC4253]. This format is as
This format is as follows: follows:
string "ssh-rsa" string "ssh-rsa"
string rsa_signature_blob string rsa_signature_blob
The value for 'rsa_signature_blob' is encoded as a string containing The value for 'rsa_signature_blob' is encoded as a string containing
s (which is an integer, without lengths or padding, unsigned and in s (which is an integer, without lengths or padding, unsigned and in
network byte order). network byte order).
4.2 x509v3-sign-dss-sha1 4.3. x509v3-sign-dss-sha1
Certificates that use the DSA public key algorithm SHOULD use the Certificates that use the DSA public key algorithm MAY use the
"x509v3-sign-dss-sha1" key-type-name. "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 Signing and verifying using this key format, uses the certificate's
private key, in exactly the same manner specified for "ssh-dss" private key, in exactly the same manner specified for "ssh-dss"
public keys in [I-D.ietf-secsh-transport]. That is to say, signing public keys in [RFC4253]. That is to say, signing and verifying
and verifying using this key format is done according to the Digital using this key format is done according to the Digital Signature
Signature Standard [FIPS-186-2] using the SHA-1 hash [FIPS-180-2]. Standard [FIPS-186-2] using the SHA-1 hash [FIPS-180-2].
The signature format for x509v3-sign-dss-sha1 certificates is the The signature format for x509v3-sign-dss-sha1 certificates is the
"ssh-dss" signing format specified in [I-D.ietf-secsh-transport]. "ssh-dss" signing format specified in [RFC4253]. This format is as
This format is as follows: follows:
string "ssh-dss" string "ssh-dss"
string dss_signature_blob string dss_signature_blob
The value for 'dss_signature_blob' is encoded as a string containing The value for 'dss_signature_blob' is encoded as a string containing
r followed by s (which are 160-bit integers, without lengths or r followed by s (which are 160-bit integers, without lengths or
padding, unsigned and in network byte order). 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 5. Implementation Considerations
Implementations should be careful when using x.509v3 certificates as Implementations should be careful when using X.509v3 certificates as
hostkeys. If the peer does not implement the required algorithms to hostkeys. If the peer does not implement the required algorithms to
validate both the x.509v3 certificate and all certificates in the validate both the end-entity certificate and all certificates in the
chain, it MUST disconnect. There is no way to renegotiate the key chain, it MUST disconnect. There is no way to renegotiate the key
during key exchange. during key exchange.
This is especially true when using the "x509v3-sign" key type, since This is especially true when using the "x509v3-sign" key type, since
in this case the peer has no knowledge whatsoever of required in this case the peer has no knowledge whatsoever of required
algorithms. algorithms. The peer might also refuse a "x509v3-sign" key if the
required intermediate certificates and OCSP responses are not
included.
6. IANA Considerations 6. IANA Considerations
This document reserves all key types beginning with "x509v3-sign" in This document reserves all key types beginning with "x509v3-sign" in
the SSH publickey type registry. the SSH publickey type registry.
This document specifically adds "x509v3-sign-rsa-sha1", "x509v3-sign- This document specifically adds "x509v3-sign-rsa-sha1", "x509v3-sign-
dss-sha1", and "x509v3-sign" to the SSH publickey type registry. dss-sha1", and "x509v3-sign" to the SSH publickey type registry.
This document adds "x509v3-sign-rsa" and "x509v3-sign-dss" to the SSH This document adds "x509v3-sign-rsa" and "x509v3-sign-dss" to the SSH
publickey type registry as "poisoned" by historical use. publickey type registry as "poisoned" by historical use.
7. Security Considerations 7. Security Considerations
PKI is an extremely complex topic, and care must be taken by both PKI is an extremely complex topic, and care must be taken by both
implementors and deployers to understand the complex interactions implementors and deployers to understand the complex interactions
involved. involved.
Implementations should carefully validate the certificate, including, This document suggests that validation of the ExtendedKeyUsage
but not limited to, certificate expiration, certificate signature, extension MAY be disabled by configuration in the implementations.
certificate revocation lists, etc. Disabling validation of other extensions such as KeyUsage or
BasicConstraints MUST NOT be done, as that might lead into invalid
trust paths being established.
For more information, implementors should refer to [ITU.X509.2000] Implementations should carefully validate the certificate, including
and [RFC3280]. 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. References
8.1 Normative 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 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 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280, Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002. April 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003. Version 2.1", RFC 3447, February 2003.
[I-D.ietf-secsh-transport] [RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Lonvick, C., "SSH Transport Layer Protocol", Authentication Protocol", RFC 4252, January 2006.
draft-ietf-secsh-transport-24 (work in progress),
March 2005.
[PKCS.7.1993] [RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
RSA Laboratories, "Cryptographic Message Syntax Standard. Transport Layer Protocol", RFC 4253, January 2006.
Version 1.5", PKCS 7, November 1993.
[FIPS-180-2] [FIPS-180-2]
National Institute of Standards and Technology, "Secure National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", Federal Information Processing Hash Standard (SHS)", Federal Information Processing
Standards Publication 180-2, August 2002. Standards Publication 180-2, August 2002.
[FIPS-186-2] [FIPS-186-2]
National Institute of Standards and Technology, "Digital National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", Federal Information Processing Signature Standard (DSS)", Federal Information Processing
Standards Publication 186-2, January 2000. Standards Publication 186-2, January 2000.
[ITU.X509.2000] [ITU.X509.2000]
International Telecommunications Union, "Information International Telecommunications Union, "Information
technology - Open Systems Interconnection - The Directory: technology - Open Systems Interconnection - The Directory:
Public-key and attribute certificate frameworks", ITU- Public-key and attribute certificate frameworks", ITU-
T Recommendation X.509, ISO Standard 9594-8, March 2000. T Recommendation X.509, ISO Standard 9594-8, March 2000.
8.2 Informative References [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", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [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.
Authors' Addresses Authors' Addresses
Oskari Saarenmaa
F-Secure
Tammasaarenkatu 7
PL 24
Helsinki 00181
FI
Email: oskari.saarenmaa@f-secure.com
Joseph Galbraith Joseph Galbraith
VanDyke Software VanDyke Software
4848 Tramway Ridge Blvd 4848 Tramway Ridge Blvd
Suite 101 Suite 101
Albuquerque, NM 87111 Albuquerque, NM 87111
US US
Phone: +1 505 332 5700 Phone: +1 505 332 5700
Email: galb-list@vandyke.com 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.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
skipping to change at page 8, line 41 skipping to change at page 10, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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