Network Working Group O. Saarenmaa Internet-Draft F-Secure Intended status: Informational J. Galbraith Expires: August 11, 2007 VanDyke Software February 7, 2007 X.509 authentication in SSH draft-saarenmaa-ssh-x509-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 11, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document specifies how X.509 certificates and signatures are used within the Secure Shell protocol for user and server authentication. Saarenmaa & Galbraith Expires August 11, 2007 [Page 1] Internet-Draft X.509 authentication in SSH February 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . . 3 3. Certificate validation . . . . . . . . . . . . . . . . . . . . 3 4. Use in SSH Protocol . . . . . . . . . . . . . . . . . . . . . . 3 4.1. x509v3-sign-rsa . . . . . . . . . . . . . . . . . . . . . . 3 4.2. x509v3-sign-dss . . . . . . . . . . . . . . . . . . . . . . 4 5. Implementation Considerations . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . . 5 8.2. Informative References . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . . . 7 Saarenmaa & Galbraith Expires August 11, 2007 [Page 2] Internet-Draft X.509 authentication in SSH February 2007 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 new X.509v3 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. 4. Use in SSH Protocol Key type names are of the form "x509v3-sign*". This document defines two new key formats. As opposed to formats defined in [RFC4253], these new formats do NOT include the key type or length of any fields inside the key blob. Implementations are expected to have obtained these values out-of-band. 4.1. x509v3-sign-rsa Certificates that use the RSA public key algorithm MUST use the "x509v3-sign-rsa" key format. This format is as follows: byte[n] DER encoded X.509v3 certificate data Saarenmaa & Galbraith Expires August 11, 2007 [Page 3] Internet-Draft X.509 authentication in SSH February 2007 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). 4.2. x509v3-sign-dss Certificates that use the DSA public key algorithm MUST use the "x509v3-sign-dss" key format. This format is as follows: byte[n] 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 Saarenmaa & Galbraith Expires August 11, 2007 [Page 4] Internet-Draft X.509 authentication in SSH February 2007 during key exchange. 6. IANA Considerations This document adds "x509v3-sign-rsa" and "x509v3-sign-dss" to the SSH publickey type registry. 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 intentionally leaves the details of certificate validation and PKI profile to use unspecified. 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [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. Saarenmaa & Galbraith Expires August 11, 2007 [Page 5] Internet-Draft X.509 authentication in SSH February 2007 8.2. Informative References [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. [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. Trademark notice "ssh" is a registered trademark in the United States and/or other countries. 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 Saarenmaa & Galbraith Expires August 11, 2007 [Page 6] Internet-Draft X.509 authentication in SSH February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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