Network Working Group O. Sury Internet-Draft CZ.NIC Updates: 4255 (if approved) December 5, 2011 Intended status: Standards Track Expires: June 7, 2012 Use of SHA-256 Algorithm with RSA, DSA and ECDSA in SSHFP Resource Records draft-os-ietf-sshfp-ecdsa-sha2-04 Abstract This document updates RFC 4255, which defines a DNS resource record - SSHFP that contains a standard SSH key fingerprint used to verify Secure Shell (SSH) host keys using Domain Name System Security (DNSSEC). This document defines additional options supporting Secure Shell (SSH) public keys using the Elliptic Curve Digital Signature Algorithm (ECDSA) and the use of fingerprints computed using the SHA- 256 message digest algorithm in SSHFP resource records. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on June 7, 2012. 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 Sury Expires June 7, 2012 [Page 1]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 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. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3 3. SSHFP Resource Records . . . . . . . . . . . . . . . . . . . . 3 3.1. SSHFP Fingerprint Type Specification . . . . . . . . . . . 4 3.1.1. SHA-256 SSHFP Fingerprint Type Specification . . . . . 4 3.2. SSHFP Algorithm Number Specification . . . . . . . . . . . 4 3.2.1. ECDSA SSHFP Algorithm Number Specification . . . . . . 4 4. Implementation Considerations . . . . . . . . . . . . . . . . . 4 4.1. Support for SHA-256 fingerprints . . . . . . . . . . . . . 4 4.2. Support for ECDSA . . . . . . . . . . . . . . . . . . . . . 4 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. RSA public key . . . . . . . . . . . . . . . . . . . . . . 5 5.1.1. RSA public key with SHA1 fingerprint . . . . . . . . . 5 5.1.2. RSA public key with SHA256 fingerprint . . . . . . . . 5 5.2. DSA public key . . . . . . . . . . . . . . . . . . . . . . 6 5.2.1. DSA public key with SHA1 fingerprint . . . . . . . . . 6 5.2.2. DSA public key with SHA256 fingerprint . . . . . . . . 6 5.3. ECDSA public key . . . . . . . . . . . . . . . . . . . . . 6 5.3.1. ECDSA public key with SHA256 fingerprint . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 6.1. SSHFP RR Types for public key algorithms . . . . . . . . . 7 6.2. SSHFP RR types for fingerprint types . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 9 Sury Expires June 7, 2012 [Page 2]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 1. Introduction The Domain Name System (DNS) is the global, hierarchical distributed database for Internet Naming. The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network. RFC 4253 [RFC4253] defines Public Key Algorithms for the Secure Shell server public keys. The DNS has been extended to store fingerprints in a DNS resource record named SSHFP [RFC4255], which provides out-of-band verification by looking up a fingerprint of the server public key in the DNS [RFC1034], [RFC1035] and using Domain Name System Security Extensions (DNSSEC) [RFC4033], [RFC4034], [RFC4035] to verify the lookup. RFC 4255 [RFC4255] describes how to store the cryptographic fingerprint of SSH public keys in SSHFP resource records. SSHFP records contain the fingerprint and two index numbers identifying the cryptographic algorithms used 1. to link the fingerprinted public key with the corresponding private key, and 2. to derive the message digest stored as the fingerprint in the record. RFC 4255 then specifies lists of cryptographic algorithms and the corresponding index numbers used to identify them in SSHFP records. This document updates RFC 4255 by adding a new option in each list: o the Elliptic Curve Digital Signature Algorithm (ECDSA)[RFC6090] which has been added to the Secure Shell Public Key list by RFC 5656 [RFC5656] in the public key algorithms list; and o the SHA-256 algorithm [FIPS.180-3.2008] in the SSHFP Fingerprint Type list. Familiarity with DNSSEC, SSH Protocol [RFC4251], [RFC4253], [RFC4250], SSHFP [RFC4255], and the SHA-2 [FIPS.180-3.2008] family of algorithms is assumed in this document. 2. Requirements Language 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]. 3. SSHFP Resource Records The format of the SSHFP RR can be found in RFC 4255 [RFC4255]. Sury Expires June 7, 2012 [Page 3]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 3.1. SSHFP Fingerprint Type Specification The fingerprint type octet identifies the message-digest algorithm used to calculate the fingerprint of the public key. 3.1.1. SHA-256 SSHFP Fingerprint Type Specification SHA-256 fingerprints of the public keys are stored in SSHFP Resource Record with the fingerprint type 2. 3.2. SSHFP Algorithm Number Specification The SSHFP Resource Record algorithm number octet describes the algorithm of the public key. 3.2.1. ECDSA SSHFP Algorithm Number Specification ECDSA public keys are stored in SSHFP Resource Records with the algorithm number 3. ECDSA public key fingerprints MUST use the SHA-256 algorithm for the fingerprint as using the SHA-1 algorithm would weaken the security of the key. 4. Implementation Considerations 4.1. Support for SHA-256 fingerprints SSHFP-aware Secure Shell implementations SHOULD support the SHA-256 fingerprints for verification of the public key. Secure Shell implementations which support SHA-256 fingerprints MUST prefer a SHA- 256 fingerprint over SHA-1 if both are available for a server. If the SHA-256 fingerprint is tested and does not match the supplied key, then the key MUST be rejected rather than testing the alternative SHA-1 fingerprint. 4.2. Support for ECDSA SSHFP-aware Secure Shell implementations which also implement ECDSA algorithm for the public key SHOULD support SSHFP fingerprints for ECDSA public keys. 5. Examples Sury Expires June 7, 2012 [Page 4]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 5.1. RSA public key Given a private key with the following value in OpenSSH format: -----BEGIN RSA PRIVATE KEY----- MIIEpAIBAAKCAQEAwlEeCTocU4p86u0Dt20F1uI5jwgrpRbJ4fGIuzCsKTJ3fevk +7le5xMMvuvhlmLvfCMRSQciIxV1/2ugVw6d/O/MHsx9Q2drTQ/7bv3rnc+hK6Ux WJp1S8hAwEWEs1QTULiCtVA6r7wein3yXMre/BacFtu3rhpKhJGpuxmrqz0QIMF3 oQwf4DMEbV1UWftd82FpAJgGPuTgFlZnV7kFZuZI5b3Dc7aNh95t56ibQ+CfS9ZS j7klVasCa+P+oYm1yZEBL1qVL3TgFMN36yqTcGvd9n1xZN5HuK7A40P1vBspXjLS t08fLROM9cLqMF7WHugWvKtywD7P5tkuKVLHMQIDAQABAoIBAQCrZP1HSjhd/5M7 bB+RFNrHtPbsEFre3QDpCDCAW+ge1mLLcNyio9jvnL/rTwfFrDJsnknKzj3wECfq STY+U6hKyACVUe1THM9qQ6SVO+ctZUxVwPmLm4HGfDWQ4kCwJIJ8+qJf5wo8o4OU yI6UBmU0mYTILLkRGiOMVycM3xGqkUJHcjj82GLWNKakdp1CuFtmyF0aUnlDp5gm Ub0GgCgBFCO+/Eb7OoqZufhS6bisRyDEozLNO/I0Ih7lZgsaywOsjeXOZ2+zHH98 +RVrnZ6PObxPp2WmSA268gW02k2rWRGTg95boSLdxv2C1nBvdqsMXnq8hVcfKigO bYH1uIOBAoGBAOBkncI1ZYOd8mye4a+hgzBgxdzrEl3QCAm3qSw5Gsz6FwTAZAit u4lRSXb0birYKfJjcZ7Og/07r0KCMuCku/CTpbZP0gCSyd7SaeovFs1y9tUuY8r+ iT+FxFeOQ9PcYcOccivzkLwINOrG/Glm8UWUngCRDgo/CSOSTf06juY5AoGBAN2v /DQeQl/uATmIyfOGsZA4IdmAfhY8P60GVdk8zFZyDW5qmJklDA75ObepUtDnAcDd NzkNyKZBIX6aFoMkXAzwMCxk6KU3gkbciuCydCXf323fKCS7SHIk+btGa+eRhUcO HzPlzUqxrqg7ouQ1n2/zLbiN10zwWCPYzTGAwai5AoGBAJ9b9YnqQAjkEDnB8Ee5 7aBa6cpGC8oiJsM38uYcPANcjSJru99J+si/uOvJFcBJuiiRJS0CP0yFqacTLizJ 8UseoG5Ea8DKfqFHT77n6ErKHbAyfN66PCCn0FPaDiOU/L1eCttZ4+0V6vbdkH8O g8TFkhyW56CxOb1QdyCjCL9JAoGAcexxcBsowwGdkYKRPdu3PkUKaCrXIPgfRPyf e376B2afLmILP5BBTSSYm6ChVYeRaBqGuYQy2/VWkCgBb61svJ1mNDo7MESBZ4cI u4YZmCkfOehXSeEQzs/fonUDGMK4uhYwxMvQnxUGi5/yCtLft3lBwrjprrlIoktU z566ZskCgYBRFqGVaZZQgLeiEjuRtxo0MOmQvN3fwfgd7HbHoNjyalPRCUOurmDk rIpSmbeIABBWveapZwidXNRdbAqV/XZ+tEHeak4peanFGIUV5J4P9kg6eakuwC14 wU+VnpDUATpddCID+jf7ory9bCvJ4gvKlyDq5PJyR8uiut+BY0m7Hg== -----END RSA PRIVATE KEY----- 5.1.1. RSA public key with SHA1 fingerprint The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 1 1 dd465c09cfa51fb45020cc83316fff21 b9ec74ac 5.1.2. RSA public key with SHA256 fingerprint The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 1 2 b049f950d1397b8fee6a61e4d14a9acd c4721e084eff5460bbed80cfaa2ce2cb Sury Expires June 7, 2012 [Page 5]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 5.2. DSA public key Given a private key with the following value in OpenSSH format: -----BEGIN DSA PRIVATE KEY----- MIIBvAIBAAKBgQD1Ra3NFN+oFmssG3yc43L/Hn9d6gF+BCZfDWusar14dbfmgiRH Uu7KEY7byuCrDYZO/A43bZ34RIchShxzc94uv3P7PZT9FI1e5kQKOpwOwNxrOokB JW+jvRapuolUgum2FopU0gdLWHp3BBCVKGgLmvGEBf7sUcz60Xl8Rqh54wIVAML0 z+mWLxUhWYQY47TALVN5RM3jAoGBAIANhW5G23qNPrv6sPJkBThVmaU2qjaO3e46 L95mo24eS6hFQ+8k9zEtRkhoY4L74brP3oTE6s2G403NLM1DPSZ8E+8ateT9mWAy vfCFca8N9YzLbFFBJgageA1I07q7XGlpifSzWj9f5OGzKNP4aLZznDlZyD7EywRV lb3TUcVAAoGAOZcDcK01NTM1qIIYbBqCffrwjQ+9PmsuSKI6nUzfS4NysXHkdbW5 u5VxeXLcwWj5PGbRfoS2P3vwYAmakqgq502wigam18u9nAczUYl+2kOeOiIRrtSm LfpV7thLOAb8k1ESjIlkbn35jKmTcoMFRXbFmkKRTK8OEnWQ8AVg6w8CFQCS/nI5 MhAE/LKS/rJ5fSZ/j+/dNw== -----END DSA PRIVATE KEY----- 5.2.1. DSA public key with SHA1 fingerprint The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 2 1 3b6ba6110f5ffcd29469fc1ec2ee25d6 1718badd 5.2.2. DSA public key with SHA256 fingerprint The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 2 2 f9b8a6a460639306f1b38910456a6ae1 018a253c47ecec12db77d7a0878b4d83 5.3. ECDSA public key Given a private key with the following value in OpenSSH format: -----BEGIN EC PRIVATE KEY----- MHcCAQEEINFBNyh3bKEQ4CQ7MfNgbEGINuRHjaIBrZkiWbaGPCZZoAoGCCqGSM49 AwEHoUQDQgAEAP70I5SJftZiBy8g50jz52N2gUNVRPE2tyiDyxJh1sjN4b5th2yy y9zLL+dF9WFcLlAEKTwhOGqzsPj+UXFfmA== -----END EC PRIVATE KEY----- Sury Expires June 7, 2012 [Page 6]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 5.3.1. ECDSA public key with SHA256 fingerprint The SSHFP Resource Record for this key would be: server.example.net IN SSHFP 3 2 821eb6c1c98d9cc827ab7f456304c0f1 4785b7008d9e8646a8519de80849afc7 6. IANA Considerations This document updates the IANA registry "SSHFP RR Types for public key algorithms" and "SSHFP RR types for fingerprint types" [SSHFPVALS]. 6.1. SSHFP RR Types for public key algorithms The following entries are added to the "SSHFP RR Types for public key algorithms" registry: +-------+-------------+------------+ | Value | Description | Reference | +-------+-------------+------------+ | 3 | ECDSA | [This doc] | +-------+-------------+------------+ Table 1 6.2. SSHFP RR types for fingerprint types The following entries are added to the "SSHFP RR types for fingerprint types" registry: +-------+-------------+------------+ | Value | Description | Reference | +-------+-------------+------------+ | 2 | SHA-256 | [This doc] | +-------+-------------+------------+ Table 2 7. Security Considerations Please see the security considerations in [RFC4255] for SSHFP record and [RFC5656] for ECDSA algorithm. Users of SSHFP are encouraged to deploy SHA-256 as soon as software implementations allow for it. SHA-2 family of algorithms is widely believed to be more resilient to attack than SHA-1, and confidence in SHA-1's strength is being eroded by recently announced attacks [IACR Sury Expires June 7, 2012 [Page 7]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 2007/474]. Regardless of whether or not the attacks on SHA-1 will affect SSHFP, it is believed (at the time of this writing) that SHA- 256 is the better choice for use in SSHFP records. SHA-256 is considered sufficiently strong for the immediate future, but predictions about future development in cryptography and cryptanalysis are beyond the scope of this document. 8. References 8.1. Normative References [FIPS.180-3.2008] National Institute of Standards and Technology, ""Secure Hash Standard"", FIPS PUB 180-3, October 2008, <http:// csrc.nist.gov/publications/fips/fips180-3/ fips180-3_final.pdf>. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. [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. [RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006. Sury Expires June 7, 2012 [Page 8]
Internet-Draft ECDSA and SHA-256 Algorithms for SSHFP December 2011 [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, January 2006. [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, December 2009. 8.2. Informative References [IACR 2007/474] Cochran, M., ""Notes on the Wang et al. 2^63 SHA-1 Di!erential Path"", IACR 2007/474, <http://eprint.iacr.org/2007/474.pdf>. [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011. [SSHFPVALS] IANA, ""DNS SSHFP Resource Records Parameters"", IANA registry available at:, <http://www.iana.org/assignments/ dns-sshfp-rr-parameters/dns-sshfp-rr-parameters.xml>. Author's Address Ondrej Sury CZ.NIC Americka 23 120 00 Praha 2 CZ Phone: +420 222 745 110 Email: ondrej.sury@nic.cz Sury Expires June 7, 2012 [Page 9]
