Network Working Group O.S. Sury
Internet-Draft CZ.NIC
Updates: 4255 (if approved) November 23, 2011
Intended status: Standards Track
Expires: May 26, 2012

Use of SHA-256 Algorithm with RSA, DSA and ECDSA in SSHFP Resource Records
draft-os-ietf-sshfp-ecdsa-sha2-01

Abstract

This document defines how to store Secure Shell (SSH) ECDSA public keys and SHA-256 fingerprints 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/.

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This Internet-Draft will expire on May 26, 2012.

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Table of Contents

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 recource record named SSHFP [RFC4255], which provide out-of-band verification by looking up a fingerprint of the server public key in the DNS [RFC1034], [RFC1035] and using DNSSEC [RFC4033], [RFC4034], [RFC4035] to verify the lookup.

RFC 4255 [RFC4255] describes how to store SSH public keys in SSHFP resource records and specifies a list of cryptographic algorithms to use for Algorithm Number and Fingerprint Type. This document extendes the SSHFP Algorithm Number list with the ECDSA algorithm [RFC6090] which has been added to Secure Shell Public Key list in RFC 5656 [RFC5656] and the SSHFP Fingerprint Type list with SHA-256 algorithm.

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.

This document updates RFC 4255 [RFC4255].

2. SSHFP Resource Records

The format of the SSHFP RR can be found in RFC 4255 [RFC4255].

2.1. SSHFP Fingerprint Type Specification

The fingerprint type octet describes the message-digest algorithm used to calculate the fingerprint of the public key.

2.1.1. SHA-256 SSHFP Fingerprint Type Specification

SHA-256 fingerprints of the public keys are stored in SSHFP Resource Record with the fingerprint type 2.

2.2. SSHFP Algorithm Number Specification

The SSHFP Resource Record algorithm number octet describes the algorithm of the public key.

2.2.1. 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 SHA-256 algorithm fingerprint.

3. Implementation Considerations

3.1. 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 the SHA-256 fingerprints if available over SHA-1 to prevent downgrade attacks.

3.2. 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.

4. Examples

4.1. 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-----
        

4.1.1. RSA public key with SHA1 fingerprint

The SSHFP Resource Record for this key would be:

    server.example.net IN SSHFP 1 1 dd465c09cfa51fb45020cc83316fff21
                                    b9ec74ac
          

4.1.2. RSA public key with SHA256 fingerprint

The SSHFP Resource Record for this key would be:

    server.example.net IN SSHFP 1 2 b049f950d1397b8fee6a61e4d14a9acd
                                    c4721e084eff5460bbed80cfaa2ce2cb
          

4.2. 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-----
        

4.2.1. DSA public key with SHA1 fingerprint

The SSHFP Resource Record for this key would be:

    server.example.net IN SSHFP 2 1 3b6ba6110f5ffcd29469fc1ec2ee25d6
                                    1718badd
          

4.2.2. DSA public key with SHA256 fingerprint

The SSHFP Resource Record for this key would be:

    server.example.net IN SSHFP 2 2 f9b8a6a460639306f1b38910456a6ae1
                                    018a253c47ecec12db77d7a0878b4d83
          

4.3. 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-----
        

4.3.1. ECDSA public key with SHA256 fingerprint

The SSHFP Resource Record for this key would be:

    server.example.net IN SSHFP 3 2 821eb6c1c98d9cc827ab7f456304c0f1
                                    4785b7008d9e8646a8519de80849afc7
          

5. IANA Considerations

This document updates the IANA registry "SSHFP RR Types for public key algorithms" and "SSHFP RR types for fingerprint types" [SSHFPVALS].

5.1. 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]

5.2. 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]

6. 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. 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.

7. References

[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.
[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.
[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.
[RFC6090] McGrew, D., Igoe, K. and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, February 2011.
[FIPS.180-3.2008] National Institute of Standards and Technology , ""Secure Hash Standard" ", FIPS PUB 180-3, October 2008.
[SSHFPVALS] IANA , ""DNS SSHFP Resource Records Parameters" ", IANA registry available at:, .

Author's Address

Ondrej Sury CZ.NIC Americka 23 120 00 Praha 2, CZ Phone: +420 222 745 110 EMail: ondrej.sury@nic.cz