    Network Working Group                                          J. Jansen
    Request for Comments: 5702                                    NLnet Labs
    Category: Standards Track                                   October 2009

                      Use of SHA-2 Algorithms with RSA in
                  DNSKEY and RRSIG Resource Records for DNSSEC

    Abstract

       This document describes how to produce RSA/SHA-256 and RSA/SHA-512
       DNSKEY and RRSIG resource records for use in the Domain Name System
       Security Extensions (RFC 4033, RFC 4034, and RFC 4035).

    Status of This Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (c) 2009 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 BSD License.

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    Table of Contents

       1. Introduction ....................................................2
       2. DNSKEY Resource Records .........................................3
          2.1. RSA/SHA-256 DNSKEY Resource Records ........................3
          2.2. RSA/SHA-512 DNSKEY Resource Records ........................3
       3. RRSIG Resource Records ..........................................3
          3.1. RSA/SHA-256 RRSIG Resource Records .........................4
          3.2. RSA/SHA-512 RRSIG Resource Records .........................4
       4. Deployment Considerations .......................................5
          4.1. Key Sizes ..................................................5
          4.2. Signature Sizes ............................................5
       5. Implementation Considerations ...................................5
          5.1. Support for SHA-2 Signatures ...............................5
          5.2. Support for NSEC3 Denial of Existence ......................5
       6. Examples ........................................................6
          6.1. RSA/SHA-256 Key and Signature ..............................6
          6.2. RSA/SHA-512 Key and Signature ..............................7
       7. IANA Considerations .............................................8
       8. Security Considerations .........................................8
          8.1. SHA-1 versus SHA-2 Considerations for RRSIG
               Resource Records ...........................................8
          8.2. Signature Type Downgrade Attacks ...........................8
       9. Acknowledgments .................................................9
       10. References .....................................................9
          10.1. Normative References ......................................9
          10.2. Informative References ....................................9

    1.  Introduction

       The Domain Name System (DNS) is the global, hierarchical distributed
       database for Internet Naming.  The DNS has been extended to use
       cryptographic keys and digital signatures for the verification of the
       authenticity and integrity of its data.  [RFC4033], [RFC4034], and
       [RFC4035] describe these DNS Security Extensions, called DNSSEC.

       RFC 4034 describes how to store DNSKEY and RRSIG resource records,
       and specifies a list of cryptographic algorithms to use.  This
       document extends that list with the algorithms RSA/SHA-256 and RSA/
       SHA-512, and specifies how to store DNSKEY data and how to produce
       RRSIG resource records with these hash algorithms.

       Familiarity with DNSSEC, RSA, and the SHA-2 [FIPS.180-3.2008] family
       of algorithms is assumed in this document.

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

       To refer to both SHA-256 and SHA-512, this document will use the name
       SHA-2.  This is done to improve readability.  When a part of text is
       specific for either SHA-256 or SHA-512, their specific names are
       used.  The same goes for RSA/SHA-256 and RSA/SHA-512, which will be
       grouped using the name RSA/SHA-2.

       The term "SHA-2" is not officially defined but is usually used to
       refer to the collection of the algorithms SHA-224, SHA-256, SHA-384,
       and SHA-512.  Since SHA-224 and SHA-384 are not used in DNSSEC, SHA-2
       will only refer to SHA-256 and SHA-512 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].

    2.  DNSKEY Resource Records

       The format of the DNSKEY RR can be found in [RFC4034].  [RFC3110]
       describes the use of RSA/SHA-1 for DNSSEC signatures.

    2.1.  RSA/SHA-256 DNSKEY Resource Records

       RSA public keys for use with RSA/SHA-256 are stored in DNSKEY
       resource records (RRs) with the algorithm number 8.

       For interoperability, as in [RFC3110], the key size of RSA/SHA-256
       keys MUST NOT be less than 512 bits and MUST NOT be more than 4096
       bits.

    2.2.  RSA/SHA-512 DNSKEY Resource Records

       RSA public keys for use with RSA/SHA-512 are stored in DNSKEY
       resource records (RRs) with the algorithm number 10.

       The key size of RSA/SHA-512 keys MUST NOT be less than 1024 bits and
       MUST NOT be more than 4096 bits.

    3.  RRSIG Resource Records

       The value of the signature field in the RRSIG RR follows the RSASSA-
       PKCS1-v1_5 signature scheme and is calculated as follows.  The values
       for the RDATA fields that precede the signature data are specified in
       [RFC4034].

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

       hash = SHA-XXX(data)

       Here XXX is either 256 or 512, depending on the algorithm used, as
       specified in FIPS PUB 180-3; "data" is the wire format data of the
       resource record set that is signed, as specified in [RFC4034].

       signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)

       Here "|" is concatenation; "00", "01", "FF", and "00" are fixed
       octets of corresponding hexadecimal value; "e" is the private
       exponent of the signing RSA key; and "n" is the public modulus of the
       signing key.  The FF octet MUST be repeated the exact number of times
       so that the total length of the concatenated term in parentheses
       equals the length of the modulus of the signer's public key ("n").

       The "prefix" is intended to make the use of standard cryptographic
       libraries easier.  These specifications are taken directly from the
       specifications of RSASSA-PKCS1-v1_5 in PKCS #1 v2.1 (Section 8.2 of
       [RFC3447]), and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 (Section 9.2
       of [RFC3447]).  The prefixes for the different algorithms are
       specified below.

    3.1.  RSA/SHA-256 RRSIG Resource Records

       RSA/SHA-256 signatures are stored in the DNS using RRSIG resource
       records (RRs) with algorithm number 8.

       The prefix is the ASN.1 DER SHA-256 algorithm designator prefix, as
       specified in PKCS #1 v2.1 [RFC3447]:

       hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20

    3.2.  RSA/SHA-512 RRSIG Resource Records

       RSA/SHA-512 signatures are stored in the DNS using RRSIG resource
       records (RRs) with algorithm number 10.

       The prefix is the ASN.1 DER SHA-512 algorithm designator prefix, as
       specified in PKCS #1 v2.1 [RFC3447]:

       hex 30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    4.  Deployment Considerations

    4.1.  Key Sizes

       Apart from the restrictions in Section 2, this document will not
       specify what size of keys to use.  That is an operational issue and
       depends largely on the environment and intended use.  A good starting
       point for more information would be NIST SP 800-57 [NIST800-57].

    4.2.  Signature Sizes

       In this family of signing algorithms, the size of signatures is
       related to the size of the key and not to the hashing algorithm used
       in the signing process.  Therefore, RRSIG resource records produced
       with RSA/SHA-256 or RSA/SHA-512 will have the same size as those
       produced with RSA/SHA-1, if the keys have the same length.

    5.  Implementation Considerations

    5.1.  Support for SHA-2 Signatures

       DNSSEC-aware implementations SHOULD be able to support RRSIG and
       DNSKEY resource records created with the RSA/SHA-2 algorithms as
       defined in this document.

    5.2.  Support for NSEC3 Denial of Existence

       [RFC5155] defines new algorithm identifiers for existing signing
       algorithms, to indicate that zones signed with these algorithm
       identifiers can use NSEC3 as well as NSEC records to provide denial
       of existence.  That mechanism was chosen to protect implementations
       predating RFC 5155 from encountering resource records about which
       they could not know.  This document does not define such algorithm
       aliases.

       A DNSSEC validator that implements RSA/SHA-2 MUST be able to validate
       negative answers in the form of both NSEC and NSEC3 with hash
       algorithm 1, as defined in [RFC5155].  An authoritative server that
       does not implement NSEC3 MAY still serve zones that use RSA/SHA-2
       with NSEC denial of existence.

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    6.  Examples

    6.1.  RSA/SHA-256 Key and Signature

       Given a private key with the following values (in Base64):

       Private-key-format: v1.2
       Algorithm:       8 (RSASHA256)
       Modulus:         wVwaxrHF2CK64aYKRUibLiH30KpPuPBjel7E8ZydQW1HYWHfoGm
                        idzC2RnhwCC293hCzw+TFR2nqn8OVSY5t2Q==
       PublicExponent:  AQAB
       PrivateExponent: UR44xX6zB3eaeyvTRzmskHADrPCmPWnr8dxsNwiDGHzrMKLN+i/
                        HAam+97HxIKVWNDH2ba9Mf1SA8xu9dcHZAQ==
       Prime1:          4c8IvFu1AVXGWeFLLFh5vs7fbdzdC6U82fduE6KkSWk=
       Prime2:          2zZpBE8ZXVnL74QjG4zINlDfH+EOEtjJJ3RtaYDugvE=
       Exponent1:       G2xAPFfK0KGxGANDVNxd1K1c9wOmmJ51mGbzKFFNMFk=
       Exponent2:       GYxP1Pa7CAwtHm8SAGX594qZVofOMhgd6YFCNyeVpKE=
       Coefficient:     icQdNRjlZGPmuJm2TIadubcO8X7V4y07aVhX464tx8Q=

       The DNSKEY record for this key would be:

       example.net.     3600  IN  DNSKEY  (256 3 8 AwEAAcFcGsaxxdgiuuGmCkVI
                        my4h99CqT7jwY3pexPGcnUFtR2Fh36BponcwtkZ4cAgtvd4Qs8P
                        kxUdp6p/DlUmObdk= );{id = 9033 (zsk), size = 512b}

       With this key, sign the following RRSet, consisting of 1 A record:

       www.example.net. 3600  IN  A  192.0.2.91

       If the inception date is set at 00:00 hours on January 1st, 2000, and
       the expiration date at 00:00 hours on January 1st, 2030, the
       following signature should be created:

     www.example.net. 3600  IN  RRSIG  (A 8 3 3600 20300101000000
                         20000101000000 9033 example.net. kRCOH6u7l0QGy9qpC9
                         l1sLncJcOKFLJ7GhiUOibu4teYp5VE9RncriShZNz85mwlMgNEa
                         cFYK/lPtPiVYP4bwg==);{id = 9033}

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    6.2.  RSA/SHA-512 Key and Signature

       Given a private key with the following values (in Base64):

       Private-key-format: v1.2
       Algorithm:       10 (RSASHA512)
       Modulus:         0eg1M5b563zoq4k5ZEOnWmd2/BvpjzedJVdfIsDcMuuhE5SQ3pf
                        Q7qmdaeMlC6Nf8DKGoUPGPXe06cP27/WRODtxXquSUytkO0kJDk
                        8KX8PtA0+yBWwy7UnZDyCkynO00Uuk8HPVtZeMO1pHtlAGVnc8V
                        jXZlNKdyit99waaE4s=
       PublicExponent:  AQAB
       PrivateExponent: rFS1IPbJllFFgFc33B5DDlC1egO8e81P4fFadODbp56V7sphKa6
                        AZQCx8NYAew6VXFFPAKTw41QdHnK5kIYOwxvfFDjDcUGza88qbj
                        yrDPSJenkeZbISMUSSqy7AMFzEolkk6WSn6k3thUVRgSlqDoOV3
                        SEIAsrB043XzGrKIVE=
       Prime1:          8mbtsu9Tl9v7tKSHdCIeprLIQXQLzxlSZun5T1n/OjvXSUtvD7x
                        nZJ+LHqaBj1dIgMbCq2U8O04QVcK3TS9GiQ==
       Prime2:          3a6gkfs74d0Jb7yL4j4adAif4fcp7ZrGt7G5NRVDDY/Mv4TERAK
                        Ma0TKN3okKE0A7X+Rv2K84mhT4QLDlllEcw==
       Exponent1:       v3D5A9uuCn5rgVR7wgV8ba0/KSpsdSiLgsoA42GxiB1gvvs7gJM
                        MmVTDu/ZG1p1ZnpLbhh/S/Qd/MSwyNlxC+Q==
       Exponent2:       m+ezf9dsDvYQK+gzjOLWYeKq5xWYBEYFGa3BLocMiF4oxkzOZ3J
                        PZSWU/h1Fjp5RV7aPP0Vmx+hNjYMPIQ8Y5w==
       Coefficient:     Je5YhYpUron/WdOXjxNAxDubAp3i5X7UOUfhJcyIggqwY86IE0Q
                        /Bk0Dw4SC9zxnsimmdBXW2Izd8Lwuk8FQcQ==

       The DNSKEY record for this key would be:

       example.net.    3600  IN  DNSKEY  (256 3 10 AwEAAdHoNTOW+et86KuJOWRD
                       p1pndvwb6Y83nSVXXyLA3DLroROUkN6X0O6pnWnjJQujX/AyhqFD
                       xj13tOnD9u/1kTg7cV6rklMrZDtJCQ5PCl/D7QNPsgVsMu1J2Q8g
                       pMpztNFLpPBz1bWXjDtaR7ZQBlZ3PFY12ZTSncorffcGmhOL
                       );{id = 3740 (zsk), size = 1024b}

       With this key, sign the following RRSet, consisting of 1 A record:

       www.example.net. 3600  IN  A  192.0.2.91

       If the inception date is set at 00:00 hours on January 1st, 2000, and
       the expiration date at 00:00 hours on January 1st, 2030, the
       following signature should be created:

       www.example.net. 3600  IN  RRSIG  (A 10 3 3600 20300101000000
                        20000101000000 3740 example.net. tsb4wnjRUDnB1BUi+t
                        6TMTXThjVnG+eCkWqjvvjhzQL1d0YRoOe0CbxrVDYd0xDtsuJRa
                        eUw1ep94PzEWzr0iGYgZBWm/zpq+9fOuagYJRfDqfReKBzMweOL
                        DiNa8iP5g9vMhpuv6OPlvpXwm9Sa9ZXIbNl1MBGk0fthPgxdDLw
                        =);{id = 3740}

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    7.  IANA Considerations

       This document updates the IANA registry "DNS SECURITY ALGORITHM
       NUMBERS -- per [RFC4035]" (http://www.iana.org/protocols).  The
       following entries are added to the registry:

                                                 Zone  Trans.
       Value   Description       Mnemonic    Signing    Sec.   References
         8     RSA/SHA-256      RSASHA256          Y      *    RFC 5702
        10     RSA/SHA-512      RSASHA512          Y      *    RFC 5702

       * There has been no determination of standardization of the use of
         this algorithm with Transaction Security.

    8.  Security Considerations

    8.1.  SHA-1 versus SHA-2 Considerations for RRSIG Resource Records

       Users of DNSSEC are encouraged to deploy SHA-2 as soon as software
       implementations allow for it.  SHA-2 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 DNSSEC, it is believed (at the
       time of this writing) that SHA-2 is the better choice for use in
       DNSSEC records.

       SHA-2 is considered sufficiently strong for the immediate future, but
       predictions about future development in cryptography and
       cryptanalysis are beyond the scope of this document.

       The signature scheme RSASSA-PKCS1-v1_5 is chosen to match the one
       used for RSA/SHA-1 signatures.  This should ease implementation of
       the new hashing algorithms in DNSSEC software.

    8.2.  Signature Type Downgrade Attacks

       Since each RRSet MUST be signed with each algorithm present in the
       DNSKEY RRSet at the zone apex (see Section 2.2 of [RFC4035]), a
       malicious party cannot filter out the RSA/SHA-2 RRSIG and force the
       validator to use the RSA/SHA-1 signature if both are present in the
       zone.  This should provide resilience against algorithm downgrade
       attacks, if the validator supports RSA/SHA-2.

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

    9.  Acknowledgments

       This document is a minor extension to [RFC4034].  Also, we try to
       follow the documents [RFC3110] and [RFC4509] for consistency.  The
       authors of and contributors to these documents are gratefully
       acknowledged for their hard work.

       The following people provided additional feedback and text: Jaap
       Akkerhuis, Mark Andrews, Roy Arends, Rob Austein, Francis Dupont,
       Miek Gieben, Alfred Hoenes, Paul Hoffman, Peter Koch, Scott Rose,
       Michael St. Johns, and Wouter Wijngaards.

    10.  References

    10.1.  Normative References

       [FIPS.180-3.2008]
                  National Institute of Standards and Technology, "Secure
                  Hash Standard", FIPS PUB 180-3, October 2008.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC3110]  Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain
                  Name System (DNS)", RFC 3110, May 2001.

       [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.

    10.2.  Informative References

       [NIST800-57]
                  Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
                  "Recommendations for Key Management", NIST SP 800-57,
                  March 2007.

       [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                  Standards (PKCS) #1: RSA Cryptography Specifications
                  Version 2.1", RFC 3447, February 2003.

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    RFC 5702                    DNSSEC RSA/SHA-2                October 2009

       [RFC4509]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
                  (DS) Resource Records (RRs)", RFC 4509, May 2006.

       [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
                  Security (DNSSEC) Hashed Authenticated Denial of
                  Existence", RFC 5155, March 2008.

    Author's Address

       Jelte Jansen
       NLnet Labs
       Science Park 140
       1098 XG Amsterdam
       NL

       EMail: jelte@NLnetLabs.nl
       URI:   http://www.nlnetlabs.nl/

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