DNS Extensions working group                                   J. Jansen
Internet-Draft                                                NLnet Labs
Intended status: Standards Track                       December 04, 2008                        January 08, 2009
Expires: June 7, July 12, 2009

 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records
                               for DNSSEC
                 draft-ietf-dnsext-dnssec-rsasha256-09

                 draft-ietf-dnsext-dnssec-rsasha256-10

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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 (DNSSEC, RFC 4033, RFC 4034, and RFC 4035).

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  DNSKEY Resource Records . . . . . . . . . . . . . . . . . . . . 3
     2.1.  RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . 3
     2.2.  RSA/SHA-512 DNSKEY Resource Records . . . . . . . . . . . . 4 3
   3.  RRSIG Resource Records  . . . . . . . . . . . . . . . . . . . . 4
     3.1.  RSA/SHA-256 RRSIG Resource Records  . . . . . . . . . . . . 4
     3.2.  RSA/SHA-512 RRSIG Resource Records  . . . . . . . . . . . . 5 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
       5.2.1.  NSEC3 in Authoritative servers  . . . . . . . . . . . . 5
       5.2.2.  NSEC3 in Validators . . . . . . . . . . . . . . . . . . 5

   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
     7.1.  SHA-1 versus SHA-2 Considerations for RRSIG Resource
           Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.2.  Signature Type Downgrade Attacks  . . . . . . . . . . . . . 6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 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.  RFC 4033 [RFC4033], RFC 4034
   [RFC4034], and RFC 4035 [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.

   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 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 RFC 4034 [RFC4034].  RFC
   3110 [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 {TBA1}.

   For use with NSEC3 [RFC5155], the algorithm number for RSA/SHA-256
   will be {TBA2}.  The use of a different algorithm number to
   differentiate between the use of NSEC and NSEC3 is in keeping with
   the approach adopted in RFC5155.

   For interoperability, as in RFC 3110 [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 {TBA3}.

   For use with NSEC3, the algorithm number for RSA/SHA-512 will be
   {TBA4}.  The use of a different algorithm number to differentiate
   between the use of NSEC and NSEC3 is in keeping with the approach
   adopted in RFC5155. {TBA2}.

   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 RFC 4034 [RFC4034].

   hash = SHA-XXX(data)

   Here XXX is either 256 or 512, depending on the algorithm used, as
   specified in FIPS PUB 180-3 [FIPS.180-3.2008], and "data" is the wire
   format data of the resource record set that is signed, as specified
   in RFC 4034 [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
   [RFC3447], and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2
   [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 {TBA1} for use with NSEC, or
   {TBA2} for use with NSEC3. {TBA1}.

   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 {TBA3} for use with NSEC, or
   {TBA4} for use with NSEC3. {TBA2}.

   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

4.  Deployment Considerations

4.1.  Key Sizes

   Apart from the restrictions specified 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 the hashing algorithm used in
   the signing process.  Therefore, RRSIG resource records produced with
   RSA/SHA256 or RSA/SHA512 will have the same size as those produced
   with RSA/SHA1, 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 resource
   records with the RSA/SHA-2 algorithms.

5.2.  Support for NSEC3 Denial of Existence

   Note that these algorithms have no aliases to signal NSEC3 [RFC5155]
   denial of existence.  The aliases mechanism used in RFC 5155 was to
   protect implementations predating that RFC from encountering records
   they could not know about.

5.2.1.  NSEC3 in Authoritative servers

   An authoritative server that does not implement NSEC3 MAY still serve
   zones that use RSA/SHA2 with NSEC.

5.2.2.  NSEC3 in Validators

   A DNSSEC validator that implements RSA/SHA2 MUST be able to handle
   both NSEC and NSEC3 [RFC5155] negative answers.  If this is not the
   case, the validator MUST treat a zone signed with RSA/SHA256 or RSA/
   SHA512 as signed with an unknown algorithm, and thus as insecure.

6.  IANA Considerations

   This document updates the IANA registry "DNS SECURITY ALGORITHM
   NUMBERS -- per [RFC4035]" [RFC4035] "

   (http://www.iana.org/assignments/dns-sec-alg-numbers).  The following
   entries are added to the registry:

                                                     Zone
   Value      Algorithm               Mnemonic    Signing  References
   {TBA1}   RSA/SHA-256              RSASHA256          y {this memo}
   {TBA2}   RSA/SHA-256-NSEC3   RSASHA256NSEC3          y {this memo}
   {TBA3}   RSA/SHA-512              RSASHA512          y {this memo}
   {TBA4}   RSA/SHA-512-NSEC3   RSASHA512NSEC3          y {this memo}

7.  Security Considerations

7.1.  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.

7.2.  Signature Type Downgrade Attacks

   Since each RRSet MUST be signed with each algorithm present in the
   DNSKEY RRSet at the zone apex (see [RFC4035] Section 2.2), 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.

8.  Acknowledgments

   This document is a minor extension to RFC 4034 [RFC4034].  Also, we
   try to follow the documents RFC 3110 [RFC3110] and RFC 4509 [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, Roy Arends, Rob Austein, Francis Dupont, Miek Gieben,
   Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. Johns, Scott
   Rose and Wouter Wijngaards.

9.  References

9.1.  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", 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.

9.2.  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.

   [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
   Kruislaan 419
   Amsterdam  1098VA
   NL

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

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