DNSSEC Algorithms: Algorithm Implementation Requirements and Usage Guidance for DNSSEC

Internet Engineering Task Force (IETF)                        P. Wouters
Request for Comments: 8624                                       Red Hat
Obsoletes: 6944                                                  O. Sury
Category: Standards Track                    Internet Systems Consortium
ISSN: 2070-1721                                                June 2019


  Algorithm Implementation Requirements and Usage Guidance for DNSSEC

Abstract

   The DNSSEC protocol makes use of various cryptographic algorithms in
   order to provide authentication of DNS data and proof of
   nonexistence.  To ensure interoperability between DNS resolvers and
   DNS authoritative servers, it is necessary to specify a set of
   algorithm implementation requirements and usage guidelines to ensure
   that there is at least one algorithm that all implementations
   support.  This document defines the current algorithm implementation
   requirements and usage guidance for DNSSEC.  This document obsoletes
   RFC 6944.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8624.

Copyright Notice

   Copyright (c) 2019 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
   (https://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 Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Updating Algorithm Implementation Requirements and Usage
           Guidance  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Updating Algorithm Requirement Levels . . . . . . . . . .   3
     1.3.  Document Audience . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  Algorithm Selection . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  DNSKEY Algorithms . . . . . . . . . . . . . . . . . . . .   5
     3.2.  DNSKEY Algorithm Recommendation . . . . . . . . . . . . .   6
     3.3.  DS and CDS Algorithms . . . . . . . . . . . . . . . . . .   7
     3.4.  DS and CDS Algorithm Recommendation . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The DNSSEC signing algorithms are defined by various RFCs, including
   [RFC4034], [RFC5155], [RFC5702], [RFC5933], [RFC6605], and [RFC8080].
   DNSSEC is used to provide authentication of data.  To ensure
   interoperability, a set of "mandatory-to-implement" DNSKEY algorithms
   are defined.  This document obsoletes [RFC6944].

1.1.  Updating Algorithm Implementation Requirements and Usage Guidance

   The field of cryptography evolves continuously.  New, stronger
   algorithms appear, and existing algorithms are found to be less
   secure than originally thought.  Attacks previously thought to be
   computationally infeasible become more accessible as the available
   computational resources increase.  Therefore, algorithm
   implementation requirements and usage guidance need to be updated
   from time to time to reflect the new reality.  The choices for
   algorithms must be conservative to minimize the risk of algorithm
   compromise.

1.2.  Updating Algorithm Requirement Levels

   The mandatory-to-implement algorithm of tomorrow should already be
   available in most implementations of DNSSEC by the time it is made
   mandatory.  This document attempts to identify and introduce those
   algorithms for future mandatory-to-implement status.  There is no
   guarantee that algorithms in use today will become mandatory in the
   future.  Published algorithms are continuously subjected to
   cryptographic attack and may become too weak or even be completely
   broken before this document is updated.

   This document only provides recommendations with respect to
   mandatory-to-implement algorithms or algorithms so weak that they
   cannot be recommended.  Any algorithm listed in the [DNSKEY-IANA] and
   [DS-IANA] registries that are not mentioned in this document MAY be
   implemented.  For clarification and consistency, an algorithm will be
   specified as MAY in this document only when it has been downgraded
   from a MUST or a RECOMMENDED to a MAY.

   Although this document's primary purpose is to update algorithm
   recommendations to keep DNSSEC authentication secure over time, it
   also aims to do so in such a way that DNSSEC implementations remain
   interoperable.  DNSSEC interoperability is addressed by an
   incremental introduction or deprecation of algorithms.

   [RFC2119] considers the term SHOULD equivalent to RECOMMENDED, and
   SHOULD NOT equivalent to NOT RECOMMENDED.  The authors of this
   document have chosen to use the terms RECOMMENDED and NOT
   RECOMMENDED, as this more clearly expresses the intent to
   implementers.

   It is expected that deprecation of an algorithm will be performed
   gradually in a series of updates to this document.  This provides
   time for various implementations to update their implemented
   algorithms while remaining interoperable.  Unless there are strong
   security reasons, an algorithm is expected to be downgraded from MUST
   to NOT RECOMMENDED or MAY, instead of to MUST NOT.  Similarly, an
   algorithm that has not been mentioned as mandatory-to-implement is
   expected to be introduced with a RECOMMENDED instead of a MUST.

   Since the effect of using an unknown DNSKEY algorithm is that the
   zone is treated as insecure, it is recommended that algorithms
   downgraded to NOT RECOMMENDED or lower not be used by authoritative
   nameservers and DNSSEC signers to create new DNSKEYs.  This will
   allow for deprecated algorithms to become less and less common over
   time.  Once an algorithm has reached a sufficiently low level of
   deployment, it can be marked as MUST NOT so that recursive resolvers
   can remove support for validating it.

   Recursive nameservers are encouraged to retain support for all
   algorithms not marked as MUST NOT.

1.3.  Document Audience

   The recommendations of this document mostly target DNSSEC
   implementers, as implementations need to meet both high security
   expectations as well as high interoperability between various vendors
   and with different versions.  Interoperability requires a smooth
   transition to more secure algorithms.  This perspective may differ
   from that of a user who wishes to deploy and configure DNSSEC with
   only the safest algorithm.  On the other hand, the comments and
   recommendations in this document are also expected to be useful for
   such users.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Algorithm Selection

3.1.  DNSKEY Algorithms

   The following table lists the implementation recommendations for
   DNSKEY algorithms [DNSKEY-IANA].

   +--------+--------------------+-----------------+-------------------+
   | Number | Mnemonics          | DNSSEC Signing  | DNSSEC Validation |
   +--------+--------------------+-----------------+-------------------+
   | 1      | RSAMD5             | MUST NOT        | MUST NOT          |
   | 3      | DSA                | MUST NOT        | MUST NOT          |
   | 5      | RSASHA1            | NOT RECOMMENDED | MUST              |
   | 6      | DSA-NSEC3-SHA1     | MUST NOT        | MUST NOT          |
   | 7      | RSASHA1-NSEC3-SHA1 | NOT RECOMMENDED | MUST              |
   | 8      | RSASHA256          | MUST            | MUST              |
   | 10     | RSASHA512          | NOT RECOMMENDED | MUST              |
   | 12     | ECC-GOST           | MUST NOT        | MAY               |
   | 13     | ECDSAP256SHA256    | MUST            | MUST              |
   | 14     | ECDSAP384SHA384    | MAY             | RECOMMENDED       |
   | 15     | ED25519            | RECOMMENDED     | RECOMMENDED       |
   | 16     | ED448              | MAY             | RECOMMENDED       |
   +--------+--------------------+-----------------+-------------------+

   RSAMD5 is not widely deployed, and there is an industry-wide trend to
   deprecate MD5 usage.

   RSASHA1 and RSASHA1-NSEC3-SHA1 are widely deployed, although the
   zones deploying it are recommended to switch to ECDSAP256SHA256 as
   there is an industry-wide trend to move to elliptic curve
   cryptography.  RSASHA1 does not support NSEC3.  RSASHA1-NSEC3-SHA1
   can be used with or without NSEC3.

   DSA and DSA-NSEC3-SHA1 are not widely deployed and are vulnerable to
   private key compromise when generating signatures using a weak or
   compromised random number generator.

   RSASHA256 is widely used and considered strong.  It has been the
   default algorithm for a number of years and is now slowly being
   replaced with ECDSAP256SHA256 due to its shorter key and signature
   size, resulting in smaller DNS packets.

   RSASHA512 is NOT RECOMMENDED for DNSSEC signing because it has not
   seen wide deployment, but there are some deployments; hence, DNSSEC
   validation MUST implement RSASHA512 to ensure interoperability.
   There is no significant difference in cryptographic strength between
   RSASHA512 and RSASHA256; therefore, use of RSASHA512 is discouraged
   as it will only make deprecation of older algorithms harder.  People
   who wish to use a cryptographically stronger algorithm should switch
   to elliptic curve cryptography algorithms.

   ECC-GOST (GOST R 34.10-2001) has been superseded by GOST R 34.10-2012
   in [RFC7091].  GOST R 34.10-2012 hasn't been standardized for use in
   DNSSEC.

   ECDSAP256SHA256 provides more cryptographic strength with a shorter
   signature length than either RSASHA256 or RSASHA512.  ECDSAP256SHA256
   has been widely deployed; therefore, it is now at MUST level for both
   validation and signing.  It is RECOMMENDED to use the deterministic
   digital signature generation procedure of the Elliptic Curve Digital
   Signature Algorithm (ECDSA), specified in [RFC6979], when
   implementing ECDSAP256SHA256 (and ECDSAP384SHA384).

   ECDSAP384SHA384 shares the same properties as ECDSAP256SHA256 but
   offers a modest security advantage over ECDSAP256SHA256 (192 bits of
   strength versus 128 bits).  For most DNSSEC applications,
   ECDSAP256SHA256 should be satisfactory and robust for the foreseeable
   future and is therefore recommended for signing.  While it is
   unlikely for a DNSSEC use case requiring 192-bit security strength to
   arise, ECDSA384SHA384 is provided for such applications, and it MAY
   be used for signing in these cases.

   ED25519 and ED448 use the Edwards-curve Digital Security Algorithm
   (EdDSA).  There are three main advantages of EdDSA: it does not
   require the use of a unique random number for each signature, there
   are no padding or truncation issues as with ECDSA, and it is more
   resilient to side-channel attacks.  Furthermore, EdDSA cryptography
   is less prone to implementation errors ([RFC8032], [RFC8080]).  It is
   expected that ED25519 will become the future RECOMMENDED default
   algorithm once there's enough support for this algorithm in the
   deployed DNSSEC validators.

3.2.  DNSKEY Algorithm Recommendation

   Due to the industry-wide trend towards elliptic curve cryptography,
   ECDSAP256SHA256 is the RECOMMENDED DNSKEY algorithm for use by new
   DNSSEC deployments, and users of RSA-based algorithms SHOULD upgrade
   to ECDSAP256SHA256.

3.3.  DS and CDS Algorithms

   The following table lists the recommendations for Delegation Signer
   Digest Algorithms [DS-IANA].  These recommendations also apply to the
   Child Delegation Signer (CDS) RRTYPE as specified in [RFC7344].

   +--------+-----------------+-------------------+-------------------+
   | Number | Mnemonics       | DNSSEC Delegation | DNSSEC Validation |
   +--------+-----------------+-------------------+-------------------+
   | 0      | NULL (CDS only) | MUST NOT [*]      | MUST NOT [*]      |
   | 1      | SHA-1           | MUST NOT          | MUST              |
   | 2      | SHA-256         | MUST              | MUST              |
   | 3      | GOST R 34.11-94 | MUST NOT          | MAY               |
   | 4      | SHA-384         | MAY               | RECOMMENDED       |
   +--------+-----------------+-------------------+-------------------+

   [*] - This is a special type of CDS record signaling removal of DS at
                         the parent in [RFC8078].

   NULL is a special case; see [RFC8078].

   SHA-1 is still widely used for Delegation Signer (DS) records, so
   validators MUST implement validation, but it MUST NOT be used to
   generate new DS and CDS records (see "Operational Considerations" for
   caveats when upgrading from the SHA-1 to SHA-256 DS algorithm.)

   SHA-256 is widely used and considered strong.

   GOST R 34.11-94 has been superseded by GOST R 34.11-2012 in
   [RFC6986].  GOST R 34.11-2012 has not been standardized for use in
   DNSSEC.

   SHA-384 shares the same properties as SHA-256 but offers a modest
   security advantage over SHA-256 (384 bits of strength versus 256
   bits).  For most applications of DNSSEC, SHA-256 should be
   satisfactory and robust for the foreseeable future and is therefore
   recommended for DS and CDS records.  While it is unlikely for a
   DNSSEC use case requiring 384-bit security strength to arise, SHA-384
   is provided for such applications, and it MAY be used for generating
   DS and CDS records in these cases.

3.4.  DS and CDS Algorithm Recommendation

   An operational recommendation for new and existing deployments:
   SHA-256 is the RECOMMENDED DS and CDS algorithm.

4.  Security Considerations

   The security of cryptographic systems depends on both the strength of
   the cryptographic algorithms chosen and the strength of the keys used
   with those algorithms.  The security also depends on the engineering
   of the protocol used by the system to ensure that there are no non-
   cryptographic ways to bypass the security of the overall system.

   This document concerns itself with the selection of cryptographic
   algorithms for use in DNSSEC, specifically with the selection of
   "mandatory-to-implement" algorithms.  The algorithms identified in
   this document as MUST or RECOMMENDED to implement are not known to be
   broken (in the cryptographic sense) at the current time, and
   cryptographic research so far leads us to believe that they are
   likely to remain secure into the foreseeable future.  However, this
   isn't necessarily forever, and it is expected that new revisions of
   this document will be issued from time to time to reflect the current
   best practices in this area.

   Retiring an algorithm too soon would result in a zone (signed with a
   retired algorithm) being downgraded to the equivalent of an unsigned
   zone.  Therefore, algorithm deprecation must be done very slowly and
   only after careful consideration and measurement of its use.

5.  Operational Considerations

   DNSKEY algorithm rollover in a live zone is a complex process.  See
   [RFC6781] and [RFC7583] for guidelines on how to perform algorithm
   rollovers.

   DS algorithm rollover in a live zone is also a complex process.
   Upgrading an algorithm at the same time as rolling a new Key Signing
   Key (KSK) will lead to DNSSEC validation failures.  Administrators
   MUST complete the process of the DS algorithm upgrade before starting
   a rollover process for a new KSK.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <https://www.rfc-editor.org/info/rfc5155>.

   [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC", RFC 5702,
              DOI 10.17487/RFC5702, October 2009,
              <https://www.rfc-editor.org/info/rfc5702>.

   [RFC6605]  Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital
              Signature Algorithm (DSA) for DNSSEC", RFC 6605,
              DOI 10.17487/RFC6605, April 2012,
              <https://www.rfc-editor.org/info/rfc6605>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC6986]  Dolmatov, V., Ed. and A. Degtyarev, "GOST R 34.11-2012:
              Hash Function", RFC 6986, DOI 10.17487/RFC6986, August
              2013, <https://www.rfc-editor.org/info/rfc6986>.

   [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
              DNSSEC Delegation Trust Maintenance", RFC 7344,
              DOI 10.17487/RFC7344, September 2014,
              <https://www.rfc-editor.org/info/rfc7344>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8078]  Gudmundsson, O. and P. Wouters, "Managing DS Records from
              the Parent via CDS/CDNSKEY", RFC 8078,
              DOI 10.17487/RFC8078, March 2017,
              <https://www.rfc-editor.org/info/rfc8078>.

   [RFC8080]  Sury, O. and R. Edmonds, "Edwards-Curve Digital Security
              Algorithm (EdDSA) for DNSSEC", RFC 8080,
              DOI 10.17487/RFC8080, February 2017,
              <https://www.rfc-editor.org/info/rfc8080>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [RFC5933]  Dolmatov, V., Ed., Chuprina, A., and I. Ustinov, "Use of
              GOST Signature Algorithms in DNSKEY and RRSIG Resource
              Records for DNSSEC", RFC 5933, DOI 10.17487/RFC5933, July
              2010, <https://www.rfc-editor.org/info/rfc5933>.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,
              <https://www.rfc-editor.org/info/rfc6781>.

   [RFC6944]  Rose, S., "Applicability Statement: DNS Security (DNSSEC)
              DNSKEY Algorithm Implementation Status", RFC 6944,
              DOI 10.17487/RFC6944, April 2013,
              <https://www.rfc-editor.org/info/rfc6944>.

   [RFC7091]  Dolmatov, V., Ed. and A. Degtyarev, "GOST R 34.10-2012:
              Digital Signature Algorithm", RFC 7091,
              DOI 10.17487/RFC7091, December 2013,
              <https://www.rfc-editor.org/info/rfc7091>.

   [RFC7583]  Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
              "DNSSEC Key Rollover Timing Considerations", RFC 7583,
              DOI 10.17487/RFC7583, October 2015,
              <https://www.rfc-editor.org/info/rfc7583>.

   [DNSKEY-IANA]
              IANA, "Domain Name System Security (DNSSEC) Algorithm
              Numbers",
              <http://www.iana.org/assignments/dns-sec-alg-numbers>.

   [DS-IANA]  IANA, "Delegation Signer (DS) Resource Record (RR) Type
              Digest Algorithms",
              <http://www.iana.org/assignments/ds-rr-types>.

Acknowledgements

   This document borrows text from RFC 4307 by Jeffrey I. Schiller of
   the Massachusetts Institute of Technology (MIT) and RFC 8247 by Yoav
   Nir, Tero Kivinen, Paul Wouters, and Daniel Migault.  Much of the
   original text has been copied verbatim.

   We wish to thank Michael Sinatra, Roland van Rijswijk-Deij, Olafur
   Gudmundsson, Paul Hoffman, Evan Hunt, and Peter Yee for their
   feedback.

   Kudos to Roy Arends for bringing the DS rollover issue to light.

Authors' Addresses

   Paul Wouters
   Red Hat
   Canada

   Email: pwouters@redhat.com


   Ondrej Sury
   Internet Systems Consortium
   Czech Republic

   Email: ondrej@isc.org