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Internet Engineering Task Force (IETF)                        P. WoutersRequest for Comments: 8624                                       Red HatObsoletes:6944                                                  O. SuryCategory: Standards Track                    Internet Systems ConsortiumISSN: 2070-1721                                                June 2019Algorithm Implementation Requirements and Usage Guidance for DNSSECAbstract   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 obsoletesRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8624.Wouters & Sury               Standards Track                    [Page 1]

RFC 8624             DNSSEC Cryptographic Algorithms           June 2019Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .3     1.1.  Updating Algorithm Implementation Requirements and Usage           Guidance  . . . . . . . . . . . . . . . . . . . . . . . .31.2.  Updating Algorithm Requirement Levels . . . . . . . . . .31.3.  Document Audience . . . . . . . . . . . . . . . . . . . .42.  Conventions Used in This Document . . . . . . . . . . . . . .43.  Algorithm Selection . . . . . . . . . . . . . . . . . . . . .53.1.  DNSKEY Algorithms . . . . . . . . . . . . . . . . . . . .53.2.  DNSKEY Algorithm Recommendation . . . . . . . . . . . . .63.3.  DS and CDS Algorithms . . . . . . . . . . . . . . . . . .73.4.  DS and CDS Algorithm Recommendation . . . . . . . . . . .74.  Security Considerations . . . . . . . . . . . . . . . . . . .85.  Operational Considerations  . . . . . . . . . . . . . . . . .86.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .87.  References  . . . . . . . . . . . . . . . . . . . . . . . . .97.1.  Normative References  . . . . . . . . . . . . . . . . . .97.2.  Informative References  . . . . . . . . . . . . . . . . .10   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .11   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .11Wouters & Sury               Standards Track                    [Page 2]

RFC 8624             DNSSEC Cryptographic Algorithms           June 20191.  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 NOTWouters & Sury               Standards Track                    [Page 3]

RFC 8624             DNSSEC Cryptographic Algorithms           June 2019   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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.Wouters & Sury               Standards Track                    [Page 4]

RFC 8624             DNSSEC Cryptographic Algorithms           June 20193.  Algorithm Selection3.1.  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 discouragedWouters & Sury               Standards Track                    [Page 5]

RFC 8624             DNSSEC Cryptographic Algorithms           June 2019   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.Wouters & Sury               Standards Track                    [Page 6]

RFC 8624             DNSSEC Cryptographic Algorithms           June 20193.3.  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.Wouters & Sury               Standards Track                    [Page 7]

RFC 8624             DNSSEC Cryptographic Algorithms           June 20194.  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.Wouters & Sury               Standards Track                    [Page 8]

RFC 8624             DNSSEC Cryptographic Algorithms           June 20197.  References7.1.  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>.Wouters & Sury               Standards Track                    [Page 9]

RFC 8624             DNSSEC Cryptographic Algorithms           June 2019   [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 inRFC2119 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>.Wouters & Sury               Standards Track                   [Page 10]

RFC 8624             DNSSEC Cryptographic Algorithms           June 2019   [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 fromRFC 4307 by Jeffrey I. Schiller of   the Massachusetts Institute of Technology (MIT) andRFC 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.orgWouters & Sury               Standards Track                   [Page 11]