dnsop P. Wouters
Internet-Draft Red Hat
Obsoletes: 6944 (if approved) O. Sury
Intended status: Standards Track CZ.NIC
Expires: April 14, 2017 October 11, 2016

Algorithm Implementation Requirements and Usage Guidance for DNSSEC


The DNSSEC protocol makes use of various cryptographic algorithms in order to provide authentication of DNS data and proof of non-existence. To ensure interoperability between DNS resolvers and DNS authoritative servers, it is necessary to specify a set of algorithm implementation requirements and usage guidance 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 14, 2017.

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Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

The DNSSEC signing algorithms are defined by various RFCs, including [RFC4034], [RFC5155], [RFC5702], [RFC5933], [RFC6605], [I-D.ietf-curdle-dnskey-eddsa]. 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 then originally thought. 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 the algorithms in use today may become mandatory in the future. Published algorithms are continuously subjected to cryptographic attack and may become too weak or could become completely broken before this document is updated.

This document only provides recommendations for the mandatory-to-implement algorithms or algorithms too weak that are recommended not to be implemented. As a result, any algorithm listed at the [DNSKEY-IANA] and [DS-IANA] registries not mentioned in this document MAY be implemented. For clarification and consistency, an algorithm will be set to MAY only when it has been downgraded.

Although this document updates the algorithms to keep the DNSSEC authentication secure over time, it also aims at providing recommendations so that DNSSEC implementations remain interoperable. DNSSEC interoperability is addressed by an incremental introduction or deprecation of algorithms.

It is expected that deprecation of an algorithm is performed gradually. 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 MUST- or SHOULD, instead of MUST NOT. Similarly, an algorithm that has not been mentioned as mandatory-to-implement is expected to be introduced with a SHOULD instead of a MUST.

Since the effects of using an unknown DNSKEY algorithm is for the zone to be treated as insecure, it is recommended that algorithms downgraded to SHOULD- or below are no longer used by authoritative nameservers and DNSSEC signers to create new DNSKEY's. This will allow for algorithms to slowly become more unused over time. Once deployment has reached a sufficiently low point these algorithms can finally be marked as MUST NOT so that recursive nameservers can remove support for these algorithms.

Recursive nameservers are encouraged to keep 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 move to more secure algorithms. This may differ from a user point of view that may deploy and configure DNSSEC with only the safest algorithm. On the other hand, comments and recommendations from 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", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

We define some additional terms here:

SHOULD+ This term means the same as SHOULD. However, it is likely that an algorithm marked as SHOULD+ will be promoted at some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, an algorithm marked as SHOULD- may be deprecated to a MAY in a future version of this document.
MUST- This term means the same as MUST. However, it is expected at some point in the near future that this algorithm will no longer be a MUST in a future document. Although its status will be determined at a later time, it is reasonable to expect that if a future revision of a document alters the status of a MUST- algorithm, it will remain at least a SHOULD or a SHOULD-.

3. Algorithm Selection

3.1. DNSKEY Algorithms

Recommendations for DNSKEY algorithms [DNSKEY-IANA]

Number Mnemonics DNSSEC Signing DNSSEC Validation

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 zones deploying it are recommended to switch to RSASHA256 as there is an industry-wide trend to deprecate SHA1 usage. 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 vulnerable to private key compromise when generating signatures using a weak or compromised random number generator.

RSASHA512 is at the SHOULD level for DNSSEC Signing because it has not seen wide deployment, but there are some deployments hence DNSSEC Validation MUST implement RSASHA512 to ensure interoperability.

ECC-GOST is at the SHOULD NOT level because it has not seen wide deployment and the algorithm has not seen wide scrutiny in the crypto community.

ECDSAP256SHA256 and ECDSAP384SHA384 provide more strength for signature size than RSASHA256 and RSASHA512 variants. ECDSAP256SHA256 has seen increased deployment and has been raised to MUST- level for resolving and SHOULD- for signing. It is seen as a temporary improvement over RSA until the [I-D.ietf-curdle-dnskey-eddsa] algorithms are published, implemented and deployed. ECDSAP384SHA384 offers little over ECDSAP256SHA256 and has not seen wide deployment, so the use is discouraged, especially for signing.

ED25519 and ED448 uses Edwards-curve Digital Security Algorithm (EdDSA). There are three main advantages of the EdDSA algorithm: 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. Hence it is expected that these algorithms will be raised to SHOULD for signing and MUST for resolving once it has seen more implementations and deployement.

3.2. DS and CDS Algorithms

Recommendations for Delegation Signer Digest Algorithms [DNSKEY-IANA] These also apply to the CDS RRTYPE as specified in [RFC7344]

Number Mnemonics DNSSEC Delegation DNSSEC Validation
0 NULL (CDS only) MUST NOT [*] MUST NOT [*]
3 GOST R 34.11-94 MAY SHOULD

[*] - This is a special type of CDS record signaling removal of DS at the parent in [I-D.ietf-dnsop-maintain-ds]

NULL is a special case, see [I-D.ietf-dnsop-maintain-ds]

SHA-1 is in wide use for DS records, but its use is discouraged as it is an aging algorithm. Users of SHA-1 SHOULD upgrade to SHA-256.

SHA-256 is in wide use and considered strong.

GOST R 34.11-94 is not in wide use. It is still recommended to be supported in validators so that adoption can increase.

SHA-384 is not in wide use. It is still recommended to be supported in validators so that adoption can increase.

4. Security Considerations

The security of cryptographic-based 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 the use of DNSSEC, specifically with the selection of "mandatory-to-implement" algorithms. The algorithms identified in this document as "MUST implement" or "SHOULD implement" are not known to be broken at the current time, and cryptographic research so far leads us to believe that they will likely 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 practice in this area.

Retiring an algorithm too soon would result in a signed zone with such an algorithm to be downgraded to the equivalent of an unsigned zone. Therefore, algorithm deprecation must be done very slowly and only after careful consideration and measurements 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.

6. IANA Considerations

This document makes no requests of IANA.

7. Acknowledgements

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

We wish to thank Olafur Gudmundsson and Paul Hoffman for their imminent feedback.

8. References

8.1. 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.

8.2. Informative References

[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.
[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.
[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.
[RFC5933] Dolmatov, V., 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.
[RFC6605] Hoffman, P. and W. Wijngaards, "Elliptic Curve Digital Signature Algorithm (DSA) for DNSSEC", RFC 6605, DOI 10.17487/RFC6605, April 2012.
[RFC6781] Kolkman, O., Mekking, W. and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, December 2012.
[RFC6944] Rose, S., "Applicability Statement: DNS Security (DNSSEC) DNSKEY Algorithm Implementation Status", RFC 6944, DOI 10.17487/RFC6944, April 2013.
[RFC7344] Kumari, W., Gudmundsson, O. and G. Barwood, "Automating DNSSEC Delegation Trust Maintenance", RFC 7344, DOI 10.17487/RFC7344, September 2014.
[RFC7583] Morris, S., Ihren, J., Dickinson, J. and W. Mekking, "DNSSEC Key Rollover Timing Considerations", RFC 7583, DOI 10.17487/RFC7583, October 2015.
[I-D.ietf-curdle-dnskey-eddsa] Sury, O. and R. Edmonds, "EdDSA for DNSSEC", Internet-Draft draft-ietf-curdle-dnskey-eddsa-01, October 2016.
[I-D.ietf-dnsop-maintain-ds] Gudmundsson, O. and P. Wouters, "Managing DS records from parent via CDS/CDNSKEY", Internet-Draft draft-ietf-dnsop-maintain-ds-03, June 2016.
[DNSKEY-IANA] , , "DNSKEY Algorithms"
[DS-IANA] , , "Delegation Signer Digest Algorithms"

Authors' Addresses

Paul Wouters Red Hat EMail: pwouters@redhat.com
Ondrej Sury CZ.NIC EMail: ondrej.sury@nic.cz