Internet Engineering Task Force (IETF) P. Wouters

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.

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RFC 8624 DNSSEC Cryptographic Algorithms June 2019

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

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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

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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.

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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

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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.

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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.

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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.

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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>.

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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 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>.

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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 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: [email protected]

Ondrej Sury
Internet Systems Consortium
Czech Republic

Email: [email protected]

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