Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms

Internet Engineering Task Force (IETF)                       W. Hardaker
Request for Comments: 9905                                       USC/ISI
Updates: 4034, 5155                                            W. Kumari
Category: Standards Track                                         Google
ISSN: 2070-1721                                            November 2025


      Deprecating the Use of SHA-1 in DNSSEC Signature Algorithms

Abstract

   This document deprecates the use of the RSASHA1 and
   RSASHA1-NSEC3-SHA1 algorithms for the creation of DNS Public Key
   (DNSKEY) and Resource Record Signature (RRSIG) records.

   It updates RFCs 4034 and 5155 as it deprecates the use of these
   algorithms.

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/rfc9905.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Requirements Notation
   2.  Deprecating SHA-1 from DNSSEC Signatures and Delegation RRs
   3.  Security Considerations
   4.  Operational Considerations
   5.  IANA Considerations
   6.  Normative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The security of the protection provided by the SHA-1 algorithm
   [RFC3174] has been slowly diminishing over time as various forms of
   attacks have weakened its cryptographic underpinning.  DNSSEC
   [RFC9364] (originally defined in [RFC3110]) made extensive use of
   SHA-1, for example, as a cryptographic hash algorithm in Resource
   Record Signature (RRSIG) and Delegation Signer (DS) records.  Since
   then, multiple other algorithms with stronger cryptographic strength
   have become widely available for DS records and for RRSIG and DNS
   Public Key (DNSKEY) records [RFC4034].  Operators are encouraged to
   consider switching to one of the recommended algorithms listed in the
   "DNS Security Algorithm Numbers" [DNSKEY-IANA] and "DNS Security
   Algorithm Numbers" [DS-IANA] registries, respectively.  Further,
   support for validating SHA-1-based signatures has been removed from
   some systems.  As a result, SHA-1 as part of a signature algorithm is
   no longer fully interoperable in the context of DNSSEC.  As adequate
   alternatives exist, the use of SHA-1 is no longer advisable.

   This document thus deprecates the use of RSASHA1 and
   RSASHA1-NSEC3-SHA1 for DNS Security Algorithms.

1.1.  Requirements Notation

   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.

2.  Deprecating SHA-1 from DNSSEC Signatures and Delegation RRs

   The RSASHA1 [RFC4034] and RSASHA1-NSEC3-SHA1 [RFC5155] algorithms
   MUST NOT be used when creating DS records.  Operators of validating
   resolvers MUST treat RSASHA1 and RSASHA1-NSEC3-SHA1 DS records as
   insecure.  If no other DS records of accepted cryptographic
   algorithms are available, the DNS records below the delegation point
   MUST be treated as insecure.

   The RSASHA1 [RFC4034] and RSASHA1-NSEC3-SHA1 [RFC5155] algorithms
   MUST NOT be used when creating DNSKEY and RRSIG records.  Validating
   resolver implementations ([RFC9499], Section 10) MUST continue to
   support validation using these algorithms as they are diminishing in
   use but still actively in use for some domains as of this
   publication.  Operators of validating resolvers MUST treat DNSSEC
   signing algorithms RSASHA1 and RSASHA1-NSEC3-SHA1 as unsupported,
   rendering responses insecure if they cannot be validated by other
   supported signing algorithms.

3.  Security Considerations

   This document deprecates the use of RSASHA1 and RSASHA1-NSEC3-SHA1
   for DNSSEC delegation and DNSSEC signing since these algorithms are
   no longer considered to be secure.

4.  Operational Considerations

   Zone owners currently making use of SHA-1-based algorithms should
   immediately switch to algorithms with stronger cryptographic
   algorithms, such as the recommended algorithms in the IANA registries
   [DNSKEY-IANA] [DS-IANA].

   Operators should take care when deploying software packages and
   operating systems that may have already removed support for the SHA-1
   algorithm.  In these situations, software may need to be manually
   built and deployed by an operator to continue supporting the required
   levels indicated by the "Use for DNSSEC Validation" and "Implement
   for DNSSEC Validation" columns, which this document is not changing.

5.  IANA Considerations

   IANA has updated the SHA-1 (1) entry in the "Digest Algorithms"
   registry [DS-IANA] [RFC9904] as follows and has added this document
   as a reference for the entry:

   Value:  1
   Description:  SHA-1
   Use for DNSSEC Delegation:  MUST NOT
   Use for DNSSEC Validation:  RECOMMENDED
   Implement for DNSSEC Delegation:  MUST NOT
   Implement for DNSSEC Validation:  MUST

   IANA has updated the RSASHA1 (5) and RSASHA1-NSEC3-SHA1 (7) algorithm
   entries in the "DNS Security Algorithm Numbers" registry
   [DNSKEY-IANA] [RFC9904] as follows and has added this document as a
   reference for the entries:

   Number:  5
   Description:  RSA/SHA-1
   Mnemonic:  RSASHA1
   Zone Signing:  Y
   Trans.  Sec.:  Y
   Use for DNSSEC Signing:  MUST NOT
   Use for DNSSEC Validation:  RECOMMENDED
   Implement for DNSSEC Signing:  NOT RECOMMENDED
   Implement for DNSSEC Validation:  MUST

   Number:  7
   Description:  RSASHA1-NSEC3-SHA1
   Mnemonic:  RSASHA1-NSEC3-SHA1
   Zone Signing:  Y
   Trans.  Sec.:  Y
   Use for DNSSEC Signing:  MUST NOT
   Use for DNSSEC Validation:  RECOMMENDED
   Implement for DNSSEC Signing:  NOT RECOMMENDED
   Implement for DNSSEC Validation:  MUST

6.  Normative References

   [DNSKEY-IANA]
              IANA, "DNS Security Algorithm Numbers",
              <https://www.iana.org/assignments/dns-sec-alg-numbers>.

   [DS-IANA]  IANA, "Digest Algorithms",
              <http://www.iana.org/assignments/ds-rr-types>.

   [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>.

   [RFC3110]  Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
              Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110,
              May 2001, <https://www.rfc-editor.org/info/rfc3110>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <https://www.rfc-editor.org/info/rfc3174>.

   [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>.

   [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>.

   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/info/rfc9364>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [RFC9904]  Hardaker, W. and W. Kumari, "DNSSEC Cryptographic
              Algorithm Recommendation Update Process", RFC 9904,
              DOI 10.17487/RFC9904, November 2025,
              <https://www.rfc-editor.org/info/rfc9904>.

Acknowledgments

   The authors appreciate the comments and suggestions from the
   following IETF participants in helping produce this document: Mark
   Andrews, Steve Crocker, Peter Dickson, Thomas Graf, Paul Hoffman,
   Russ Housley, Shumon Huque, Barry Leiba, S. Moonesamy, Yoav Nir,
   Florian Obser, Peter Thomassen, Stefan Ubbink, Paul Wouters, Tim
   Wicinski, and the many members of the DNSOP Working Group that
   discussed this specification.

Authors' Addresses

   Wes Hardaker
   USC/ISI
   Email: ietf@hardakers.net


   Warren Kumari
   Google
   Email: warren@kumari.net