7Internet Engineering Task Force (IETF) S. Kitterman
8Request for Comments: 8301 Kitterman Technical Services
9Updates: 6376 January 2018
10Category: Standards Track
14 Cryptographic Algorithm and Key Usage Update to
15 DomainKeys Identified Mail (DKIM)
19 The cryptographic algorithm and key size requirements included when
20 DomainKeys Identified Mail (DKIM) was designed a decade ago are
21 functionally obsolete and in need of immediate revision. This
22 document updates DKIM requirements to those minimally suitable for
23 operation with currently specified algorithms.
27 This is an Internet Standards Track document.
29 This document is a product of the Internet Engineering Task Force
30 (IETF). It represents the consensus of the IETF community. It has
31 received public review and has been approved for publication by the
32 Internet Engineering Steering Group (IESG). Further information on
33 Internet Standards is available in Section 2 of RFC 7841.
35 Information about the current status of this document, any errata,
36 and how to provide feedback on it may be obtained at
37 https://www.rfc-editor.org/info/rfc8301.
41 Copyright (c) 2018 IETF Trust and the persons identified as the
42 document authors. All rights reserved.
44 This document is subject to BCP 78 and the IETF Trust's Legal
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46 (https://trustee.ietf.org/license-info) in effect on the date of
47 publication of this document. Please review these documents
48 carefully, as they describe your rights and restrictions with respect
49 to this document. Code Components extracted from this document must
50 include Simplified BSD License text as described in Section 4.e of
51 the Trust Legal Provisions and are provided without warranty as
52 described in the Simplified BSD License.
58Kitterman Standards Track [Page 1]
60RFC 8301 DKIM Crypto Usage Update January 2018
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
66 2. Conventions Used in This Document . . . . . . . . . . . . . . 2
67 3. Updates to DKIM Signing and Verification Requirements . . . . 3
68 3.1. Signing and Verification Algorithms . . . . . . . . . . . 3
69 3.2. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3
70 4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
71 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
72 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
73 6.1. Normative References . . . . . . . . . . . . . . . . . . 4
74 6.2. Informative References . . . . . . . . . . . . . . . . . 4
75 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
76 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
80 DKIM [RFC6376] signs email messages by creating hashes of the message
81 headers and content and signing the header hash with a digital
82 signature. Message recipients fetch the signature verification key
83 from the DNS where it is stored in a TXT record.
85 The defining documents, RFC 6376 [RFC6376] and its predecessors,
86 specify a single signing algorithm, RSA [RFC8017], and recommend key
87 sizes of 1024 to 2048 bits (but require verification of 512-bit
88 keys). As discussed in US-CERT Vulnerability Note VU#268267
89 [VULNOTE], the operational community has recognized that shorter keys
90 compromise the effectiveness of DKIM. While 1024-bit signatures are
91 common, stronger signatures are not. Widely used DNS configuration
92 software places a practical limit on key sizes, because the software
93 only handles a single 256-octet string in a TXT record, and RSA keys
94 significantly longer than 1024 bits don't fit in 256 octets.
96 Due to the recognized weakness of the SHA-1 hash algorithm (see
97 [RFC6194]) and the wide availability of the SHA-256 hash algorithm
98 (it has been a required part of DKIM [RFC6376] since it was
99 originally standardized in 2007), the SHA-1 hash algorithm MUST NOT
100 be used. This is being done now to allow the operational community
101 time to fully shift to SHA-256 in advance of any SHA-1-related
1042. Conventions Used in This Document
106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
108 "OPTIONAL" in this document are to be interpreted as described in
109 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
110 capitals, as shown here.
114Kitterman Standards Track [Page 2]
116RFC 8301 DKIM Crypto Usage Update January 2018
1193. Updates to DKIM Signing and Verification Requirements
121 This document updates [RFC6376] as follows:
123 o Section 3.1 of this document updates Section 3.3 of [RFC6376].
125 o Section 3.2 of this document updates Section 3.3.3 of [RFC6376].
127 o The algorithm described in Section 3.3.1 of [RFC6376] is now
128 historic and no longer used by DKIM.
130 Sections 3.3.2 and 3.3.4 of [RFC6376] are not affected.
1323.1. Signing and Verification Algorithms
134 DKIM supports multiple digital signature algorithms. Two algorithms
135 are defined by this specification at this time: rsa-sha1 and
136 rsa-sha256. Signers MUST sign using rsa-sha256. Verifiers MUST be
137 able to verify using rsa-sha256. rsa-sha1 MUST NOT be used for
138 signing or verifying.
140 DKIM signatures identified as having been signed with historic
141 algorithms (currently, rsa-sha1) have permanently failed evaluation
142 as discussed in Section 3.9 of [RFC6376].
146 Selecting appropriate key sizes is a trade-off between cost,
147 performance, and risk. Since short RSA keys more easily succumb to
148 off-line attacks, Signers MUST use RSA keys of at least 1024 bits for
149 all keys. Signers SHOULD use RSA keys of at least 2048 bits.
150 Verifiers MUST be able to validate signatures with keys ranging from
151 1024 bits to 4096 bits, and they MAY be able to validate signatures
152 with larger keys. Verifier policies can use the length of the
153 signing key as one metric for determining whether a signature is
154 acceptable. Verifiers MUST NOT consider signatures using RSA keys of
155 less than 1024 bits as valid signatures.
158 with less than 1024 bits) have permanently failed evaluation as
159 discussed in Section 3.9 of [RFC6376].
1614. Security Considerations
163 This document does not change the Security Considerations of
164 [RFC6376]. It reduces the risk of signature compromise due to weak
165 cryptography. The SHA-1 risks discussed in Section 3 of [RFC6194]
166 are resolved due to rsa-sha1 no longer being used by DKIM.
170Kitterman Standards Track [Page 3]
172RFC 8301 DKIM Crypto Usage Update January 2018
1755. IANA Considerations
177 IANA has updated the Reference and Status fields of the "sha1"
178 registration in the "DKIM Hash Algorithms" registry. The
179 registration now appears as follows:
181 +------+---------------------+----------+
182 | Type | Reference | Status |
183 +------+---------------------+----------+
184 | sha1 | [RFC6376] [RFC8301] | historic |
185 +------+---------------------+----------+
1896.1. Normative References
191 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
192 Requirement Levels", BCP 14, RFC 2119,
193 DOI 10.17487/RFC2119, March 1997,
194 <https://www.rfc-editor.org/info/rfc2119>.
196 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
197 "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
198 RFC 6376, DOI 10.17487/RFC6376, September 2011,
199 <https://www.rfc-editor.org/info/rfc6376>.
201 [RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
202 "PKCS #1: RSA Cryptography Specifications Version 2.2",
203 RFC 8017, DOI 10.17487/RFC8017, November 2016,
204 <https://www.rfc-editor.org/info/rfc8017>.
206 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
207 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
208 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
2106.2. Informative References
212 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
213 Considerations for the SHA-0 and SHA-1 Message-Digest
214 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
215 <https://www.rfc-editor.org/info/rfc6194>.
217 [VULNOTE] US-CERT, "Vulnerability Note VU#268267: DomainKeys
218 Identified Mail (DKIM) Verifiers may inappropriately
219 convey message trust", October 2012,
220 <http://www.kb.cert.org/vuls/id/268267>.
226Kitterman Standards Track [Page 4]
228RFC 8301 DKIM Crypto Usage Update January 2018
233 The author wishes to acknowledge the following individuals for their
234 review and comments on this proposal: Kurt Andersen, Murray
235 S. Kucherawy, Martin Thomson, John Levine, Russ Housley, and Jim
238 Thanks to John Levine for his DKIM Crypto Update (DCRUP) work that
239 was the source for much of the introductory material in this
245 Kitterman Technical Services
247 Ellicott City, MD 21042
248 United States of America
250 Phone: +1 301 325-5475
251 Email: scott@kitterman.com
282Kitterman Standards Track [Page 5]