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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to lack the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (May 29, 2018) is 2158 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '1' on line 224 -- Possible downref: Non-RFC (?) normative reference: ref. 'FIPS-180-4-2015' ** Downref: Normative reference to an Informational RFC: RFC 8032 -- Obsolete informational reference (is this intentional?): RFC 3447 (Obsoleted by RFC 8017) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Levine 3 Internet-Draft Taughannock Networks 4 Updates: 6376 (if approved) May 29, 2018 5 Intended status: Standards Track 6 Expires: November 30, 2018 8 A new cryptographic signature method for DKIM 9 draft-ietf-dcrup-dkim-crypto-10 11 Abstract 13 This document adds a new signing algorithm to DKIM, ed25519-sha256. 14 DKIM verifiers are required to implement this algorithm. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at https://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on November 30, 2018. 33 Copyright Notice 35 Copyright (c) 2018 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (https://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 52 3. Ed25519-SHA256 Signing Algorithm . . . . . . . . . . . . . . 3 53 4. Signature and key syntax . . . . . . . . . . . . . . . . . . 3 54 4.1. Signature syntax . . . . . . . . . . . . . . . . . . . . 3 55 4.2. Key syntax . . . . . . . . . . . . . . . . . . . . . . . 3 56 5. Key and algorithm choice and strength . . . . . . . . . . . . 4 57 6. Transition Considerations . . . . . . . . . . . . . . . . . . 4 58 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 59 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 60 8.1. DKIM Key Type registry . . . . . . . . . . . . . . . . . 4 61 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 63 9.2. Informative References . . . . . . . . . . . . . . . . . 5 64 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 Appendix A. Example of a signed message . . . . . . . . . . . . 5 66 A.1. Secret key . . . . . . . . . . . . . . . . . . . . . . . 5 67 A.2. Public key DNS record . . . . . . . . . . . . . . . . . . 6 68 A.3. Signed Message . . . . . . . . . . . . . . . . . . . . . 6 69 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 6 70 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 72 1. Introduction 74 Discussion Venue: Discussion about this draft is directed to the 75 dcrup@ietf.org [1] mailing list. 77 DKIM [RFC6376] signs e-mail messages, by creating hashes of the 78 message headers and body and signing the header hash with a digital 79 signature. Message recipients fetch the signature verification key 80 from the DNS. The defining documents specify a single signing 81 algorithm, RSA [RFC3447]. 83 This document adds a new stronger signing algorithm, Edwards-Curve 84 Digital Signature Algorithm using the Curve25519 curve (ed25519), 85 which has much shorter keys than RSA for similar levels of security. 87 2. Conventions Used in This Document 89 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 90 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 91 "OPTIONAL" in this document are to be interpreted as described in 92 [RFC2119]. 94 Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF 95 tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376]. 97 3. Ed25519-SHA256 Signing Algorithm 99 The ed25519-sha256 signing algorithm computes a message hash as 100 defined in section 3 of [RFC6376] using SHA-256 [FIPS-180-4-2015] as 101 the hash-alg, and signs it with the PureEDdSA variant Ed25519, as 102 defined in in RFC 8032 section 5.1 [RFC8032]. Example keys and 103 signatures in Appendix A below are based on the test vectors in RFC 104 8032 section 7.1 [RFC8032]. 106 The DNS record for the verification public key has a "k=ed25519" tag 107 to indicate that the key is an Ed25519 rather than RSA key. 109 This is an additional DKIM signature algorithm added to Section 3.3 110 of [RFC6376] as envisioned in Section 3.3.4 of [RFC6376]. 112 Note: since Ed25519 keys are 256 bits long, the base64 encoded key is 113 only 44 octets, so DNS key record data will generally fit in a single 114 255 byte TXT string, and will work even with DNS provisioning 115 software that doesn't handle multi-string TXT records. 117 4. Signature and key syntax 119 The syntax of DKIM signatures and DKIM keys are updated as follows. 121 4.1. Signature syntax 123 The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] is 124 updated by adding this rule to the existing rule for sig-a-tag-k: 126 ABNF: 128 sig-a-tag-k =/ "ed25519" 130 4.2. Key syntax 132 The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated 133 by adding this rule to the existing rule for key-k-tag-type: 135 ABNF: 137 key-k-tag-type =/ "ed25519" 139 The p= value in the key record is the ed25519 public key encoded in 140 base64. Since the key is 256 bits long, the base64 text is 44 octets 141 long. For example, a key record using the public key in [RFC8032] 142 Section 7.1, Test 1, mignt be: 144 s._domainkey.example TXT ( 145 "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=" 146 ) 148 5. Key and algorithm choice and strength 150 Section 3.3 of [RFC6376] describes DKIM's hash and signature 151 algorithms. It is updated as follows: 153 Signers SHOULD implement and verifiers MUST implement the 154 ed25519-sha256 algorithm. 156 6. Transition Considerations 158 For backward compatibility, signers MAY add multiple signatures that 159 use old and new signing algorithms. Since there can only be a single 160 key record in the DNS for each selector, the signatures will have to 161 use different selectors, although they can use the same d= and i= 162 identifiers. 164 7. Security Considerations 166 Ed25519 is a widely used cryptographic technique, so the security of 167 DKIM signatures using new signing algorithms should be at least as 168 good as those using old algorithms. 170 8. IANA Considerations 172 IANA is requested to update registries as follows. 174 8.1. DKIM Key Type registry 176 The following value is added to the DKIM Key Type Registry 178 +---------+-----------+--------+ 179 | TYPE | REFERENCE | STATUS | 180 +---------+-----------+--------+ 181 | ed25519 | [RFC8032] | active | 182 +---------+-----------+--------+ 184 Table 1: DKIM Key Type Registry Added Values 186 9. References 187 9.1. Normative References 189 [FIPS-180-4-2015] 190 U.S. Department of Commerce, "Secure Hash Standard", FIPS 191 PUB 180-4, August 2015, 192 . 195 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 196 Requirement Levels", BCP 14, RFC 2119, 197 DOI 10.17487/RFC2119, March 1997, 198 . 200 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 201 Specifications: ABNF", STD 68, RFC 5234, 202 DOI 10.17487/RFC5234, January 2008, 203 . 205 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 206 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 207 RFC 6376, DOI 10.17487/RFC6376, September 2011, 208 . 210 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 211 Signature Algorithm (EdDSA)", RFC 8032, 212 DOI 10.17487/RFC8032, January 2017, 213 . 215 9.2. Informative References 217 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 218 Standards (PKCS) #1: RSA Cryptography Specifications 219 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 220 2003, . 222 9.3. URIs 224 [1] mailto:dcrup@ietf.org 226 Appendix A. Example of a signed message 228 This is a small message with an ed25519-rsa DKIM signature. 230 A.1. Secret key 232 Ed25519 secret key in base64. 234 fL+5V9EquCZAovKik3pA6Lk9zwCzoEtjIuIqK9ZXHHA= 236 A.2. Public key DNS record 238 test._domainkey.example.com IN TXT 239 "k=ed25519; p=yi50DjK5O9pqbFpNHklsv9lqaS0ArSYu02qp1S0DW1Y=" 241 A.3. Signed Message 243 The text in each line of the message start at the first position 244 except for the five continuation lines on the DKIM-Signature which 245 start with a single space. 247 DKIM-Signature: v=1; a=ed25519; c=relaxed/simple; d=example.com; 248 i=@example.com; q=dns/txt; s=test; t=5; h=message-id : 249 date : from : to : subject : date : from : subject; 250 bh=wE7NXSkgnx9PGiavN4OZhJztvkqPDlemV3OGuEnLwNo=; 251 b=wt7P+9DoBwcln1RKE3LN7069ZEEiSyVE/NH1YXnqnJy4JcrSCZUbeIEh 252 vXssPHelX4yNSXG9eTGTwwk5NxYqBw== 253 Received: from localhost 254 Message-ID: 255 Date: Mon, 01 Jan 2011 01:02:03 +0400 256 From: Test User 257 To: somebody@example.com 258 Subject: Testing 260 This is a test message. 262 Appendix B. Change log 264 09 to 10 Improve abstract, minor nits. 266 08 to 09 Specify sha-256 for the extremely literal minded. Take out 267 the prehash stuff. Add example. 269 07 to 08 Specify base64 key records. Style edits per Dave C. 271 06 to 07: Remove RSA fingerprints. Change Pure to hashed eddsa. 273 05 to 06: Editorial changes only. 275 04 to 05: Remove deprecation cruft and inconsistent key advice. Fix 276 p= and k= text. 278 03 to 04: Change eddsa to ed25519. Add Martin's key regeneration 279 issue. Remove hashed ed25519 keys. Fix typos and clarify text. 280 Move syntax updates to separate section. Take out SHA-1 stuff. 282 01 to 02: Clarify EdDSA algorithm is ed25519 with Pure version of 283 the signing. Make references to tags and fields consistent. 285 Author's Address 287 John Levine 288 Taughannock Networks 289 PO Box 727 290 Trumansburg, NY 14886 292 Phone: +883.5100.01196712 293 Email: standards@taugh.com