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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 210 ** Obsolete normative reference: RFC 3447 (Obsoleted by RFC 8017) ** Downref: Normative reference to an Informational RFC: RFC 8032 Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 3 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) January 22, 2018 5 Intended status: Standards Track 6 Expires: July 26, 2018 8 A new cryptographic signature method for DKIM 9 draft-ietf-dcrup-dkim-crypto-08 11 Abstract 13 This document adds a new signing algorithm to DKIM. 15 Status of This Memo 17 This Internet-Draft is submitted in full conformance with the 18 provisions of BCP 78 and BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF). Note that other groups may also distribute 22 working documents as Internet-Drafts. The list of current Internet- 23 Drafts is at https://datatracker.ietf.org/drafts/current/. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 This Internet-Draft will expire on July 26, 2018. 32 Copyright Notice 34 Copyright (c) 2018 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (https://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with respect 42 to this document. Code Components extracted from this document must 43 include Simplified BSD License text as described in Section 4.e of 44 the Trust Legal Provisions and are provided without warranty as 45 described in the Simplified BSD License. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 50 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 51 3. Ed25519-SHA256 Signing Algorithm . . . . . . . . . . . . . . 3 52 4. Signature and key syntax . . . . . . . . . . . . . . . . . . 3 53 4.1. Signature syntax . . . . . . . . . . . . . . . . . . . . 3 54 4.2. Key syntax . . . . . . . . . . . . . . . . . . . . . . . 3 55 5. Key and algorithm choice and strength . . . . . . . . . . . . 4 56 6. Transition Considerations . . . . . . . . . . . . . . . . . . 4 57 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 58 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 59 8.1. DKIM Key Type registry . . . . . . . . . . . . . . . . . 4 60 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 61 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 62 9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5 63 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 5 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 66 1. Introduction 68 Discussion Venue: Discussion about this draft is directed to the 69 dcrup@ietf.org [1] mailing list. 71 DKIM [RFC6376] signs e-mail messages, by creating hashes of the 72 message headers and body and signing the header hash with a digital 73 signature. Message recipients fetch the signature verification key 74 from the DNS. The defining documents specify a single signing 75 algorithm, RSA [RFC3447]. 77 This document adds a new stronger signing algorithm, Edwards-Curve 78 Digital Signature Algorithm using the Curve25519 curve (ed25519), 79 which has much shorter keys than RSA for similar levels of security. 81 2. Conventions Used in This Document 83 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 84 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 85 "OPTIONAL" in this document are to be interpreted as described in 86 [RFC2119]. 88 Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF 89 tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376]. 91 3. Ed25519-SHA256 Signing Algorithm 93 The ed25519-sha256 signing algorithm computes a message hash as 94 defined in section 3 of [RFC6376], and signs it with the Hash variant 95 of Ed25519, as defined in in RFC 8032 section 5.1 [RFC8032]. The 96 signing algorithm is HashEdDSA. 98 Even though the input to the signing algorithm has already been 99 hashed, the PureEdDSA version of the algorithm is not widely 100 implemented, and the extra hash causes no problems beyond a minor 101 computational slowdown. 103 The DNS record for the verification public key has a "k=ed25519" tag 104 to indicate that the key is an Ed25519 rather than RSA key. 106 Note: since Ed25519 keys are 256 bits long, the base64 encoded key is 107 only 44 octets, so DNS key record data will generally fit in a single 108 255 byte TXT string, and will work even with DNS provisioning 109 software that doesn't handle multi-string TXT records. 111 4. Signature and key syntax 113 The syntax of DKIM signatures and DKIM keys are updated as follows. 115 4.1. Signature syntax 117 The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] is 118 updated by adding this rule to the existing rule for sig-a-tag-k: 120 ABNF: 122 sig-a-tag-k =/ "ed25519" 124 4.2. Key syntax 126 The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated 127 by adding this rule to the existing rule for key-k-tag-type: 129 ABNF: 131 key-k-tag-type =/ "ed25519" 133 The p= value in the key record is the ed25519 public key encoded in 134 base64. Since the key is 256 bits long, the base64 text is 44 octets 135 long. For example, a key record using the public key in [RFC8032] 136 Section 7.1, Test 1, mignt be: 138 s._domainkey.example TXT ( 139 "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=" 140 ) 142 5. Key and algorithm choice and strength 144 Section 3.3 of [RFC6376] describes DKIM's hash and signature 145 algorithms. It is updated as follows: 147 Signers SHOULD implement and verifiers MUST implement the 148 ed25519-sha256 algorithm. 150 6. Transition Considerations 152 For backward compatibility, signers MAY add multiple signatures that 153 use old and new signing algorithms. Since there can only be a single 154 key record in the DNS for each selector, the signatures will have to 155 use different selectors, although they can use the same d= and i= 156 identifiers. 158 7. Security Considerations 160 Ed25519 is a widely used cryptographic technique, so the security of 161 DKIM signatures using new signing algorithms should be at least as 162 good as those using old algorithms. 164 8. IANA Considerations 166 IANA is requested to update registries as follows. 168 8.1. DKIM Key Type registry 170 The following value is added to the DKIM Key Type Registry 172 +---------+-----------+--------+ 173 | TYPE | REFERENCE | STATUS | 174 +---------+-----------+--------+ 175 | ed25519 | [RFC8032] | active | 176 +---------+-----------+--------+ 178 Table 1: DKIM Key Type Registry Added Values 180 9. References 181 9.1. Normative References 183 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 184 Requirement Levels", BCP 14, RFC 2119, 185 DOI 10.17487/RFC2119, March 1997, 186 . 188 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 189 Standards (PKCS) #1: RSA Cryptography Specifications 190 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 191 2003, . 193 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 194 Specifications: ABNF", STD 68, RFC 5234, 195 DOI 10.17487/RFC5234, January 2008, 196 . 198 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 199 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 200 RFC 6376, DOI 10.17487/RFC6376, September 2011, 201 . 203 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 204 Signature Algorithm (EdDSA)", RFC 8032, 205 DOI 10.17487/RFC8032, January 2017, 206 . 208 9.2. URIs 210 [1] mailto:dcrup@ietf.org 212 Appendix A. Change log 214 07 to 08 Specify base64 key records. Style edits per Dave C. 216 06 to 07: Remove RSA fingerprints. Change Pure to hashed eddsa. 218 05 to 06: Editorial changes only. 220 04 to 05: Remove deprecation cruft and inconsistent key advice. Fix 221 p= and k= text. 223 03 to 04: Change eddsa to ed25519. Add Martin's key regeneration 224 issue. Remove hashed ed25519 keys. Fix typos and clarify text. 225 Move syntax updates to separate section. Take out SHA-1 stuff. 227 01 to 02: Clarify EdDSA algorithm is ed25519 with Pure version of 228 the signing. Make references to tags and fields consistent. 230 Author's Address 232 John Levine 233 Taughannock Networks 234 PO Box 727 235 Trumansburg, NY 14886 237 Phone: +1 831 480 2300 238 Email: standards@taugh.com