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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 201 ** 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) December 1, 2017 5 Intended status: Standards Track 6 Expires: June 4, 2018 8 A new cryptographic signature method for DKIM 9 draft-ietf-dcrup-dkim-crypto-07 11 Abstract 13 DKIM was designed to allow new cryptographic algorithms to be added. 14 This document adds a new signing 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 June 4, 2018. 33 Copyright Notice 35 Copyright (c) 2017 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 . . . . . . . . . . . . 3 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 . . . . . . . . . . . . . . . . . . 4 63 9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 5 65 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 67 1. Introduction 69 Discussion Venue: Discussion about this draft is directed to the 70 dcrup@ietf.org [1] mailing list. 72 DKIM [RFC6376] signs e-mail messages, by creating hashes of the 73 message headers and body and signing the header hash with a digital 74 signature. Message recipients fetch the signature verification key 75 from the DNS where it is stored in a TXT record. The defining 76 documents specify a single signing algorithm, RSA [RFC3447], and 77 recommend key sizes of 1024 to 2048 bits. 79 This document adds a new stronger signing algorithm, Edwards-Curve 80 Digital Signature Algorithm using the Curve25519 curve (ed25519), 81 which has much shorter keys than RSA for similar levels of security. 83 2. Conventions Used in This Document 85 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 86 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 87 "OPTIONAL" in this document are to be interpreted as described in 88 [RFC2119]. 90 Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF 91 tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376]. 93 3. Ed25519-SHA256 Signing Algorithm 95 The ed25519-sha256 signing algorithm computes a message hash as 96 defined in section 3 of [RFC6376], and signs it with the Hash variant 97 of Ed25519, as defined in in RFC 8032 section 5.1 [RFC8032]. The 98 signing algorithm is HashEdDSA. (Even though the input to the 99 signing algorithm has already been hashed, the PureEdDSA which does 100 not do an additional hash is not widely implemented, and the extra 101 hash causes no problems other than an insignificant slowdown.) The 102 DNS record for the verification public key MUST have a "k=ed25519" 103 tag to indicate that the key is an Ed25519 rather than RSA key. 105 Note: since Ed25519 keys are 256 bits long, DNS key record data will 106 generally fit in a single 255 byte TXT string, and will work even 107 with DNS provisioning software that doesn't handle multi-string TXT 108 records. 110 4. Signature and key syntax 112 The syntax of DKIM signatures and DKIM keys are updated as follows. 114 4.1. Signature syntax 116 The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] is 117 updated by adding this rule to the existing rule for sig-a-tag-k: 119 ABNF: 121 sig-a-tag-k =/ "ed25519" 123 4.2. Key syntax 125 The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated 126 by adding this rule to the existing rule for key-k-tag-type: 128 ABNF: 130 key-k-tag-type =/ "ed25519" 132 5. Key and algorithm choice and strength 134 Section 3.3 of [RFC6376] describes DKIM's hash and signature 135 algorithms. It is updated as follows: 137 Signers SHOULD implement and verifiers MUST implement the 138 ed25519-sha256 algorithm. 140 6. Transition Considerations 142 For backward compatibility, signers MAY add multiple signatures that 143 use old and new signing algorithms. Since there can only be a single 144 key record in the DNS for each selector, the signatures will have to 145 use different selectors, although they can use the same d= and i= 146 identifiers. 148 7. Security Considerations 150 Ed25519 is a widely used cryptographic technique, so the security of 151 DKIM signatures using new signing algorithms should be at least as 152 good as those using old algorithms. 154 8. IANA Considerations 156 IANA is requested to update registries as follows. 158 8.1. DKIM Key Type registry 160 The following value is added to the DKIM Key Type Registry 162 +---------+-----------+--------+ 163 | TYPE | REFERENCE | STATUS | 164 +---------+-----------+--------+ 165 | ed25519 | [RFC8032] | active | 166 +---------+-----------+--------+ 168 Table 1: DKIM Key Type Registry Added Values 170 9. References 172 9.1. Normative References 174 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 175 Requirement Levels", BCP 14, RFC 2119, 176 DOI 10.17487/RFC2119, March 1997, 177 . 179 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 180 Standards (PKCS) #1: RSA Cryptography Specifications 181 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 182 2003, . 184 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 185 Specifications: ABNF", STD 68, RFC 5234, 186 DOI 10.17487/RFC5234, January 2008, 187 . 189 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 190 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 191 RFC 6376, DOI 10.17487/RFC6376, September 2011, 192 . 194 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 195 Signature Algorithm (EdDSA)", RFC 8032, 196 DOI 10.17487/RFC8032, January 2017, 197 . 199 9.2. URIs 201 [1] mailto:dcrup@ietf.org 203 Appendix A. Change log 205 06 to 07: Remove RSA fingerprints. Change Pure to hashed eddsa. 207 05 to 06: Editorial changes only. 209 04 to 05: Remove deprecation cruft and inconsistent key advice. Fix 210 p= and k= text. 212 03 to 04: Change eddsa to ed25519. Add Martin's key regeneration 213 issue. Remove hashed ed25519 keys. Fix typos and clarify text. 214 Move syntax updates to separate section. Take out SHA-1 stuff. 216 01 to 02: Clarify EdDSA algorithm is ed25519 with Pure version of 217 the signing. Make references to tags and fields consistent. 219 Author's Address 221 John Levine 222 Taughannock Networks 223 PO Box 727 224 Trumansburg, NY 14886 226 Phone: +1 831 480 2300 227 Email: standards@taugh.com