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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 (June 11, 2017) is 2510 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 247 == Missing Reference: 'RFC5234' is mentioned on line 97, but not defined == Missing Reference: 'FIPS-180-3-2008' is mentioned on line 202, but not defined ** Obsolete normative reference: RFC 3447 (Obsoleted by RFC 8017) ** Downref: Normative reference to an Informational RFC: RFC 8032 Summary: 2 errors (**), 0 flaws (~~), 4 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) June 11, 2017 5 Intended status: Standards Track 6 Expires: December 13, 2017 8 Cryptographic Update to DKIM 9 draft-ietf-dcrup-dkim-crypto-01 11 Abstract 13 DKIM was designed to allow new cryptographic algorithms to be added. 14 This document adds a new algorithm and a new way to represent 15 signature validation keys, and deprecates obsolete signing 16 algorithms. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on December 13, 2017. 35 Copyright Notice 37 Copyright (c) 2017 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 54 3. EdDSA-SHA256 Signing Algorithm . . . . . . . . . . . . . . . 3 55 4. Public key fingerprints . . . . . . . . . . . . . . . . . . . 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 Signature Tag Registry . . . . . . . . . . . . . . . 4 61 8.2. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 5 62 8.3. DKIM Key Type registry . . . . . . . . . . . . . . . . . 5 63 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 65 9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6 66 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 68 1. Introduction 70 Discussion Venue: Discussion about this draft is directed to the 71 dcrup@ietf.org [1] mailing list. 73 DKIM [RFC6376] signs e-mail messages, by creating hashes of the 74 message headers and content and signing the header hash with a 75 digital signature. Message recipients fetch the signature 76 verification key from the DNS where it is stored in a TXT record. 77 The defining documents specify a single signing algorithm, RSA 78 [RFC3447], and recommends key sizes of 1024 to 2048 bits. While 1024 79 bit signatures are common, stronger signatures are not. Widely used 80 DNS configuration software places a practical limit on key sizes, 81 because the software only handles a single 256 octet string in a TXT 82 record, and RSA keys longer than 1156 bits don't fit in 256 octets. 84 This document adds a new signing algorithm, Edwards-Curve Digital 85 Signature Algorithm (EdDSA), which has much shorter keys than RSA for 86 similar levels of security. It also adds a new key representation, 87 with the key itself in the signature and a shorter key fingerprint in 88 the DNS. 90 2. Conventions Used in This Document 92 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 93 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 94 "OPTIONAL" in this document are to be interpreted as described in 95 [RFC2119]. 97 Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF 98 tokens sig-a-tag-k, key-k-tag-type, and base64tring are imported from 99 [RFC6376]. 101 3. EdDSA-SHA256 Signing Algorithm 103 The eddsa-sha256 signing algorithm computes a message hash as defined 104 in section 3 of [RFC6376], and signs it using the EdDSA algorithm 105 defined in [RFC8032]. The DNS record for the verification public key 106 MUST have a "k=ecdh" or "k=ecdhfp" tag to indicate that the key is an 107 ECDH rather than RSA key. 109 The syntax of DKIM signature tags is updated as follows: 111 ABNF: 113 sig-a-tag-k = "rsa" / "rsafp" / "eddsa" / "eddsafp" x-sig-a-tag-k 115 The syntax of DKIM key tags is updated as follows: 117 ABNF: 119 key-k-tag-type = "rsa" / "rsafp" / "eddsa" / "eddsafp" 120 / x-key-k-tag-type 122 4. Public key fingerprints 124 Rather than using a public key stored in the DNS, a signature MAY 125 include the corresponding public key, with a fingerprint in the DNS. 126 For an RSA signature, the Signing Algorithm is rsafp-sha256, or for 127 an EdDSA signature the Signing Algorithm is eddsafp-sha256. The 128 corresponding public key is included in the signature as a base64 129 encoded string with a k= tag. The key in the tag is the same one 130 that would be published in a non-fingerprint key record. 132 The DNS record contains a sha-256 hash of the public key, stored in 133 base64 in the p= tag. The key type tag MUST be present and contains 134 k=rsafp or k=ecdhfp. 136 k= The public key (base64; REQUIRED). Whitespace is ignored in 137 this value and MUST be ignored when reassembling the original 138 key. 140 ABNF: 142 sig-k-tag = %x6b [FWS] "=" [FWS] sig-k-tag-data 143 sig-k-tag-data = base64string 145 5. Key and algorithm choice and strength 147 [[ splice in text from Scott K's draft to taste here ]] 149 Section 3.3 of [RFC6376] describes DKIM's hash and signature 150 algorithms. It is updated as follows: 152 Signers MUST NOT implement and verifiers SHOULD NOT implement the 153 rsa-sha1 algorithm. Signers SHOULD implement and verifiers MUST 154 implement the rsafp-256, ecdh-sha256, and ecdhfp-sha256 algorithms. 156 Signers that use rsa-sha256 signatures MUST use keys at least 1024 157 bits long and SHOULD use keys 2048 bits long. Verifiers MUST NOT 158 accept rsa-sha256 signatures with keys less than 1024 bits long. 160 [[ is there any reason to allow or require RSA keys longer than 2048 161 ? ]] 163 6. Transition Considerations 165 For backward compatibility, signers MAY add multiple signatures that 166 use old and new signing algorithms or key representations. Since 167 there can only be a single key record in the DNS for each selector, 168 the signatures will have to use different selectors, although they 169 can use the same d= and i= identifiers. 171 7. Security Considerations 173 EdDSA and key fingerprints are widely used cryptographic techniques, 174 so the security of DKIM signatures using new signing algorithms 175 should be at least as good as those using old algorithms. Since key 176 fingerprints make it possible to publish verification records for RSA 177 keys of any length, rsafp signatures SHOULD use key lengths of 1536 178 or 2048 bits. 180 8. IANA Considerations 182 IANA is requested to update registries as follows. 184 8.1. DKIM Signature Tag Registry 186 The following value is added to the DKIM Signature Tag Registry 187 +------+-----------------+--------+ 188 | TYPE | REFERENCE | STATUS | 189 +------+-----------------+--------+ 190 | k | (this document) | active | 191 +------+-----------------+--------+ 193 Table 1: DKIM Signature Tag Registry Added Value 195 8.2. DKIM Hash Algorithms Registry 197 The following value is updated in the DKIM Hash Algorithms Registry 199 +------+-------------------+----------+ 200 | TYPE | REFERENCE | STATUS | 201 +------+-------------------+----------+ 202 | sha1 | [FIPS-180-3-2008] | historic | 203 +------+-------------------+----------+ 205 Table 2: DKIM Hash Algorithms Registry Updated Value 207 8.3. DKIM Key Type registry 209 The following values is added to the DKIM Key Type Registry 211 +---------+-----------+--------+ 212 | TYPE | REFERENCE | STATUS | 213 +---------+-----------+--------+ 214 | rsafp | [RFC3447] | active | 215 | eddsa | [RFC8032] | active | 216 | eddsafp | [RFC8032] | active | 217 +---------+-----------+--------+ 219 Table 3: DKIM Key Type Registry Added Values 221 9. References 223 9.1. Normative References 225 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 226 Requirement Levels", BCP 14, RFC 2119, 227 DOI 10.17487/RFC2119, March 1997, 228 . 230 [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography 231 Standards (PKCS) #1: RSA Cryptography Specifications 232 Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 233 2003, . 235 [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 236 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 237 RFC 6376, DOI 10.17487/RFC6376, September 2011, 238 . 240 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 241 Signature Algorithm (EdDSA)", RFC 8032, 242 DOI 10.17487/RFC8032, January 2017, 243 . 245 9.2. URIs 247 [1] mailto:dcrup@ietf.org 249 Author's Address 251 John Levine 252 Taughannock Networks 253 PO Box 727 254 Trumansburg, NY 14886 256 Phone: +1 831 480 2300 257 Email: standards@taugh.com