Network Working Group J. Levine Internet-Draft Taughannock Networks Updates: 6376 (if approved) June 20, 2017 Intended status: Standards Track Expires: December 22, 2017 Cryptographic Update to DKIM draft-ietf-dcrup-dkim-crypto-02 Abstract DKIM was designed to allow new cryptographic algorithms to be added. This document adds a new signing algorithm and a new way to represent signature validation keys, and deprecates an obsolete signing algorithm. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 22, 2017. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Levine Expires December 22, 2017 [Page 1] Internet-Draft DKIM Crypto Update June 2017 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 3. EdDSA-SHA256 Signing Algorithm . . . . . . . . . . . . . . . 3 4. Public key fingerprints . . . . . . . . . . . . . . . . . . . 3 5. Key and algorithm choice and strength . . . . . . . . . . . . 4 6. Transition Considerations . . . . . . . . . . . . . . . . . . 4 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 8.1. DKIM Signature Tag Registry . . . . . . . . . . . . . . . 5 8.2. DKIM Hash Algorithms Registry . . . . . . . . . . . . . . 5 8.3. DKIM Key Type registry . . . . . . . . . . . . . . . . . 5 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . 6 9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction Discussion Venue: Discussion about this draft is directed to the dcrup@ietf.org [1] mailing list. DKIM [RFC6376] signs e-mail messages, by creating hashes of the message headers and content and signing the header hash with a digital signature. Message recipients fetch the signature verification key from the DNS where it is stored in a TXT record. The defining documents specify a single signing algorithm, RSA [RFC3447], and recommends key sizes of 1024 to 2048 bits. While 1024 bit signatures are common, stronger signatures are not. Widely used DNS configuration software places a practical limit on key sizes, because the software only handles a single 256 octet string in a TXT record, and RSA keys longer than 1156 bits don't fit in 256 octets. This document adds a new signing algorithm, Edwards-Curve Digital Signature Algorithm (EdDSA), which has much shorter keys than RSA for similar levels of security. It also adds a new key representation, with the key itself in the signature and a shorter key fingerprint in the DNS. 2. Conventions Used in This Document The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Levine Expires December 22, 2017 [Page 2] Internet-Draft DKIM Crypto Update June 2017 Syntax descriptions use Augmented BNF (ABNF) [RFC5234]. The ABNF tokens sig-a-tag-k, key-k-tag-type, and base64string are imported from [RFC6376]. 3. EdDSA-SHA256 Signing Algorithm The eddsa-sha256 signing algorithm computes a message hash as defined in section 3 of [RFC6376], and signs it with Ed25519, the EdDSA algorithm using the edwards25519 curve, as defined in in RFC 8032 section 5.1 [RFC8032]. The signing algorithm is PureEdDSA as defined in RFC 8032 section 4, since the input to the signing algorithm has already been hashed. The DNS record for the verification public key MUST have a "k=eddsa" or "k=eddsafp" tag to indicate that the key is an EdDSA rather than RSA key. The syntax of DKIM signature tags is updated as follows, where this rule replaces the existing rule for sig-a-tag-k: ABNF: sig-a-tag-k = "rsa" / "rsafp" / "eddsa" / "eddsafp" / x-sig-a-tag-k The syntax of DKIM key tags is updated as follows, where this rule replaces the existing rule for key-k-tag-type: ABNF: key-k-tag-type = "rsa" / "rsafp" / "eddsa" / "eddsafp" / x-key-k-tag-type 4. Public key fingerprints Rather than using a public key stored in the DNS, a signature MAY include the corresponding public key, with a fingerprint in the DNS. For an RSA signature, the Signing Algorithm is rsafp-sha256, or for an EdDSA signature the Signing Algorithm is eddsafp-sha256. The DNS record contains a SHA-256 hash of the public key, stored in base64 in the p= tag. The key type tag MUST be present and contains k=rsafp or k=eddsafp. k= The public key (base64; REQUIRED). White space is ignored in this value and MUST be ignored when reassembling the original key. Levine Expires December 22, 2017 [Page 3] Internet-Draft DKIM Crypto Update June 2017 ABNF: sig-k-tag = %x6b [FWS] "=" [FWS] sig-k-tag-data sig-k-tag-data = base64string Section 5.5 of [RFC6376], on computing the message hash and signature, is modified as follows: When creating a signature with a signing algorithm that uses a key fingerprint, the signer includes the public key in the signature as a base64 encoded string with a k= tag. The key in the tag is the same one that would be published in a non-fingerprint key record. Section 3.7 of [RFC6376], on computing the message hashes, is not modified. Since the key in the k= tag is known in advance, it included in the signature in the same manner as all of the other signature fields other than b=. Section 6.1.3 of [RFC6376], to compute the verification, is modified as follows: In item 4, if the signing algorithm uses a key fingerprint, extract the verification key from the k= tag. If there is no such tag, the signature does not validate. Extract the key hash from the p= tag of the key record. If there is no such tag or the tag is empty, the signature does not validate. Compute the SHA-256 hash of the verification key, and compare it to the value of the key hash. If they are not the same, the signature does not validate. Otherwise proceed to verify the signature using the validation key and the algorithm described in the "a=" tag. 5. Key and algorithm choice and strength Section 3.3 of [RFC6376] describes DKIM's hash and signature algorithms. It is updated as follows: Signers MUST NOT implement and verifiers SHOULD NOT implement the rsa-sha1 algorithm. Signers SHOULD implement and verifiers MUST implement the rsafp-256, eddsa-sha256, and eddsafp-sha256 algorithms. Signers that use rsa-sha256 signatures MUST use keys at least 1024 bits long and SHOULD use keys 2048 bits long. Verifiers MUST NOT accept rsa-sha256 signatures with keys less than 1024 bits long. 6. Transition Considerations For backward compatibility, signers MAY add multiple signatures that use old and new signing algorithms or key representations. Since there can only be a single key record in the DNS for each selector, the signatures will have to use different selectors, although they can use the same d= and i= identifiers. Levine Expires December 22, 2017 [Page 4] Internet-Draft DKIM Crypto Update June 2017 7. Security Considerations EdDSA and key fingerprints are widely used cryptographic techniques, so the security of DKIM signatures using new signing algorithms should be at least as good as those using old algorithms. Since key fingerprints make it possible to publish verification records for RSA keys of any length, rsafp signatures SHOULD use key lengths of 1536 or 2048 bits. 8. IANA Considerations IANA is requested to update registries as follows. 8.1. DKIM Signature Tag Registry The following value is added to the DKIM Signature Tag Registry +------+-----------------+--------+ | TYPE | REFERENCE | STATUS | +------+-----------------+--------+ | k | (this document) | active | +------+-----------------+--------+ Table 1: DKIM Signature Tag Registry Added Value 8.2. DKIM Hash Algorithms Registry The following value is updated in the DKIM Hash Algorithms Registry +------+-------------------+----------+ | TYPE | REFERENCE | STATUS | +------+-------------------+----------+ | sha1 | [FIPS-180-3-2008] | historic | +------+-------------------+----------+ Table 2: DKIM Hash Algorithms Registry Updated Value 8.3. DKIM Key Type registry The following values are added to the DKIM Key Type Registry Levine Expires December 22, 2017 [Page 5] Internet-Draft DKIM Crypto Update June 2017 +---------+-----------+--------+ | TYPE | REFERENCE | STATUS | +---------+-----------+--------+ | rsafp | [RFC3447] | active | | eddsa | [RFC8032] | active | | eddsafp | [RFC8032] | active | +---------+-----------+--------+ Table 3: DKIM Key Type Registry Added Values 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, . [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, . 9.2. URIs [1] mailto:dcrup@ietf.org Appendix A. Change log 01 to 02: Clarify EdDSA algorithm is ed25519 with Pure version of the signing. Make references to tags and fields consistent. Levine Expires December 22, 2017 [Page 6] Internet-Draft DKIM Crypto Update June 2017 Author's Address John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 Email: standards@taugh.com Levine Expires December 22, 2017 [Page 7]