| Network Working Group | J. Levine |
| Internet-Draft | Taughannock Networks |
| Updates: 6376 (if approved) | June 5, 2018 |
| Intended status: Standards Track | |
| Expires: December 7, 2018 |
A new cryptographic signature method for DKIM
draft-ietf-dcrup-dkim-crypto-11
This document adds a new signing algorithm to DKIM, ed25519-sha256. DKIM verifiers are required to implement this algorithm.
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DKIM signs e-mail messages, by creating hashes of the message headers and body and signing the header hash with a digital signature. Message recipients fetch the signature verification key from the DNS. The defining documents specify a single signing algorithm, RSA.
This document adds a new stronger signing algorithm, Edwards-Curve Digital Signature Algorithm using the Curve25519 curve (ed25519), which has much shorter keys than RSA for similar levels of security.
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].
Syntax descriptions use Augmented BNF (ABNF). The ABNF tokens sig-a-tag-k and key-k-tag-type are imported from [RFC6376].
The ed25519-sha256 signing algorithm computes a message hash as defined in section 3 of [RFC6376] using SHA-256 [FIPS-180-4-2015] as the hash-alg, and signs it with the PureEdDSA variant Ed25519, as defined in in RFC 8032 section 5.1. Example keys and signatures in Appendix A below are based on the test vectors in RFC 8032 section 7.1.
The DNS record for the verification public key has a "k=ed25519" tag to indicate that the key is an Ed25519 rather than RSA key.
This is an additional DKIM signature algorithm added to Section 3.3 of [RFC6376] as envisioned in Section 3.3.4 of [RFC6376].
Note: since Ed25519 keys are 256 bits long, the base64 encoded key is only 44 octets, so DNS key record data will generally fit in a single 255 byte TXT string, and will work even with DNS provisioning software that doesn't handle multi-string TXT records.
The syntax of DKIM signatures and DKIM keys are updated as follows.
The syntax of DKIM algorithm tags in section 3.5 of [RFC6376] is updated by adding this rule to the existing rule for sig-a-tag-k:
ABNF:
sig-a-tag-k =/ "ed25519"
The syntax of DKIM key tags in section 3.6.1 of [RFC6376] is updated by adding this rule to the existing rule for key-k-tag-type:
ABNF:
key-k-tag-type =/ "ed25519"
The p= value in the key record is the ed25519 public key encoded in base64. Since the key is 256 bits long, the base64 text is 44 octets long. For example, a key record using the public key in [RFC8032] Section 7.1, Test 1, mignt be:
s._domainkey.example TXT ( "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo=" )
Section 3.3 of [RFC6376] describes DKIM's hash and signature algorithms. It is updated as follows:
Signers SHOULD implement and verifiers MUST implement the ed25519-sha256 algorithm.
For backward compatibility, signers can add multiple signatures that use old and new signing algorithms. Since there can only be a single key record in the DNS for each selector, the signatures have to use different selectors, although they can use the same d= and i= identifiers.
Ed25519 is a widely used cryptographic technique, so the security of DKIM signatures using new signing algorithms should be at least as good as those using old algorithms.
All of the security advice in [RFC6376] continues to apply except that the advice in Section 8 of [RFC8032] supplants the advice about RSA threats.
IANA is requested to update registries as follows.
The following value is added to the DKIM Key Type Registry
| TYPE | REFERENCE | STATUS |
|---|---|---|
| ed25519 | [RFC8032] | active |
| [FIPS-180-4-2015] | U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-4, August 2015. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008. |
| [RFC6376] | Crocker, D., Hansen, T. and M. Kucherawy, "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. |
| [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. |
This is a small message with both rsa-sha256 and ed25519-sha256 DKIM signatures. The signatures are independent of each other, so either signature would be valid if the other were not present.
Ed25519 secret key in base64.
fL+5V9EquCZAovKik3pA6Lk9zwCzoEtjIuIqK9ZXHHA=
RSA secret key in PEM format.
-----BEGIN RSA PRIVATE KEY----- MIICXQIBAAKBgQDkHlOQoBTzWRiGs5V6NpP3idY6Wk08a5qhdR6wy5bdOKb2jLQi Y/J16JYi0Qvx/byYzCNb3W91y3FutACDfzwQ/BC/e/8uBsCR+yz1Lxj+PL6lHvqM KrM3rG4hstT5QjvHO9PzoxZyVYLzBfO2EeC3Ip3G+2kryOTIKT+l/K4w3QIDAQAB AoGAH0cxOhFZDgzXWhDhnAJDw5s4roOXN4OhjiXa8W7Y3rhX3FJqmJSPuC8N9vQm 6SVbaLAE4SG5mLMueHlh4KXffEpuLEiNp9Ss3O4YfLiQpbRqE7Tm5SxKjvvQoZZe zHorimOaChRL2it47iuWxzxSiRMv4c+j70GiWdxXnxe4UoECQQDzJB/0U58W7RZy 6enGVj2kWF732CoWFZWzi1FicudrBFoy63QwcowpoCazKtvZGMNlPWnC7x/6o8Gc uSe0ga2xAkEA8C7PipPm1/1fTRQvj1o/dDmZp243044ZNyxjg+/OPN0oWCbXIGxy WvmZbXriOWoSALJTjExEgraHEgnXssuk7QJBALl5ICsYMu6hMxO73gnfNayNgPxd WFV6Z7ULnKyV7HSVYF0hgYOHjeYe9gaMtiJYoo0zGN+L3AAtNP9huqkWlzECQE1a licIeVlo1e+qJ6Mgqr0Q7Aa7falZ448ccbSFYEPD6oFxiOl9Y9se9iYHZKKfIcst o7DUw1/hz2Ck4N5JrgUCQQCyKveNvjzkkd8HjYs0SwM0fPjK16//5qDZ2UiDGnOe uEzxBDAr518Z8VFbR41in3W4Y3yCDgQlLlcETrS+zYcL -----END RSA PRIVATE KEY-----
brisbane._domainkey.example.com IN TXT ( "v=dkim1; k=ed25519; p=yi50DjK5O9pqbFpNHklsv9lqaS0ArSYu02qp1S0DW1Y=") test._domainkey.example.com IN TXT ( "v=dkim1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDkHlOQoBTzWR" "iGs5V6NpP3idY6Wk08a5qhdR6wy5bdOKb2jLQiY/J16JYi0Qvx/byYzCNb3W91y3FutAC" "DfzwQ/BC/e/8uBsCR+yz1Lxj+PL6lHvqMKrM3rG4hstT5QjvHO9PzoxZyVYLzBfO2EeC3" "Ip3G+2kryOTIKT+l/K4w3QIDAQAB")
The text in each line of the message starts at the first position except for the continuation lines on the DKIM-Signature headers which start with a single space.
DKIM-Signature: v=1; a=ed25519-sha256; c=simple/simple; d=football.example.com; i=@football.example.com; q=dns/txt; s=brisbane; t=1518460054; h=from : to : subject : date : message-id : from : subject : date; bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=; b=9/dsDChY0YMTtD5Eyw3wx7x22BlSJP7M5ECbJ7GWrR45nXlTCGb8l0YB o0wBLR++X5LqmsxXaOYLLJe46l10AQ== DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=football.example.com; i=@football.example.com; q=dns/txt; s=test; t=1527915362; h=from : to : subject : date : message-id : from : subject : date; bh=4bLNXImK9drULnmePzZNEBleUanJCX5PIsDIFoH4KTQ=; b=icKcLSEZYXJ95flvWE8FT6hl5iqd8MC/LEKYH0QjsqYy6MO/4pgVNCZH l/RAXAuADxE/40Fg7uTlxwwD1hjN2Ple6J//cJfslBdDOq6zTVbne1dqtl NOat7iamJ1AfRqyG+ja7a2AZsrpUuJ7VA6O+0zRYPqpwMEkEFIzI9i/Xk= From: Joe SixPack <joe@football.example.com> To: Suzie Q <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi. We lost the game. Are you hungry yet? Joe.