Internet DRAFT - draft-ietf-dcrup-dkim-ecc
draft-ietf-dcrup-dkim-ecc
DCRUP S. Rose
Internet-Draft NIST
Updates: 6376 (if approved) June 21, 2017
Intended status: Standards Track
Expires: December 23, 2017
Defining Elliptic Curve Cryptography Algorithms for use with DKIM
draft-ietf-dcrup-dkim-ecc-01
Abstract
DomainKeys Identified Mail (DKIM) uses digital signature to associate
a message with a given sending domain. Currently, there is only one
cryptography algorithm defined for use with DKIM (RSA). This
document defines four new elliptic curve cryptography algorithms for
use with DKIM. This will allow for algorithm agility if a weakness
is found in RSA, and allows for smaller key length to provide the
same digital signature strength.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Defining New ECC algorithms for Use with DKIM . . . . . . . . 3
3. Changes to ABNF Definitions of DKIM Keys and Signatures . . . 3
3.1. Changes to DKIM Key Definition . . . . . . . . . . . . . 3
3.2. Changes to DKIM Signature Definition . . . . . . . . . . 4
4. Sender Considerations . . . . . . . . . . . . . . . . . . . . 5
5. Receiver Considerations . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
DomainKeys Identified Mail (DKIM)[RFC6376] uses digital signatures to
associate a sending domain with a given message. Each DKIM signed
email message as a digital signature in its header, that can be
validated by a receiver by obtaining the appropriate public key
stored in the DNS. Currently, DKIM has only one cryptographic
algorithm defined for use (RSA) and two digital signature algorithms
(RSA/SHA-1 and RSA/SHA-256). In the past, 1024-bit RSA keys were
common, equating to (roughly) a security key strength of 80 bits
[NIST.800-57.2016]. Today, a minimum of 112 bits is recommended,
which equates to 2048 bit RSA keys.
The public portion of 2048 bit RSA keys are still small enough to fit
into a DNS TXT RR without issues in performance. The encoded public
key is too large to fit into the maximum allowed characters in a
single string, but a DNS TXT RR allows for multiple strings, so the
key can be broken into "chunks" to allow it to be served. However,
some code components may not correctly handle TXT RRs with multiple
strings which will result in errors in validation.
Elliptic Curve Cryptography (ECC) has shown to have the same
(roughly) equivalent key strength with smaller sizes. A 224 to 255
bit ECDSA key has (roughly) the same key strength as a 2048 bit RSA
key (112 bits of strength). This means smaller keys can be used to
achieve the same DKIM security strength, as well as being easier to
manage in the DNS.
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Having additional digital signature algorithms defined for use with
DKIM also permits algorithm agility. If a weakness is discovered in
one digital signature algorithm, email senders can quickly migrate to
another algorithm without waiting for a standards action and
subsequent software update.
This document defines a ECDSA as a new algorithms for DKIM. This
document also defines a new hash algorithm for use with DKIM
signatures. This document updates the IANA registry with new values
for the algorithms. This document does not change the DKIM key or
signature formats, but only defines new algorithm values using those
formats.
1.1. Requirements Language
The 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 RFC 2119 [RFC2119].
2. Defining New ECC algorithms for Use with DKIM
This document defines a new digital signature algorithm for use with
DKIM:
algorithm | mnemonic
-------------+-------------
ECDSA P-256 | ecdsa256
For ECDSA, the SHA-1 hash algorithm MUST NOT be used.
3. Changes to ABNF Definitions of DKIM Keys and Signatures
The original definition of DKIM signatues and keys are defined in
[RFC6376]. The following are changes to the definition to include
the new digital signature algorithm and secure hash algorithm.
3.1. Changes to DKIM Key Definition
The original definition of the textual representation of DKIM keys is
found in section 3.6.1 of [RFC6376]. The only changes to the
definition is below. The entire key:tag definition is included for
clarity. All other tags:value pairs are unchanged. References to
the definitions below have also been updated to reflect the current
state of the art.
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults to
"sha256"). A colon-separated list of hash algorithms that might
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be used. Unrecognized algorithms MUST be ignored. Refer to
[RFC6376]Section 3.3 for a discussion of the hash algorithms
implemented by Signers and Verifiers. The set of algorithms
listed in this tag in each record is an operational choice made by
the Signer.
ABNF:
key-h-tag = %x68 [FWS] "=" [FWS] key-h-tag-alg
*( [FWS] ":" [FWS] key-h-tag-alg )
key-h-tag-alg = "sha1" / "sha256" / x-key-h-tag-alg
x-key-h-tag-alg = hyphenated-word ; for future extension
k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers and
Verifiers MUST support the "rsa" key type. The "rsa" key type
indicates that an ASN.1 DER-encoded [UTI.X680.2002] RSAPublicKey
(see [RFC8017], Sections 3.1 and A.1.1) is being used in the "p="
tag. The "ecdsa256" key type indicates an ASN.1 DER-encoded
[UTI.X680.2002] PublicKey (see [RFC5480], Section 2.2) is being
used in the "p=" tag. (Note: the "p=" tag further encodes the
value using the base64 algorithm.) Unrecognized key types MUST be
ignored.
ABNF:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
key-k-tag-type = "rsa" / "ecdsa256" / x-key-k-tag-type
x-key-k-tag-type = hyphenated-word ; for future extension
3.2. Changes to DKIM Signature Definition
The original definition of the textual representation of DKIM
signatures is found in section 3.5 of [RFC6376]. The only changes to
the definition is below. The entire key:tag definition is included
for clarity. All other tags:value pairs are unchanged. References
to the definitions below have also been updated to reflect the
current state of the art.
a= The algorithm used to generate the signature (plain-text;
REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256" and
SHOULD support "ecdsa256-sha256"; Signers MUST NOT use "sha1" with
"ecdsa256". See [RFC6376] Section 3.3 for a description of RSA
and [FIPS.186-4.2013] Section 6 for a brief description of ECDSA.
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ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / "ecdsa256" / x-sig-a-tag-k
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
; for later extension
4. Sender Considerations
New algorithms for an established protocols take some time to gain
wide deployment. There will be a period of time where new algorithms
are in operation side by side with older algorithms. There will also
be a sizable percentage of DKIM validators that will not understand
new algorithms until they are upgraded. This will lead to a period
of time where multiple DKIM signature algorithms are in use for a
sender. Email administrators MAY want to also sign with RSA/SHA-1 or
RSA/SHA-256 for a period of time. This period of time is difficult
to measure, but DMARC [RFC7960] aggregate reports could provide a
view on DKIM validation rates by receivers.
5. Receiver Considerations
These requirements are for DKIM verifiers (as defined it [RFC6376]).
These entities would be the consumers of any end-to-end email
security policy and would be the entity responsible for validating
DKIM signatures.
DKIM verifiers claiming conformance to this document MUST implement
all of the above cryptographic algorithms.
This document does NOT change the behavior of the core DKIM
specification in that verifiers MUST ignore unknown algorithms in
DKIM signatures.
6. Security Considerations
This document defines the use of new elliptic curve cryptographic
algorithms for use with DomainKey Identified Mail (DKIM). This
document is not a discussion of the relative strengths or weaknesses
of these algorithms, but only defines their use.
There is a risk for mail receivers that do not understand or
implement the new algorithms. Attackers could modify or spoof
messages from sending zones using one of the newly defined algorithms
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and it would not be detectable as an attack by ECC-ignorant
receivers. Likewise, ECC-ignorant receivers may mark valid DKIM
signed email messages as invalid due to unknown algorithms.
7. IANA Considerations
This draft defines the use of a new algorithm for DKIM. This draft
updates the "DKIM Key Tag" registry to include the following new
value:
algorithm | mnemonic | Reference
-------------+------------+--------------
ECDSA P-256 | ecdsa256 | This document
The current DKIM Key Tag registry is located at
https://www.iana.org/assignments/dkim-parameters/dkim-
parameters.xhtml#dkim-parameters-6
8. References
8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<http://www.rfc-editor.org/info/rfc5480>.
[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,
<http://www.rfc-editor.org/info/rfc6376>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<http://www.rfc-editor.org/info/rfc8017>.
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[UTI.X680.2002]
"ITU-T Recommendation X.680 (2002) | ISO/IEC 8825-1:2002,
Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU X680, 2002.
8.2. Informative References
[FIPS.186-4.2013]
National Institute of Standards and Technology, "Digital
Signature Standard", FIPS PUB 186-4, July 2013,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-4.pdf>.
[NIST.800-57.2016]
National Institute of Standards and Technology,
"Recommendations for Key Management Part 1: General",
NIST 800-57, January 2016.
[RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
E., Ed., and K. Andersen, Ed., "Interoperability Issues
between Domain-based Message Authentication, Reporting,
and Conformance (DMARC) and Indirect Email Flows",
RFC 7960, DOI 10.17487/RFC7960, September 2016,
<http://www.rfc-editor.org/info/rfc7960>.
Author's Address
Scott Rose
NIST
100 Bureau Dr.
Gaithersburg, MD 20899
USA
Phone: +1 301-975-8439
Email: scott.rose@nist.gov
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