Internet DRAFT - draft-vesely-smooth-canon
draft-vesely-smooth-canon
IETF A. Vesely
Internet-Draft June 23, 2014
Intended status: Informational
Expires: December 25, 2014
DKIM "smooth" header canonicalization
draft-vesely-smooth-canon-00
Abstract
This document describes a new canonicalization algorithm for DKIM,
designed to be better able to survive transit through intermediaries.
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
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This Internet-Draft will expire on December 25, 2014.
Copyright Notice
Copyright (c) 2014 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The "smooth" Header Canonicalization Algorithm . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 4
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1. Introduction
DomainKeys Identified Mail (DKIM [RFC6376]) Signatures only provide
for two canonicalization algorithms, "simple" and "relaxed". This
document describes a new algorithm, "smooth", which is more relaxed
then the "relaxed" algorithm. The algorithm is being specified for
headers only, for the time being.
The reason to introduce a new algorithm is twofold. On the one hand,
many mail transfer agents (MTAs) bestow slight changes on header
fields syntax, in an attempt to unformalize it. Such activity does
not impede local DKIM verification, provided that the latter occurs
before the former, but breaks verification at further hops.
On the other hand, using a more robust canonicalization is not enough
to have the original DKIM Signature survive the transit. In some
cases, the original DKIM Signature has to be significantly weaker
than [RFC6376] recommendations, and additional verifications MAY be
needed to recover any semantics of trust that might have been
associated with a signer's role. Thus, this document can be used as
a reference for a class of semantics alterations that are being
introduced.
2. Terms
The terms WSP, CRLF, DQUOTE, "comment", and "quoted-string" are
imported from [RFC5322].
3. The "smooth" Header Canonicalization Algorithm
The "smooth" header canonicalization algorithm MUST apply the
following steps in order:
o Convert any word encoded as specified by [RFC2047] into its plain
UTF-8 value. For example, convert "=?UTF-8?B?Zm9vCg?=" to
"foo". Note that character conversion is needed if the
original charset is not UTF-8. The format of the resulting value
is described in Internationalized Email Headers ([RFC6532]).
o Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject: AbC".
o Unfold all header field continuation lines as described in
[RFC5322]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed by
WSP) MUST be interpreted without the CRLF. Implementations MUST
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NOT remove the CRLF at the end of the header field value.
o Delete all comments, that is parenthesized text not in a quoted
string.
o Delete all DQUOTE characters defining quoted-strings. For
example, convert 'charset="us-ascii"' to 'charset=us-ascii'.
Escaped DQUOTE characters inside a quoted-string MUST be
unescaped. For example, '"an \"example\" of escaped DQUOTE"' to
'an "example" of escaped DQUOTE'.
o Delete all sequences of one or more WSP characters. WSP
characters here include those before and after a line folding
boundary, as well as those before or after the colon separating
the header field name from the header field value.
4. IANA Considerations
There is a registry of DKIM-Signature Canonicalization Header created
by RFC6376.
5. Security Considerations
to be done.
6. Normative References
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
September 2011.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
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Author's Address
Alessandro Vesely
v. L. Anelli 13
Milano, MI 20122
IT
Email: vesely@tana.it
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