On May 29, 2011, at 9:04 PM, Murray S. Kucherawy wrote:
-----Original Message-----
From: ietf-dkim-bounces(_at_)mipassoc(_dot_)org
[mailto:ietf-dkim-bounces(_at_)mipassoc(_dot_)org] On Behalf Of Alessandro
Vesely
Sent: Saturday, May 28, 2011 9:29 AM
To: ietf-dkim(_at_)mipassoc(_dot_)org
Subject: Re: [ietf-dkim] New canonicalizations
On 27/May/11 19:16, Murray S. Kucherawy wrote:
I'm all for including experimental code in future releases of our
stuff, especially if it's an experiment other implementations are
trying. But I need to see a spec first, or enough detail that I
could write one.
For the body, I brought some ideas[1]. For MIME header fields,
punctuation and boundaries need to be omitted as well. For other
header fields, including the DKIM-Signature, it is probably enough to
remove just any white space.
[1] http://mipassoc.org/pipermail/ietf-dkim/2011q2/016692.html
IMHO, the "hard parts" of the code are (i) selecting a MIME parser,
and (ii) finding a good way to structure experimental C14Ns and handle
double (triple?) signatures in the existing code.
One of the elegant things about the current canonicalizations is that they
can stream. I think a system that's MIME-aware can too, but possibly not,
and in any case having to teach a DKIM implementation about MIME will make it
a lot more complicated and expensive. If we have to go down that road, I
think working on DOSETA and MIMEAUTH is the way to go.
Streaming MIME parsers are possible, but they're not simple. Also, MIME is more
complex than most people think it is, and even coming up with a
canonicalization specification that is both MIME aware and does what the author
intended is likely to be quite difficult.
If we want the lower-hanging fruits, we might take the list of things MLMs
like to do to messages and find ways to canonicalize those. Fortunately, we
made a list of the common ones in the MLM document.
Even that seems to be adding complexity simply for the sake of adding
complexity, rather than because there's an obvious benefit.
The most obvious thing that MLMs do that invalidate signatures are 1. append
content to the body and 2. prepend content to the subject line. Any approach
that allows me to replay messages while making those changes seems to open the
door to abuse.
Cheers,
Steve
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