-----Original Message-----
From: John R. Levine [mailto:johnl(_at_)iecc(_dot_)com]
Sent: Monday, October 25, 2010 8:07 AM
To: Murray S. Kucherawy
Cc: ietf-dkim(_at_)mipassoc(_dot_)org
Subject: Re: [ietf-dkim] Statistics about DKIM and MIME
The one that stands out is "multipart/signed" (from RFC1847) which drops
to about a 65% survival rate. I don't know much about how this is
typically formatted or treated enroute, but it was easily the biggest
outlier in the report. Not sure if that should be a surprise to us or not.
I'm surprised. That suggests something often adds the S/MIME signature
after the DKIM signature, but as far as I know, S/MIME signatures are
usually applied by the MUA.
Do the stats say what kind of failure it was, e.g. body hash or header
hash?
Actually it's worse than I said originally. We track pass/fail in two bits,
one being whether or not the crypto lined up and the other being whether or not
the body hashes matched. Thus, it's possible to get a "pass" coupled with a
body hash change. I had only selected for the first bit.
So here are the stats again. The first column is obviously the media type; the
second is the count of signatures covering a message with that type as the
outermost MIME part; the third column is the number of those that passed in
both the crypto and the body hash sense, and the fourth is the pass percentage.
application/ms-tnef 26 23 88.5%
application/pdf 16 16 100%
message/disposition-notification 10 10 100%
message/rfc822 2 2 100%
multipart/alternative 290865 265270 91.2%
multipart/mixed 38509 35370 91.8%
multipart/related 7959 7149 89.8%
multipart/report 958 883 92.2%
multipart/signed 314 86 27.4%
text 13 13 100%
text/calendar 34 32 94.1%
text/html 63144 55880 88.5%
text/plain 72195 55415 76.8%
In the particular case of multipart/signed there were 106 messages where the
RSA verification failed, but 122 where it passed but the body hash at the
verifier didn't match the one in the signature. So more failures occur from
body changes than do from header changes.
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