-----BEGIN PGP SIGNED MESSAGE-----
Dominikus Scherkl wrote:
Carl Ellison <cme(_at_)acm(_dot_)org> wrote:
Y'know, there's an even simpler attack with the same premise. You
intercept an encrypted e-mail from Alice to Bob. You take the mail
body out of the message and send that body to Bob under your e-mail
address (or under some address you control that Bob might mistake for
Alice's, which would be even better). Bob decrypts the message and
replies to it, including the original message body by default.
In that case Bob sees the original message, and at least has the possibility
of noting that it is not consistent with the reply-to address. If he sees
garbage, that could be consistent with any reply-to address, unless Bob
knows about this attack.
This is all part of the same problem that has been pointed out before in
the context of signing: the message content and the headers (including
the reply-to address and hence the public key to be used to encrypt replies),
are not treated as a unit cryptographically.
The mistake here, on Bob's part, is to reply to a message without
paying attention to the e-mail address being used
The Flaw I see (on the whole attack) is:
Why should anybody reply cleartext to an encrypted message?
The attack does not depend on the victim replying in cleartext.
If the message is encrypted, it would be encrypted to the attacker's key.
Peter Gutmann wrote:
On the grand scale of things, it has curiosity value, but not much more.
There
are a pile of other attacks which fall into the same class, e.g. concern over
the Bleichenbacher attack on SSL being used against S/MIME email (come to
think
of it, that one never came up on open-pgp). My thoughts on this at the time,
which also apply to this attack, were:
-- Snip --
[...] this attack requires that an attacker send you around a million pieces
of CMS encrypted email with attached receipt requests, that you respond with
a million receipts indicating to the attacker the exact details of why the
decrypt failed, that you reuse the same per-message key for each of those
million messages.
What on earth does this attack have to do with sending millions of messages?
It requires one message, and is considerably more plausible than applying the
Bleichenbacher attack to email (or would be, if it is wasn't prevented in
practice by compression).
- --
David Hopwood <david(_dot_)hopwood(_at_)zetnet(_dot_)co(_dot_)uk>
Home page & PGP public key: http://www.users.zetnet.co.uk/hopwood/
RSA 2048-bit; fingerprint 71 8E A6 23 0E D3 4C E5 0F 69 8C D4 FA 66 15 01
Nothing in this message is intended to be legally binding. If I revoke a
public key but refuse to specify why, it is because the private key has been
seized under the Regulation of Investigatory Powers Act; see www.fipr.org/rip
-----BEGIN PGP SIGNATURE-----
Version: 2.6.3i
Charset: noconv
iQEVAwUBPWE6izkCAxeYt5gVAQH7sAf6AklABDur8W+Aoq6FAMlSwprTkS9/ds6d
jFk8vNqlF2RYQApMGmGCSBcoayNS4o9WwYBP0hIEaqv/9jTcZXHGnz11IoUoFbR8
fQIQEh5egiGeqyt43n1kojWEptA1MHN5VNBC+WeYMV0sJYvqiSM61NjIHJMUV94Y
3ueWpee4drXCYgjVRMH8PhXj1IoqIyhzzPtzaQ46s0hVaZcQIOE6vVuSqAwyXLmr
qW52cjRZ8wIJjA5I4PPQcW8/IXSMcMvAkFLeG5HFcl9COmC+wRqJVgzhq6Q2du+8
qqLHAs23g/FsKIckBNaWeU0DSkIp0oZcxCcOjsAB3JFLkMiInhUE5w==
=gZJl
-----END PGP SIGNATURE-----