[openpgp] Reducing the meta-data leak

"Neal H. Walfield" <neal@walfield.org> Tue, 03 November 2015 10:02 UTC

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Date: Tue, 03 Nov 2015 11:02:25 +0100
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From: "Neal H. Walfield" <neal@walfield.org>
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Subject: [openpgp] Reducing the meta-data leak
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Hi,

At the IETF 94 OpenPGP WG session, Bryan, if I recall correctly,
suggested that we should try and hide more meta-data.  For instance,
instead of listing the recipients, someone decrypting a message would
try each of their available secret keys in turn.  Werner pointed out
that these probes are a pain for people who use a passphrase protected
key and I mentioned that it is a pain for people who use a smartcard,
in paritcular, those who use more than one smartcard.

What about using a bloom filter for encoding the recipients?  This, of
course, doesn't eliminate the meta-data leak and it can lead to false
positives (= gratuitious passphrase prompts / smartcard prompts), but
it should reduce the metadata leak a fair amount, I think.  Thoughts?

Thanks,

:) Neal