Re: [openpgp] Reducing the meta-data leak

"Neal H. Walfield" <neal@walfield.org> Tue, 03 November 2015 14:37 UTC

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Date: Tue, 03 Nov 2015 15:37:06 +0100
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From: "Neal H. Walfield" <neal@walfield.org>
To: Derek Atkins <derek@ihtfp.com>
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Subject: Re: [openpgp] Reducing the meta-data leak
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At Tue, 03 Nov 2015 09:30:48 -0500,
Derek Atkins wrote:
> "Neal H. Walfield" <neal@walfield.org> writes:
> > 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?
> 
> There was an extension at one point where you use the string 0x00...00
> for the keyID and that forced you to test all your secret keys.  There
> are certainly times where that is warranted; there are other times where
> it is not.
> 
> I wasn't at the meeting (in person or virtually) so I'm not sure I
> completely understand what the use-case is where the above solution
> doesn't work?

Bryan Ford proposed getting rid of all unencrypted meta-data.  In
particular, he wanted to get rid of the recipients / number of
recipients.

There are some practical difficulties with this approach,
which I mentioned above.

My proposal is a blue sky idea to avoid having to try to decrypt a
message with every secret key while (hopefully) making it more
difficult to get at the list of recipients.

Neal