Re: [openpgp] Intended Recipient Fingerprint signature subpacket

Vincent Breitmoser <look@my.amazin.horse> Mon, 21 May 2018 20:04 UTC

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Date: Mon, 21 May 2018 22:04:10 +0200
From: Vincent Breitmoser <look@my.amazin.horse>
To: openpgp@ietf.org
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Subject: Re: [openpgp] Intended Recipient Fingerprint signature subpacket
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No feedback on this at all?  Should I maybe create a website and logo for
a surreptitious forwarding attack?

I'll add some more motivation: There is currently no way to distinguish
signatures made for plaintext messages from signatures made for encrypted
messages.

This opens up a scenario where a message is sent as signed cleartext (which many
people do by default), and only encrypted at a later point, for example by an
inbound message encryption feature. At that point, there is no way for a mail
client to tell whether this was actually an e2e encrypted message, or sent in
the clear.

As a straightforward fix, I propose an additional "sent in the clear" subpacket
that indicates when a signature was made over a message that is sent in the
clear, and wasn't intended to authenticate an encrypted message.

 - V

Vincent Breitmoser(look@my.amazin.horse)@Tue, Mar 06, 2018 at 12:19:51AM +0100:
> Hey folks,
> 
> dkg and I have been discussing an "Intended Recipient Fingerprint"
> subpacket, that pins a signature to be valid only in an encrypted
> context to the indicated recipient.
> 
> Use of this subpacket removes some wiggling room for signed+encrypted
> messages.  This can be used to prevent replay attacks, where a signature
> is taken out of its context and forwarded to a different recipient.
> 
> Please see https://0xacab.org/schleuder/schleuder/issues/158 for a
> complete description of an attack scenario in the context of the
> Schleuder remailer.  The given scenario is solved with this subpacket on
> the openpgp layer.
> 
> Diff attached for rfc4880bis, please comment.
> 
>  - V
>