Re: [openpgp] Intended Recipient Fingerprint signature subpacket

Justus Winter <justuswinter@gmail.com> Tue, 19 March 2019 10:27 UTC

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From: Justus Winter <justuswinter@gmail.com>
To: Vincent Breitmoser <look@my.amazin.horse>, openpgp@ietf.org
In-Reply-To: <20180521200410.kxq7nj6gyki5yvhx@calamity>
References: <20180305231951.GA21944@calamity> <20180521200410.kxq7nj6gyki5yvhx@calamity>
Date: Tue, 19 Mar 2019 11:27:19 +0100
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Subject: Re: [openpgp] Intended Recipient Fingerprint signature subpacket
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Vincent Breitmoser <look@my.amazin.horse> writes:

> No feedback on this at all?  Should I maybe create a website and logo for
> a surreptitious forwarding attack?

I agree that it is a useful feature.  It is implemented as proposed in
Sequoia, you can designate recipients while encrypting a message, and
during signature verification it constrains the validity of the
signatures.

> 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.

I support this proposal.


Thanks,
Justus