Re: [openpgp] Issuer Fingerprint

Sounds like it doesn't make sense to make this optional for signatures as
implicit signature identity could result in attacks where the attacker
changes an implicit identity and signature verification fails?

On Monday, June 13, 2016, Werner Koch <wk@gnupg.org> wrote:

> Hi!
>
> It is a long time problem in OpenPGP that signatures have no way to
> unambiguously specify the key used to create the signature.  The specs
> suggest the use of the Issuer subpacket to convey the long keyid of the
> issuing key.  However, it is possible to create colliding 64 bit keyids
> and thus it is possible that a user downloads the wrong key for a
> signature; this will yield a bad signature status and the user has no
> easy means to decide whether this is is really a bad signature, or due
> to the use of a colliding public key.
>
> This can easily be solved by including the full fingerprint of the key
> in the signature.  Introducing such a feature can be made orthogonal to
> a new fingerprint format.  I propose this change:
>
> --8<---------------cut here---------------start------------->8---
> @@ -1055,6 +1055,7 @@ #### {5.2.3.1} Signature Subpacket Specification
>            30   Features
>            31   Signature Target
>            32   Embedded Signature
> +          33   Issuer Fingerprint
>    100 to 110   Private or experimental
>
>  An implementation SHOULD ignore any subpacket of a type that it does
> @@ -1615,6 +1616,16 @@ #### {5.2.3.26} Embedded Signature
>  in Section 5.2 above.  It is useful when one signature needs to refer
>  to, or be incorporated in, another signature.
>
> +#### Issuer Fingerprint
> +
> +(1 octet key version number, N octets of fingerprint)
> +
> +The OpenPGP Key fingerprint of the key issuing the signature.  The
> +only possible key version number is 4 and thus N must be 20.  This
> +subpacket is intended to eventually replace the issuer subpacket which
> +does not not unambiguously specify the key.  It SHOULD be part of all
> +signatures.
> +
>  ### {5.2.4} Computing Signatures
>
>  All signatures are formed by producing a hash over the signature data,
> --8<---------------cut here---------------end--------------->8---
>
>
>
> Shalom-Salam,
>
>    Werner
>
>
> --
> Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.
>     /* EFH in Erkrath: https://alt-hochdahl.de/haus */
>
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-- 
Joseph Lorenzo Hall
Chief Technologist, Center for Democracy & Technology [https://www.cdt.org]
1401 K ST NW STE 200, Washington DC 20005-3497
e: joe@cdt.org, p: 202.407.8825, pgp: https://josephhall.org/gpg-key
Fingerprint: 3CA2 8D7B 9F6D DBD3 4B10  1607 5F86 6987 40A9 A871