[openpgp] Re: v4+v6

Hi Paul,

Thanks for taking a look.

On Sun, 09 Feb 2025 17:53:43 +0100,
Paul Schaub wrote:
> > By using the same key material and the same meta-data, it is possible
> to transform a v4 key packet into a v6 key packet and vica-versa [...]
> 
> I'm not sure about the vice-versa part. My instinct would tell me there might be dragons when "downgrading" a v6
> certificate into a v4 one, even though I cannot come up with a concrete idea for an attack. So treat this as just an
> articulation of my gut feelings :D

I'm also less sure about this part, and I hope others who understand
the cryptography better than I do can comment.

> All in all its a nice idea, although its not universally applicable
> to all certs (e.g. legacy/weak keys).
>
> Also, some popular key types (legacy x25519, legacy ed25519) MUST NOT be used with v6, so I guess you'd
> transform them to the new key type, which is no longer a bijection, as the v4 cert might also use the new
> non-legacy algorithms.

You're right.  I failed to note that this proposal only applies to the
intersection of algorithms and constraints specified by RRC 4880 and
RFC 9580.

:) Neal

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