[openpgp] Constrained Legacy Fingerprint Storage for v5 Keys

["Neal H. Walfield" <[e-mail]>]

> 15.1.  Constrained Legacy Fingerprint Storage for v5 Keys
>
>    Some OpenPGP implementations have fixed length constraints for key
>    fingerprint storage that will not fit all 32 octets of a v5
>    fingerprint.  For example, [OPENPGPCARD] reserves 20 octets for each
>    stored fingerprint.
>
>    An OpenPGP implementation MUST NOT attempt to map any part of a v5
>    fingerprint to such a constrained field unless the relevant spec for
>    the constrained environment has explicit guidance for storing a v5
>    fingerprint that distinguishes it from a v4 fingerprint.  An
>    implementation interacting with such a constrained field SHOULD
>    directly calculate the v5 fingerprint from public key material and
>    associated metadata instead of relying on the constrained field.

This text forbids (MUST NOT) an OpenPGP implementation from storing
part of a v5 fingerprint in constrained space.  But, it doesn't say
what should be stored there instead.  Should an OpenPGP implementation
that stores a v5 key on an OpenPGP card leave the fingerprint field as
is?  Should it set it to all zeros?  I feel some guidance should be
provided.


I'm also a bit confused by the guidance.  This text specifically
references the OpenPGP card spec as an example of a constrained
environment.  As far as I know, the OpenPGP card spec doesn't provide
space for the public key.  What should an implementation do?


Finally, what is so bad about storing a truncated fingerprint?  What's
the attack model?  An implementation can set the wrong fingerprint
field.  So, as I understand it, the fingerprint is only a hint anyway.

Thanks,

Neal