Re: [openpgp] Hashing literal (meta)data fields

[Justus Winter <[e-mail]>]

Hi Daniel,

Daniel Huigens <d.huigens=[e-mail]> writes:

> In draft-koch-openpgp-2015-rfc4880bis-00, v5 signatures hash the
> metadata fields, and v4 signatures can optionally hash the metadata
> fields by adding a "Literal Data Meta Hash" signature subpacket, which
> includes a hash of the metadata fields (and the subpacket is then
> hashed into the signature). It is disallowed for v5 signatures. [3]
>
>
> Now, for my concrete proposal: let's use the "Literal Data Meta Hash"
> subpacket for both v4 and v5 signatures, to allow (but not force) the
> signer to hash these fields. This maintains the possibility to convert
> a detached signature (that doesn't have this subpacket) to an attached
> one (if we don't require it), and similarly, to convert an attached
> signature (that does have it) to a detached one (if we ignore this
> subpacket for detached signatures).
>
> Let me know what you think!

The proposed scheme seems complicated to me, retains the
shortcomings of the literal packet's metadata, and breaks re-attaching
the signature.

It is complicated because you need to specify a way to hash these
fields.

It retains the shortcomings of not defining what date means and the
limitation of the file name length.

Instead, one could simply add notations to the signature to specify say
creation time, modification time, and filename.  With this scheme, one
can better specify the semantics, lift the file name length restriction,
it doesn't require specifying how to hash the metadata, and it isn't an
all-or-nothing thing.  As a bonus, if you detach the signature, the
information is still available, so you can make use of it, and
re-attaching the signature is possible and easy.

Best,
Justus