Signing a file given its hash only

[Richard Stoughton]

On Wed, May 14, 2025 at 12:59 AM Jay Acuna <mysidia at gmail.com> wrote:
> Your options with GPG are essentially to sign a text file or message
> that lists the hash.
> Then have L  verify the GPG signature and then verify the hash listed
> in the signed file matches the file to be verified.

To "sign" the hash on M, it would be necessary to inject a one-time
secret  (e.g. a OpenPGP private key, or a symmetric key) known only to
H into the build process on M. Then M would sign the hash or create a
MAC that could later be checked by H before actually signing the
artifact.

Unfortunately, this is currently not possible. Our best option at this
time is to compare the hashes of the artifacts deployed on L with
their hashes written to a log on M during the build process, before H
creates the final signatures. This could be done in a much more
efficient way if GnuPG would be able to create signatures with hashes
instead of  the complete file content as input.

> Or you can forward the Gpg-agent from H to M  using remote gpg agent
> forwarding over SSH,
> and run the Gpg signing command on M,  so that  M performs the hashing
> and  H performs the key operation.
>
> Or files on M could possibly be made available to H  using a
> network-based mount, such as  SSHFS or NFS.
> Other than that;  the GPG client had to have access to a file in order
> for it to be capable of signing that file.

Unfortunately, there is no way to connect H with M, neither via agent
forwarding nor via SSH/NFS/SMB.

On Wed, May 14, 2025 at 12:59 AM Jay Acuna <mysidia at gmail.com> wrote:
>
> On Tue, May 13, 2025 at 5:20 PM Richard Stoughton via Gnupg-users
> <gnupg-users at gnupg.org> wrote:
> > Hi,
> >
> > We have three servers H -> M -> L with high, medium, and low security.
> > The private signature key is known to H only and must never leave H.
> >
> > The question is: Is it possible to gpg-sign a file given its hash only?
>
>
> Your options with GPG are essentially to sign a text file or message
> that lists the hash.
> Then have L  verify the GPG signature and then verify the hash listed
> in the signed file matches the file to be verified.
>
> Or you can forward the Gpg-agent from H to M  using remote gpg agent
> forwarding over SSH,
> and run the Gpg signing command on M,  so that  M performs the hashing
> and  H performs the key operation.
>
> Or files on M could possibly be made available to H  using a
> network-based mount, such as  SSHFS or NFS.
> Other than that;  the GPG client had to have access to a file in order
> for it to be capable of signing that file.
>
> --
> -JA