Fun (?) with GnuPG

We use GnuPG as part of the infrastructure to certify authenticity of development history in Git in various places:

• Signed tags created by git tag -s is to say "This tag was created by me, the holder of the private GnuPG key that signed this object". Because the object name of any Git object is computed as a cryptographic hash over what the object records, and because a signed tag object records the object name of a tagged object (typically a commit) and the human readable name (typically a release number or name) the tagger wants to give the tagged object, an attacker cannot forge a phony tag that points at a different commit signed with the private key the attacker does not have. You are saying "You can verify that it is true that I wanted to make that commit release X" safely because of this. Also, because the commit object records all the objects and their location in a project tree, and the parent commit objects, such a signed tag also ensures that all the development history behind such a tagged commit cannot be tampered with.
• When you merge a signed tag (either done by git merge or git pull), the content of the tag with its GnuPG signature is copied to the resulting commit object. This lets you ensure that the history behind the side branch that was merged to the history cannot be tampered with and the signature certifies that it came from the signer (typically a subsystem lieutenant).
• Signed commits created by git commit -S is a way to say "This commit was created by me", and ensures that the history behind the commit cannot be tampered with and certifies that the change it introduces came from the signer.
• Still under development is git push --signed, a way to certify that you wanted to put a particular commit at the tip of a particular branch.

GnuPG is also used as a mechanism to ensure the integrity and authenticity of tarballs that are sent to the kernel.org servers, which is a common distribution point for open source projects like the Linux kernel and Git itself. A maintainer prepares a tarball and a detached signature, uploads them, and the receiving end will verify that the signature is good.

It is a common practice to specify the expiration date when creating a signing key. For example, the key I have been using to sign Git release tags was originally set up to expire in 3 years since the key was created. But the thing is, a project may outlive that expiry date. An interesting question is what happens to the existing tags when the key expires.

Unluckily, the right thing happens. If the holder of the key does not do anything, the key becomes expired, and the signatures in the signed tags stops validating. Luckily, the validity of a key can be extended by the holder of the key, and once it is done, the signatures made before the key's original expiration date will continue to validate fine.

At least, that is the theory ;-)

As my key was originally set to expire early next month, I've extended the lifespan of the key 96AFE6CB I have been using a few days ago and uploaded the updated key to pgp keyservers, so existing signed tags (e.g. v2.0.0) should continue to be valid.

A few tips:

• Although this page is a specific instruction to Debian contributors, it was very helpful when I had to figure out how to futz with GnuPG subkeys. It does not talk about how to update the expiration date for a subkey, though (you use "gpg --edit-key" and then choose the subkey with the "key N" command and finally use the "expire" command).
• In order to force a specific subkey to be used when signing for Git, you would need to use the ! suffix to the GnuPG key-id, e.g. in my ~/.gitconfig file:
    [user] signingkey = 96AFE6CB!
  Without the ! suffix, GnuPG tries to use the newest subkey you have associated with the same primary key, which may not be the subkey you would want to use.

I signed a new v2.1.2 maintenance release with the same key today. Hopefully it will validate OK for you (otherwise, you may have to fetch the public key from the keyserver).