FR: support signing commits with X.509 certificates

[pylbrecht]

This feature request came up on Discord recently. I am opening this issue to track it.
Right now we only support signing commits using GnuPG or SSH (see commit signing docs).

Git is shelling out to gpgsm for signing commits using X.509 certs. We could do something similar by implementing a SigningBackend for X.509.

Resources

• https://github.com/github/smimesign
• https://linux.die.net/man/1/gpgsm
• lib/src/gpg_signing.rs
• lib/src/ssh_signing.rs
• lib/src/signing.rs

Related

• support for signed commits #58