This has been available for contributors in the United States for some years now, and I've used it to sign the assignment documents in the past. It might not be available in all countries though.

> If you use GPG, you can sign your assignment using a detached signature in the following manner:

> gpg -a --detach-sig ASSIGNMENT

> Where ASSIGNMENT is the PDF file(s) as you have received it from us.

> Then simply email the assignment, key ID, and signature file back to us at assign@gnu.org. Please make sure that your key is listed on a public keyserver.