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.
I'm actually surprised that no one mentioned this sooner. It's too bad that this isn't an option for most other legal documents that one has to submit in a lot of situations.
> 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.