Apparently, the Brexit Agreement has been in negotiation a lot longer than we’ve been led to believe because it references technologies that were introduced in 1997 and haven’t been updated since 2002. On page 921 of the agreement:

s/MIME functionality is built into the vast majority of modern e-mail software packages including Outlook, Mozilla Mail as well as Netscape Communicator 4.x and inter-operates among all major e- mail software packages.

Also, the now-defunct—since 2011—SHA-1 encryption algorithm shall be used:

the hash algorithm SHA-1 shall be applied

There’s no excuse for this appearing in a 2020 Trade Agreement. It’s embarrassing.