There’s no reason fax couldn’t be authenticated, it just isn’t. Modern fax is just a JPEG in a weird wrapper format. Email would arguably be worse, because email leaks tons of metadata in the wrapper.

Fax was pretty secure in the day of circuit switched analogue phone lines these days it’s all digital, though. There was an almost direct physical electrical connection between your fax machine and the recipient, something that we never get anymore. Your carrier and the government could listen in on the connection, of course, but that’s not really part of most people’s threat model.

There’s no reason to use fax today, but up to the mid-2000s there was a good reason to use fax over email.

Today, encrypted email is only used by privacy nerds and big businesses. Privacy nerds use PGP, big businesses use S/MIME. The latter is much easier to work with and is supported by basically every mail client out there, the former is free. Neither are usable safely by general consumers, they’re both full of technical details and concepts that very few people care about. L

The most infuriating part is that various governments use smart cards for digital ID that could be used to sign and encrypt emails like these (what’s better proof of ownership of a government ID than an email that can only be signed by the ID in question?), but the technology remains woefully underused.