I saw that people on the dark web would sign their posts with a PGP key to prove that their account has not been compromised. I think I understand the concept of how private and public keys work but I must be missing something because I don’t see how it proves anything.
I created a key and ran gpg --export --armor fizz@… and I ran that twice and both blocks were identical. If I posted my public key block couldn’t someone copy and paste that under their message and claim to be me?
If this is some kind of messaging board, you’d probably put your public key in your profile (I assume that since OP is talking about the dark web that the posters there would rather not share their actual identity).
Let’s talk about Alice, Bob and Eve. Alice is an active poster on a dark web forum. She puts her public key on her profile and uses the corresponding private key to sign her messages. If Eve wants to pretend to be Alice, Eve can simply put her own public key on her profile and sign messages with her own private key. But Bob is smart. Rather than just looking at the profile of the poster and copying their key every time, Bob saved it in his key store and assigned it to Alice (possibly even marked it as trusted). When Bob sees a post by Eve, he’ll try to validate it. This validation might succeed (if Bob has access to Eve’s public key), but it will be clear that the message wasn’t signed by Alice’s key.
Of course, this all assumes that Bob has quite some knowledge of how this works and is vigilant enough to perform all these validations correctly.
As for the regular internet, there are some services where you can share your public key: keys.openpgp.org is one of these. Of course, as /u/perviouslyiner@lemm.ee says, there’s still the matter of trust. You need to make sure that the public key you’re using is actually from the right person.
That’s exactly my point. The gold standard would be a key signing party, but given that humans don’t tend to talk to each other in meat space much these days, it’s more of a rare occurrence than it used to be. I don’t really know what the ideal solution would be that would be a good mix of trust, privacy, and ease of use though.