It is good idea but isn't it better to encrypt locally?
Does it make a difference? Its much easier if the forum software did it automatically.
I think encrypting locally is better than encrypting messages server-side. I can't see the point sending messages to server to encrypt. My taste maybe different from yours.
Well, PGP has you make a private and public key. The public key is given to others, so they can send you an encrypted message. Only you can decrypt it with your private key. So if you uploaded a public key to your account on bitcointalk, any pm anybody sends to your, could then be encrypted with this key, and then the plaintext deleted. However then the sender would not be allowed to keep the message in his outbox unencrypted as that would defeat the purpose of encryption.
Also, if the forum comes under attack, as it already has (ref. recent BFL subpoena), there's no guarantee that there will not be installed any software to circumvent the encryption, pretending for the users that all is fine, while in reality the clear text message is siphoned off behind the scenes.
In short, as long as the message goes to the bitcointalk server in clear-text it must be assumed it is compromised, even though they promise to encrypt it, then delete the clear text message. Of course, it might be possible to do the entire task client-side, without the clear-text message touching bitcointalks server, but that again could be circumvented temporarily or permanently by changing javascript in the forum code.
Handling encryption yourself might be the best idea. So perhaps the very best idea would be to just give your PGP-key to those you wish to communicate securely with. However, there might also be MITM-attacks here.
A gives PGP pubkey to C, however B intercepts the communication and gives C his pubkey.
C encrypts a message to A with A's pubkey, then sends it to A. It is again intercepted by B, B decrypts it with his pubkey, as the pubkey C holds and thinks is A's pubkey in reality is B's pubkey, then B decrypts message from C with his own privkey, then encrypts it with A's pubkey and send it to A. That interception could go both ways, but it could be revealed if great care is taken in exchange of the keys.
To actually know that you have the correct key, you should compare fingerprints.
This could be worth reading:
https://futureboy.us/pgp.htmlAs for communication with "persons of interest", it's not a wild stretch to imagine that there are active MITM attacks in this area. As for what constitutes "persons of interest", once you start looking at anything online that's outside the ordinary, for example you show an interest in TOR, Tails or anything else that's 'suspicious', you most likely end up on some list of "persons of interest". As for persons of "major interest", as major security researches and the like, it's not unthinkable that MITM-attacks in deed is taking place. For example I'm pretty sure that encrypted messages going to bitcoin devs are "of interest". I have not read about it happening, but for an entity with enough resources, I guess it's already happening. So in this regard, verification is as important as encryption itself.
If you let a forum do all the encryption, a MITM-attack is very easy to set up, and might give the users a false sense of security, as the forum says "it's all good", while in reality they have a gun to their head.
Call me paranoid, but if the target is interesting enough, and it is possible to do surveilance, it will be done.
Taking all of this into consideration, I would say that doing everything locally is the best method.
