This is a chosen ciphertext attack, so yes, you have to decrypt the attacker-provided payload. That being the case, there are a lot of systems where that is ea

This is a chosen ciphertext attack, so yes, you have to decrypt the attacker-provided payload. That being the case, there are a lot of systems where that is easy to achieve. If you have GPG in your mail client, it will decrypt anything anybody sends you. Another thing to keep in mind is that attacks always get better, they never get worse. So today's chosen ciphertext attack is tomorrow's zero-knowledge attack.