active cracking

[farmwife]

"Active cracking" means indirectly changing a plaintext message so that it becomes a totally different, plaintext message that says something totally different but still makes sense, by creating a brand new encryption key that is applied to the ciphertext itself and changes the ciphertext rather than the original plaintext. I know this can be done without ever knowing the original plaintext message because you don't have to decrypt the ciphertext in order to change it, but can it be done without ever determining the original encryption key, or do you have to determine the original encryption key in order to create the second, active-crack key?