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Suppose
that an intruder has an encrypted message as well as the decrypted
version of that message. Can the intruder mount a ciphertext-only
attack, a known-plaintext attack, or a chosen-plaintext attack?
Although an intruder intercepting Alice’s encrypted message will see only gibberish, the intruder knows both the key (Bob’s public key, which is available for all the world to see) and the algorithm that Alice used for encryption.
Therefore. “Yes”, Trudy can thus mount a chosen-plaintext attack, using the known standardized encryption algorithm and Bob’s publicly available encryption key to encode any message she chooses! (6th ed, p684)
Other cite references...
In this case,,, a known plaintext attack is performed. If, somehow, the message encrypted by the sender was chosen by the attacker, then this would be a chosen-plaintext attack.