Passive attack
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A passive attack on a cryptosystem is one in which the cryptanalyst cannot interact with any of the parties involved, attempting to break the system solely based upon observed data (i.e. the ciphertext). This can also include known plaintext attacks where both the plaintext and its corresponding ciphertext are known.
While most classical ciphers are vulnerable to this form of attack, most modern ciphers are designed to prevent this type of attack above all others.
See also: Chosen plaintext attack, Chosen ciphertext attack, Adaptive chosen ciphertext attack, topics in cryptography.