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Ciphertext-only attack

In cryptography, a ciphertext-only attack is a form of cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. The attack is successful if the corresponding plaintexts can be deduced, or even better, the key. Every cipher attempts to provide protection against ciphertext-only attacks.

In the history of cryptography, early ciphers, implemented using pen-and-paper, were routinely broken using ciphertexts alone. Mechanical encryption devices such as Enigma made this much more difficult, and the eventual introduction of computers and electronics into cryptography has resulted in ciphertext-only attacks becoming very rare.

See also

• Known-plaintext attack
• Chosen plaintext attack.