Knownplaintext

Known-plaintext attack, or known-plaintext cryptanalysis, is a cryptanalytic model in which the attacker has access to one or more pairs of plaintext and the corresponding ciphertext produced by a secret key. The goal is to deduce the secret key itself or to decrypt additional ciphertexts produced with the same key.

In contrast to other attack models, such as ciphertext-only (where the attacker only observes ciphertext) and chosen-plaintext or chosen-ciphertext (where the attacker can obtain ciphertexts from chosen plaintexts or vice versa), known-plaintext assumes some knowledge of plaintexts. The attack is generally more feasible when the encryption is deterministic and a given plaintext always yields the same ciphertext; with probabilistic encryption, multiple ciphertexts per plaintext require more samples or more sophisticated analysis.

In practice, modern secure schemes are designed to be resistant to known-plaintext attacks under standard threat models, but KPA remains a relevant consideration in scenarios where keys are reused or where partial information about plaintexts is available. The strongest widely used security notions for encryption are indistinguishability under chosen-plaintext (IND-CPA) and under chosen-ciphertext attacks (IND-CCA), which provide guarantees even when an attacker has broader access than in a simple known-plaintext setting and thereby imply resistance beyond what a KPA guarantees.

Historically, known-plaintext attacks influenced the cryptanalysis of classical ciphers and early block ciphers, guiding the development of more robust designs. Today, the model remains a foundational concept in the study of cryptographic security and in the evaluation of encryption schemes.