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Known-plaintext attack
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<p> The known-plaintext attack (KPA) is an <a href="/facts/Attack_model/0UGyn7ha">attack model</a> for <a href="/facts/Cryptanalysis/7CPEQlJI">cryptanalysis</a> where the attacker has access to both the <a href="/facts/Plaintext/tWnBDCgE">plaintext</a> (called a crib) and its <a href="/facts/Encryption/16q29Fdv">encrypted</a> version (<a href="/facts/Ciphertext/luNTs4sY">ciphertext</a>). These can be used to reveal <a href="/facts/Cryptographic_key/5W5PAmT4">secret keys</a> and <a href="/facts/Code_book/X78YE7AS">code books</a>. The term "crib" originated at <a href="/facts/Bletchley_Park/ID0B04WB">Bletchley Park</a>, the British <a href="/facts/World_War_II/2efV5L1U">World War II</a> decryption operation, where it was defined as:</p><blockquote><p>A plain language (or code) passage of any length, usually obtained by solving one or more cipher or code messages, and occurring or believed likely to occur in a different cipher or code message, which it may provide a means of solving.</p>— <a href="https://www.codesandciphers.org.uk/documents/cryptdict/cryptxtt.pdf"><i>The Bletchley Park 1944 Cryptographic Dictionary formatted by Tony Sale, 2001</i></a> (PDF), p. 22</blockquote>