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A keyword cipher is a form of monoalphabetic substitution. A keyword is used as the key, and it determines the letter matchings of the cipher alphabet to the plain alphabet. Repeats of letters in the word are removed, then the cipher alphabet is generated with the keyword matching to A,B,C etc. until the keyword is used up, whereupon the rest of the ciphertext letters are used in alphabetical order, excluding those already used in the key.
Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Encrypted: K R Y P T O S A B C D E F G H I J L M N Q U V W X Z
With KRYPTOS as the keyword, all As become Ks, all Bs become Rs and so on. Encrypting the message "knowledge is power" using the keyword "kryptos":
Plaintext: K N O W L E D G E I S P O W E R Encoded: D G H V E T P S T B M I H V T L
Only one alphabet is used here, so the cipher is monoalphabetic.
The best ways to attack a keyword cipher without knowing the keyword are through known-plaintext attack, frequency analysis and discovery of the keyword (often a cryptanalyst will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.