cipher, any method of transforming a message to conceal its meaning. The term is also used synonymously with ciphertext or cryptogram in reference to the encrypted form of the message. A brief treatment of ciphers follows. For full treatment, see cryptology. All ciphers involve either transposition or substitution, or a combination of these two mathematical operations—i.e., product ciphers. In transposition cipher systems, elements of the plaintext (e.g., a letter, word, or string of symbols) are rearranged without any change in the identity of the elements. In substitution systems, such elements are replaced by other objects or groups of objects without