625. The downside to this method is reduced longevity of the current DRYAD page, since the page is getting twice as much use as a single-authentication scheme. Encryption The second mode is used to encrypt short numeric information (such as map coordinates or a new radio frequency). The coder arbitrarily selects two letters at random. The first letter designates a row in the current active page. The second one is used similarly to the authentication mode, except that the letter immediately to the right is selected. This selected letter is called the "SET LETTER." Numbers are enciphered one digit at a time. A ciphertext letter is chosen from the selected row (the row designated by the SET LETTER) in the column under the plain text digit. If the digit occurs more than once in the number, the coder is instructed to choose a different letter in the same column. All the digits in a single plaintext number are encoded from the same row. (There is also a provision for encoding letters associated with map grid coordinates.) See also BATCO — similar paper based tactical cipher used by the British forces M-94 — tactical cipher used in World War II Dryad — original meaning in mythology Polyalphabetic cipher Substitution cipher References Further explanation of the DRYAD Numeral Cipher/Authentication System U.S. Army Field Manual FM 24-35 Classical ciphers National Security Agency encryption devices