The DRYAD Numeral Cipher/Authentication System (KTC 1400 D) is a simple, paper cryptographic system employed by the U.S. military for authentication and for encryption of short, numerical messages. Each unit with a radio is given a set of matching DRYAD code sheets. A single sheet is valid for a limited time (e.g. 6 hours), called a cryptoperiod. A DRYAD cipher sheet contains 25 lines or rows of scrambled letters. Each line is labeled by the letters A to Y in a column on the left of the page. Each row contains a random permutation of the letters A through Y. The letters in each row are grouped into 10 columns labeled 0 through 9. The columns under 0, 1, 2 and 5 have more letters than the other digits, which have just two each. While crude, the DRYAD Numeral Cipher/Authentication System has the advantage of being fast, relatively easy and requires no extra equipment (such as a pencil). The presence of more cipher-text columns under the digits 0, 1, 2 and 5, is apparently intended to make ciphertext frequency analysis more difficult. But much of the security comes from keeping the cryptoperiod short. DRYAD can be used in two modes, authentication or encryption. Authentication For authentication, a challenging station selects a letter at random from the left most column followed by a second (randomly selected) letter in the row of the first chosen letter. The station being challenged would then authenticate by picking the letter directly below the row and position of the second letter selected. For example, using the example cipher sheet to the right and the NATO phonetic alphabet, Jason could challenge Peggy by transmitting "authenticate Alpha Bravo". Peggy's correct response would then be "authenticate Yankee". Another form used involves selecting the third letter to the right of the second letter chosen by the challenging station (Jason's "Bravo" letter). Both the directional offset (up, down, left or right) and numeral offset can be values different from the examples given here, but must be agreed upon and understood by both parties before authentication. One problem presented is that an enemy impersonator has a one in 25 chance of guessing the correct response (one in 24 if a letter is selected from the same row). A solution to this is for Jason to require Peggy to authenticate twice; lowering the impersonator's odds of guessing the correct response to one in