"The chart consists of a square 26 x 26 containing 26 random alphabets with no repeats in either rows or columns. There are two different random alphabets on the outside of the square, on placed vertically to the left and the other placed vertically to the right. Below the chart there is a substitution table to be used in case numbers occur in the challenge instead of letters in some instances. This table consists of 24 letters (a complete alphabet with the exception of X and Y) each of which is assigned a number from 0 through 23."


"There are two methods of using the above-described chart, viz., the Challenge and Reply Method and the Message (Direct) Authentication Method.

(1) Challenge and Reply Method: The authentication consists of 3 letters and a reply of one letter. The three letters of the challenge are any three letters chosen at random. Locate the first letter of the challenge in the left-hand outside alphabet, proceed horizontally to the second letter (within the square), then proceed vertically up or down to the third letter in the right-hand outside alphabet. This letter is the reply letter.

(2) Message (Direct) Authentication Method: The authenticator consists of a single letter repeated. The elements used are: hour of origin, as shown in the time group (or hour of transmission if a time group is not used); the last character of the originator's call; and the first letter of the text. Select the letter in the left-hand outside column representing the time in hours (as taken from the substitution table), proceed horizontally to the last character of the originator's call sign (substitute from the table if this is a digit), then vertically to the first letter of the text (substitute from the table if the text consists entirely of numbers), then horizontally to the outside right-hand alphabet. The letter thus designated is the reply letter."

See CSP 1272(A), Instructions for Use of Aircraft Code No. 16 for more detail.


The description enclosed in quotes above is from:
Survey Of Authentication Systems 1942-45 (1945) This is available from NARA; NSA Historical Collections 190/37/7/1, NR 3526 CBRK24 12960A 19420728.

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