In 1910, the U.S. Department of State published a codebook detailing The Green Cipher. In its 1400 pages were an extensive list of words, laid out alphabetically like in a dictionary, alongside each term's corresponding five-letter code. Pages were designated three-letter "root words", consisting of a consonant, a vowel, and a consonant in that order. The columns on the page lined up the entries with two-letter suffixes, either a consonant followed by a vowel or the reverse order. For example, on a page with the root word "Caf", the first entry would be the solution for the word "Cafba", and across the page would be the solution for "Cafab". Messages would be transmitted with each pair of words melded into one ten-letter string.
Additionally, an ingenious method of dating telegrams was also included, allowing a date and time to be condensed into a five-letter sequence. The first letter of the sequence would indicate the month. The code letter for January would be B, followed by C for February, D for March, F for April, G for May, K for June, L for July, M for August, P for September, R for October, S for November, and V for December.
Dates would be two digits, so January 1 would become "January 01". A vowel represented the first digit: O for 0, A for 1, E for 2, and U for 3. This would be the second letter in the sequence. The second digit of the two-digit date would become the third letter, following the same alphabetic sequence of consonants as the months, starting with B as 1 and ending with R as 0.
The time was given as an hour. The same sequence of consonants was used, with B as 1 and progressing through to end with V as 12. Generally, the last letter would either be I to designate A.M. or Y for P.M., but noon was also coded as an I, and midnight as Y.
For instance, this post is marked below as July 24 at 12:00 AM (midnight). In the green cipher, this comes out as LEFVY.
In 1919, the Green Cipher's name was officially changed to the more accurate Department of State Code A-1.
source: Wrixon, Fred B. Codes, Ciphers and Other Cryptic and Clandestine Communication. Black Dog & Leventhal, 1998. pp. 342-344.
