Feature: Use of the One-Time pad in Vietnam

14 November 2005

Sgt. Ron Hibbard Ret.

Special Forces were one of (if not the only) units in Vietnam to utilize Morse code on a regular basis. We used a method of encryption called the Diana Cryptosystem.

The basis of these "One-Time Pads", is that there were only two matching pads in existence, and they would only be used one time. They were booklets that contained randomly generated groups of 5-letter "words;” 30 words to a page. The person sending a message would first write the letters to the message, over these random groups of words. Included in the front of each one-time pad was a one-page encryption table. If I wanted to send the letter "P", and the letter under the "P" was an "A", then I would send a "K". The person listening on the frequency at the other end, would have the other matching pad. They would write the letter they received (a "K") over the letter in their one-time pad (an "A"), and decipher it based on the table, yielding the original letter "P".

Each communication site in Vietnam (we had over 100 A-Camps along the Cambodian / Laotian border, and some 20 B-detachment sites spread over the country) had a different pad, depending on the location they were having the commo-check with. It obviously was very important that both people were using the appropriate matching pads, or the deciphered messages would not make any sense.

After a while, most of us became so proficient with the system, that we actually learned the deciphering matrix by heart. No matter what pads anyone had, the combinations always were the same. i.e. Any 3 letters always went together, regardless of the order; "BKO"/"KOB"/"OBK"/"BOK". After listening to thousands and thousands of transmissions, it really got quite simple. If I was listening to code, and a letter "B" was sent (now remember, we usually sent around 20-25 "words" (5 letters per word) a minute, hence the importance of the "speed" keys!), and the letter it was associated with was an "O", most of us would decipher as we heard it, and just write the "K". That may sound like quite a yarn, but it is absolutely true.

A funny story... there was a guy that I trained with at Ft. Bragg in the 7th SF group for over a year before we went to Vietnam. He had a penchant for simply sending too many dits for the letter "H". It was so obvious, that even in Vietnam, all of us on the commo circuit, always knew when Smitty was transmitting! Thank god he never had a problem with the "4"s! We were always wary of him sending any coordinates for air strikes!

Copyright © VietnamGear.com. All rights reserved. This material is intended solely for internal use within VietnamGear.com. Any other reproduction, publication or redistribution of this material without the written agreement of the copyright owner is strictly forbidden and any breach of copyright will be considered actionable