I was researching the US military's DIANA one-time-pad system and came across the following quote purportedly from a former US Special Forces soldier:

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.

(source: https://dodona.ugent.be/en/exercises/2088793301/)

Wouldn't the use of random words as reported by the soldier rather than groups of random letters diminish the security of the system, as the letters in 5 letter English words are not randomly positioned?

I am aware that "secure" in this context--messages between Special Forces camps during wartime--may just mean keeping messages secure long enough that they are no longer useful even if broken.


1 Answer 1


Do those look like English words to you?


(Quoted from the page.)

When they say ‘5-letter words’, they don't mean words chosen from an English dictionary. They mean that they rolled a d26 five times independently and strung the resulting letters together, with spaces between every five letters and line breaks between every four ‘words’ for legibility. This method is just as secure as using a d2 (i.e., flipping a coin) to choose the bits of a binary one-time pad; it just uses a different alphabet.

  • $\begingroup$ That's an example from the page author, not an actual Vietnam-era pad which may be different. $\endgroup$
    – user40185
    Jun 16, 2019 at 17:01
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    $\begingroup$ OK, but why do you suspect that the system being described was not actually…the system being described? Is your question ‘Does this source accurately describe the history?’, or is your question ‘Is the system described in this source secure?’? $\endgroup$ Jun 16, 2019 at 17:07
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    $\begingroup$ @ jsfierro Yes, the system used by the SF soldier is secure if the key is at least as long as the message, the key is not compromised, and the key is used once. Squeamish answered your question perfectly: "word" means a group of five letters generated in a truly random manner. I have actually used that system in Special Forces--many moons ago--and yes, it provides confidentiality. $\endgroup$
    – Patriot
    Jun 16, 2019 at 18:32
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    $\begingroup$ The National Security Agency has an extremely interesting history of VC SIGINT on their website. You might like that. VC Special Forces cryptographic teams were small and used Vernam Cipher and HF too, sometimes in Near Vertical Incidence Skywave. $\endgroup$
    – Patriot
    Jun 16, 2019 at 18:39
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    $\begingroup$ @Patriot and Squeamish Ossifrage thanks, that makes much more sense than using a page of actual words. $\endgroup$
    – user40185
    Jun 16, 2019 at 19:19

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