Diana Cryptosystem

Programming Praxis

[ Today’s exercise comes from the blog of long-time reader Ben Simon, after he declined my offer to guest-author the exercise and told me to steal it. The code and most of the text is his, so he gets the credit, not me. ]

It’s been a while since we had an exercise from our cryptography topic. Today’s exercise is the Diana Cryptosystem, also known as the trigraph cipher, which was used militarily by US forces during the Vietnam War and is theoretically unbreakable when used properly.

There are two pieces to the system: the trigraph itself, which is pictured at right, and a one-time pad, like this:

WHTVI AUCFU RETFK OMSAL
MYMNE ZIEGP UKVTF WZHOK
GORWY WETFR COYET OOWHY
ZPDDA CMMXT VYTJI RRQGU

To use the cipher, a section of the one-time pad is chosen and the two five-character groups that begin the section are transmitted unchanged. Thereafter, the…

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