The Index of Coincidence:

If you study this you'll learn that it has the handy ability to render values that can be used to determine the language of the plaintext enciphered before the plaintext is revealed. This would only be true of multiple substitution tables and probably not capable of that in quadradic encryptions. However you could still use it to test your run-results for validity. Its a lot more robust than scanning for keywords.

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You question: Is a known payload in an encrypted message useful in uncovering the encryption?

I can't think of any exception where that is not the case in principle. You might not have enough data to make it useful through. Human choice, repetition and duration are the Achilles' Heels of ciphers.

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If you are doing sigint on local PAGER transmissions, let me know. I think pagers transmissions are quite sporting to analyze. Its not so much a case of decrypting as it is deducing their packet structure. Fun! Fun! Fun!

If you are messing with Cellular Phones, I think that is illegal.

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In the early 1980s, I worked on digital radio modem communications. We bit-banged directly out of a 8085 micro to/from the radio audio signal without an UART in a completely proprietary protocol at 4800 baud. We had a fairly secure system that was used in Mobile Data Terminals (MDTs). I still see those units on TV and in the Movies (We did the system for Los Angeles).

That company never believed our warnings that flat screen displays would get cheaper and that laptop computers would eventually replace their MDTs and so they never evolved their design. I assume that company eventually went out of business.

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Someday I'd like to get my PC sound card input hooked up to my radio scanner and write PC programs to translate live radio packets.

I can do this much easier in an 8051 than a PC, so I'll probably program one on my lab prototypes to do this. The last time I did this, it was a Intel 386 running under DOS. :)

aka j