I am pleased to offer this month's guest essay, written by one of the newest members of the Quiller Mailing List, Scryer[9]. You might already have seen, on the <QML>, Scryer's excellent decrypting of Quiller's intended message to Control in

For your convenience, I have reproduced at the bottom of the page the cryptogram used by Scryer as an example.

Rick Holt (Iron Mouth)

The Index of Coincidence is a sensitive test that allows us to determine (well, OK,

To use it to determine the period, break the cipher up into each period you would like to test; in this case, we may as well start with the four we identified as likely from the Kasiski test. Write the cryptogram four times, once in each period, then do a frequency count on each column. For each one, add up the quantity f[i] * (f[i] - 1), where f[A] is the frequency of ciphertext A and so on. Divide each of these results by N * (N - 1), where N is the number of letters in that column. This is simple to do by hand, and even simpler to do with a computer.

To test whether the July Vig is period 5, write it as period 5 and calculate the IC on each column: it comes out as 0.0303 for the first column (pretty random), 0.0563 for the second (not bad), 0.0433, 0.0476, and 0.0390 (pretty pathetic). Then take the average of these five numbers, and we get the overall score for period five: 0.0433. This is not encouraging: we want about 0.065.

Well, then let's try period 6: the average of these is 0.0487, still not good. Period 9? 0.0455. Period 10? 0.0491.

The best of the lot (and it's not all that great) is period 10, 0.0491. Usually the best value will show up much better than this, but occasionally the cryptanalyst will have bad luck, especially with shortish cryptograms such as this one. In any case, we have our candidate: try period 10 first, and start dragging your cribs through it looking for likely pieces of the key to pop out the other side. If it doesn't work out, then try period 6, and so on. The important thing is to keep trying stuff, and don't be discouraged with failure: you can often solve something if you just keep trying new ideas.

This is the July Vigenere cryptogram Scryer refers to in his article:

Level of difficulty: High

Enhancements: None

Clue: For the Messiah's travel plans