TheCodeBreakers
Lists are given of letters that are never found together in
one word, of letters that rarely come together in a word, of combinations
of letters that are not possible ("Thus tha' may not precede shin."), and
so on. Finally, the exposition gives a list of letters in order of "frequency
of usage in Arabic in the light of what a perusal of the Noble Koran
reveals." The writers even note that "In non-Koranic writings, the
frequency may be different from this." Following which, Qalqashandi
explains lucidly the principles of cryptanalysis and demonstrates with
two examples. But this knowledge vanished in the Arab decline.
The technique of cryptanalysis rests on two phenomena. One is that
all letters are not used equally in any language. In other words, the first
26 letters of say, the Gettysburg Address do not contain one a, one b, one
c, so on. Rather, some letters occur more often than others. The second
phenomenon is that the proportions in which the letters occur remain
constant
annaabi.ee 
