Cryptology - Unofficial St. Mary's College of California Web Site

170 CHAPTER 9. DIGRAPHIC CIPHERS
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Figure 9.2: A More Complicated Digraphic Substitution Chart
for etaoinshr, as we would in a monographic cipher, we look for th-he-in-erre-....
But the basic idea is the same: do a (very large) frequency count, guess
that the top occurring pairs come from therein, and start working. So while
we need more data, the techniques are very similar to the ones with which we
are comfortable.
How to make this system better I guess a 26 × 26 × 26 hyper-chart listing
all 17576 three-letter triples. Maybe one would do this as 26 charts, where the
first letter of the triple was the name of the chart, the second one was on the left
of the chart and the third one was one the top The extreme cumbersomeness
of a system like this would limits its use to those who have a computer to help. 4
9.2 Hill Ciphers
Lester S. Hill (1881-1961) was an assistant professor of mathematics at Hunter
College in New York when, in 1931, he published the method now named for him
as “Cryptography in an Algebraic Alphabet” in The American Mathematical
4 The idea of enciphering letters not individually but in groups, so-called block ciphers,
reappears in almost every modern cipher system.