1.5. PROBLEMS 17
are r rows and c columns, this means she can send a message of rc letters.
In terms of security and frustrating an attacker, which of the following is
the best choice for rc and why: 1331, 1369, 1800, or 10201?
Exercise 1.5.15. Research and write a brief description about one of
the following:
The Black Chamber.
The technological treason in the Falcon and the Snowman case.
Cryptography during Prohibition and the role of Elizabeth Smith
Friedman.
Echelon.
The Kryptos sculpture at the NSA.
Exercise 1.5.16. A major theme of this book is the need to do compu-
tations quickly. The Babylonians worked base 60; this meant they needed to
know multiplication tables from 0 × 0 all the way to 59 × 59, far more than
we learn today (since we work base 10, we only go up to 9 × 9).
(a) Calculate how many multiplications Babylonians must memorize or
write down.
(b) The number in part (a) can almost be cut in half, as xy = yx. Using
this observation, how many multiplications must be memorized or written
down?
(c) As it is painful and expensive to lug clay tablets around, there was a
pressing need to trim these numbers as much as possible. The Babylonians
made the remarkable observation that
xy =
(x + y)2 x2 y2
2
.
Show this formula is true, and thus reduces multiplication to squaring, sub-
tracting, and division by 2.
Remark. The above formula shows that the Babylonians need only
learn the squares and can deduce the remaining products by elementary
operations. This is an early example of a “look-up table”, where some
calculations are done and stored, then used to deduce the rest. This exercise
shows that the standard way to do a problem is sometimes not the most
practical.
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