
Parallels Between Cryptanalysis and Crossword Solving - gprasanth
http://www.crosswordunclued.com/2015/06/parallels-between-cryptanalysis-and.html
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Animats
Ah, they cite Friedman. Friedman was the real genius in cryptanalysis. Before
Friedman, cryptanalysis was about puzzles. After Friedman, it was about
number-crunching.

Friedman's first big development was the "index of coincidence". This is a
distance measure between a crypto key being tried and the actual key. If you
can tell if you're getting closer, you can hill-climb to a solution, and
computers are good at this. If a distance measure can be found for a
cryptosystem, it's thus easily breakable. WWII rotor machines are vulnerable
to this approach. So are the classic paper-based substation cyphers, except
for one-time ones where the key is as long as the text. (Use the same one-time
key twice, and the cypher can be broken. See VENONA.)

Modern cryptosystem design requires that wrong decryptions look random even if
the key is off by only one bit. That's a necessary, but not a sufficient,
condition.

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thaumasiotes
> WWII rotor machines are vulnerable to this approach. So are the classic
> paper-based substation cyphers, except for one-time ones where the key is as
> long as the text. (Use the same one-time key twice, and the cypher can be
> broken. See VENONA.)

Well yeah. If you reuse the key, then you've encrypted a message using a key
shorter than the text.

