CryptologyStatistical Analysis of Ciphertext |
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Natural languages such as
This allows automating the cryptanalysis based on letter frequencies to a large extent. The web offers several such programs, for example see here or here, and in the ACA Crypto Dropbox.
Exercises:
EJGGZ TGWOF IPOHI HONAW OCIAO TQUPO HZTHI EFOTQ QCHIO TNAIO IOHHZ TGUJP QRAOT QCGWO FIIJP ROTQR OTQNA VJHOT RJQJQ EOPHint: The letter frequencies will mislead you. A better approach uses a probable word.
For a description in mathematical terms see here.
The statistical analysis of ciphertext
To escape statistical attacks people invented lots of methods that flatten the distribution of frequencies, for example homophonic, polygraphic, or polyalphabetic ciphers, or compression before encryption. But these methods give little protection against attacks by pattern search or probable words.
There are refined methods of statistical analysis that also apply to more complicated ciphers. These are the subject of Chapter 3.