Cryptanalysis of General Polygraphic Substitutions
Polygraphic systems do provide a significant improvement in security over monoalphabetic substitutions. Given an individual letter 'E' in a message, it could be encrypted using any of 52 instructions depending on its location and neighbors, which can be used to great advantage to mask the frequency of individual letters. However, the security boost is limited; while it generally requires a larger sample of text to crack, it can still be done by hand.
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“If men would avoid that general language and general manner in which they strive to hide all that is peculiar and would say only what was uppermost in their own minds after their own individual manner, every man would be interesting.”
—Ralph Waldo Emerson (1803–1882)