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Alice has a long message to send. She is using mono alphabetic substitution cipher. She thinks that if she compresses the message it may protect the text from single letter frequency attack by Eve. Does the compression help? Should she compress the message before the encryption or after the encryption? Defend.

Will compression help defeat single letter frequency attack against a mono alphabetic substitution cipher?

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  • $\begingroup$ Hints for these nice homework questions: There are answers valid for all common compression algorithms. A simple reason why compression is usually done before encryption is that ciphertext is usually not compressible; but does it apply in the context? Assume that the attacker knows that Alice used compression (and which) wherever she used it, per second Kerckhoffs's principle. $\endgroup$ – fgrieu Nov 1 '16 at 12:11
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The procedure for doing this is:

plain text $\rightarrow$ compression $\rightarrow$ encryption $\rightarrow$ cipher text

Since the cipher text is not compressible, we do compression before encryption. The secret key of a mono-alphabetic substitution cipher is not effected by this compression. So compression does not avoid single-letter frequency analysis, and mono-alphabetic ciphers can be easily broken by frequency analysis.

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