Ciphers invented before the era of computers and modern cryptographic theory. Classical ciphers typically operate on letters instead of bits and are usually designed to be implemented by hand or using simple mechanical devices.

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How can one break a monoalphbetic substitution chipher at pseudorandom text?

Does anybody know how to break monoalphbetic substitution cipher, if it is applied to some pseudorandom text (for example to some surrogate key filed in a database)? Let us assume that we have only ...
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How to attack a classical cipher using known partial plaintext?

I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. What are the cryptanalysis approaches for ...
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About Cryptography in a Character Language

Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it. Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...