Nihilist and Checkerboard (Polybius) Ciphers

In the history of cryptography, the Nihilist cipher is a manually operated symmetric encryption cipher, originally used by Russian Nihilists in the 1880s to organize terrorism against the tsarist regime. The term is sometimes extended to several improved algorithms used much later for communication by the First Chief Directorate with its spies. First the encipherer constructs a Polybius square using a mixed alphabet. This is used to convert both the plaintext and a keyword to a series of two digit numbers. These numbers are then added together in the normal way to get the ciphertext, with the key numbers repeated as required.

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