Nihilist cipher
nounEtymology The cipher was originally used by Russian Nihilists in the 1880s to organise terrorism against the tsarist regime.
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Computing, Cryptography, Engineering, Mathematics, Natural sciences, Physical sciences, Sciences Historical A symmetric cipher in which a Polybius square using a mixed alphabet is used to convert both the plain text and a keyword to a series of two-digit numbers, which are then summed in the normal way to get the ciphertext, with the key numbers repeated as required; (by extension) any of several improved algorithms based on this.