Nihilist cipher

The Nihilist cipher is a manually operated symmetric encryption cipher, originally used by Russian Nihilists in the 1880s to organise terrorism against the tsarist regime. The encipherer builds a Polybius square from a mixed alphabet and uses it to turn both the plaintext and a keyword into a series of two-digit numbers. The key numbers are repeated as required and added to the plaintext numbers in the ordinary way to produce the ciphertext. Because each symbol is used as a whole number with no fractionation, the basic cipher is little more than a numerical version of the Vigenere cipher and can be attacked by very similar methods, and the use of normal rather than modular addition leaks extra information. The term is sometimes extended to improved algorithms that Soviet spy rings used much later, including versions built on a straddling checkerboard, and the line of development ended with the VIC cipher of the 1950s.