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Simple ciphers

Simple encryption algorithms, which were invented long before first computers, are based on substitution and transposition of single plaintext characters. Meanwhile, the operations performed in modern encryption algorithms are usually similar but they affect single bits and bytes.

Substitution ciphers

The substitution ciphers are about replacing each group of plaintext letters with another predefined group. For decrypting, one should use a reverse substitution. There are four main categories of substitution ciphers:

Transposition ciphers

Transposition ciphers do not substitute any plaintext characters but they change positions of the letters.

Rotor machines

First electro-mechanical machines started to be used for encryption as early as at the end of the 19th century. Rotor machines, which were developed after World War I, were equipped with internal movable rotors that implemented polyalphabetic substitution ciphers.

Simple XOR

It is a simple substitution cipher which operates on single bits and bytes, instead of on leters.