Reciprocal cipher

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A reciprocal cipher is a shared-key cryptographic system in which there is no difference between "encoding" and "decoding". If a person "encrypts" some ciphertext (with the same key) with the system, cleartext comes out. (In non-reciprocal ciphers, repeatedly encrypting does *not* result in the original plaintext).

Some famous reciprocal ciphers include

Most mechanical cipher machines use a reciprocal cipher, so it wouldn't need a seperate "encode mode" and "decode mode".