Reciprocal cipher

From CryptoDox, The Online Encyclopedia on Cryptography and Information Security

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

• The Enigma machine was a reciprocal cipher machine
• ROT13
• Beaufort cipher
• the Kama-sutra cipher
• Enigma
• ...

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