# Is there a cipher that uses part of the already-computed ciphertext in the encryption scheme?

Are there any famous ciphers that, to determine the encrypted letter at position n in the ciphertext, it uses the encrypted letter at position n-1 in the ciphertext?

I tried google but I couldn't figure out how to phrase my search terms correctly to find something like this.

• Would CBC mode count? – poncho Jul 30 '16 at 3:04
• For classic (pre-electronic) ciphers this was not uncommon and was mostly called 'autokey' (auto being Greek for self) en.wikipedia.org/wiki/Autokey_cipher – dave_thompson_085 Jul 31 '16 at 3:41

The concept you mentioned is used in self synchronising stream ciphers. For example, CFB mode of a block cipher uses $$C_0=IV$$ $$C_n=E_K(C_{n-1})\oplus P_n, \quad n \geq 1$$ with $C_n,P_n$ denoting the $n^{th}$ plaintext and ciphertext blocks. So if you substitute "block" for "symbol", this is the same as what you want. CBC mode also gives a self synchronising stream cipher.
If we can extend "letter" to "block", then CBC mode is quite an obvious answer. It chains the blocks together so that each plaintext block is XOR'ed with the previous ciphertext block before encryption, i.e. $C_{n} = E_k(P_{n} \oplus C_{n-1})$.