2
$\begingroup$

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.

$\endgroup$
2
  • 3
    $\begingroup$ Would CBC mode count? $\endgroup$
    – poncho
    Jul 30, 2016 at 3:04
  • 2
    $\begingroup$ 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 $\endgroup$ Jul 31, 2016 at 3:41

2 Answers 2

3
$\begingroup$

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.

Moustique is a self synchronising stream cipher designed by J Daemen.

$\endgroup$
3
$\begingroup$

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}) $.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.