cipher block chaining

cipher block chaining is a way to nondeterministically encrypt an arbitrary-length message using a block cipher.

1. pad your message to a multiple of the block length
2. generate a random initial vector (IV) $C_0$. this is the first ciphertext.
3. before encrypting block $M_i$, XOR it with $C_{i-1}$, i.e. $C_i:=E_K(M_i\oplus C_{i-1})$
4. and then output the ciphertext $C:=C_0||C_1||\dots ||C_m$.

in diagram form:

note that you expect to see a block cipher input repeated after $\sqrt{2^n}$ blocks have been encrypted with the same key $K$, where $n$ is the block size in bits (TODO: why? so what?)