Counter Mode

What
The counter mode of operation is a mode that relies on a counter with an initial value. Then, for each successive block, the underlying block cipher algorithm encrypts the counter value (incremented) and passes it into a vernam cipher with the original message block.

Advantages

 * It can be parallelized because it does not contain any feedback loops

Counter Encryption
$$C_i = P_i \oplus E(Ctr_i)$$

Counter Decryptiong
$$P_i = C_i \oplus E(Ctr_i)$$