📝Counter (CTR)

CTR is a block cipher mode of operation that turns a block cipher into a stream cipher.

Similar to OFB, it generates a keystream. Nonce (IV) + counter are used as keystream input for each block.

• The counter can be any predictable function that produces a sequence that does not repeat for a long time. In practice, a simple increment is the most simple and popular implementation.

• The IV predictability allows random access when encrypting/decrypting blocks, and can be done in parallel.

If IV/Nonce are random, they can be combined with the counter using any invertible operation (concatenation, addition, or XOR).

• If Nonce is non-random, they should be combined by concatenation. Simply Xoring nonce and counter would break security under a Chosen-plaintext attack.

CTR requires that offset/location information is not corrupt. Otherwise, it is impossible to know the counter and decrypt a block.

Algorithm:

• Encrypt Nonce+Counter, Xor it with plaintext/ciphertext.

This algorithm is similar to OFB.