I've been reading a bit about block cipher modes and I have a relatively straightforward question regarding CTR. In essence, I was hoping you guys would be kind enough to validate my understanding of things.
As I understand it, CTR does the following:
- Take the first output of the nonce (let us assume an incrementing int for our purposes)
- Encrypt the nonce with the key. CTR requires that the key and the nonce be of identical size.
Perform some lossless operation between encrypted nonce and a portion of plaintext (e.g.: XOR)
Repeate ad libitum
Here's what I don't understand. CTR is supposed to transform a block cipher into a stream cipher, but don't you still have to operate block-by-block for step 3?
What am I missing?