Modes of Operation: CTR Mode

Learn about the counter mode of operation and how, unlike some of the other modes, it is able to benefit from parallelization.

Counter mode

The last of the modes of operation we will describe in full is counter (CTR) mode.

Encryption using CTR mode

Counter mode can be considered a counter-based version of CFB mode without feedback. The main difference is that we assume both the sender and receiver have access to a reliable counter, which computes a new shared value each time a ciphertext block is exchanged. This shared counter is not necessarily a secret value, but both sides must synchronize the counter. Both encryption and decryption in CTR mode are depicted in the illustration below:

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