CTM

A block cipher mode that turns a block cipher into a stream cipher by encrypting successive counter values.

Counter mode (CTR) runs a block cipher such as AES over an incrementing counter and XORs the result with the plaintext, effectively making a stream cipher that can encrypt blocks in parallel and in any order. Because identical counter values must never repeat under the same key, the nonce and counter handling are critical to its security. It underpins authenticated modes like AES-GCM, where counter mode provides confidentiality and a separate tag provides integrity. On the exam, associate CTR with high-performance, parallelizable encryption.