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Security Fundamentals & Crypto · Advanced

Counter Mode (CTR)

Block cipher mode that encrypts a nonce-plus-counter value and XORs the result with plaintext, acting like a stream cipher.

Definition

CTR mode turns a block cipher into a stream cipher, supports parallel encryption and decryption, and allows random access to any block. It requires a unique nonce and counter combination for every block under a given key. It provides confidentiality only and is the basis of GCM.

Example

AES-CTR encrypts disk sectors in parallel by using the block number as part of the counter.

Exam tip

Nonce reuse in CTR mode exposes plaintext, just like reusing a stream cipher keystream.

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