Counter Mode Encryption, also known as Counter (CTR) mode, is a mode of operation for block ciphers that enables the encryption of individual blocks of plaintext using a counter value. It transforms a block cipher into a stream cipher, allowing it to encrypt data of arbitrary length.
In Counter Mode Encryption, a unique counter value is generated for each block of plaintext. The counter value is combined with a secret encryption key using the block cipher algorithm to produce a stream of pseudo-random bits, known as the keystream. The keystream is then XORed with the plaintext block to produce the corresponding ciphertext block.
One of the main advantages of Counter Mode Encryption is its parallelizability, as each block of plaintext can be encrypted independently of others. This property allows for faster encryption and makes it well-suited for hardware and software implementations that can take advantage of parallel processing.
To ensure security, it is essential to use a unique and unpredictable counter value for each block of plaintext. This counter value should never be reused, as doing so can lead to serious security vulnerabilities, such as keystream reuse or encryption key recovery.