Counter Encryption Mode
Counter (CTR) mode is a method of encryption that transforms a block cipher into a stream cipher. It’s a mode of operation used for cryptographic block ciphers, which are algorithms that encrypt data in fixed-size blocks. Unlike some traditional block cipher modes, which encrypt or decrypt one block of data at a time, CTR mode enables the encryption of smaller units of data, making it more versatile and efficient, particularly for applications where data lengths aren’t always a multiple of the block size.
In CTR mode, the encryption process involves generating a unique counter value for each block of plaintext data. This counter, often combined with a nonce (a number used once) to ensure its uniqueness, is then encrypted using the block cipher. The resulting encrypted counter is then bitwise XORed (exclusive OR operation) with the plaintext to produce the ciphertext. For decryption, the same process is applied: the counter is encrypted and then XORed with the ciphertext to retrieve the original plaintext.
One of the key advantages of CTR mode is that it allows for parallel processing, significantly speeding up encryption and decryption, especially in hardware implementations. Moreover, since it converts a block cipher into a stream cipher, it eliminates the need for padding, making it useful for encrypting data of arbitrary lengths. However, it’s crucial to never reuse the same counter value with the same key, as this can lead to vulnerabilities and compromise the security of the encrypted data. In secure implementations, CTR mode is often favored for its efficiency, parallelizability, and flexibility.
Links
- Educative: https://www.educative.io/answers/what-is-ctr
- IncludeHelp: https://www.includehelp.com/cryptography/counter-ctr-mode-in-cryptography.aspx
- University of California, Davis: https://www.cs.ucdavis.edu/~rogaway/papers/ctr.pdf