Block cipher modes of operation are techniques used to apply a block cipher algorithm to encrypt or decrypt data that is larger than the block size of the cipher. These modes define how multiple blocks of data are processed and combined to provide confidentiality and integrity. Here are some commonly used block cipher modes of operation:
- Electronic Codebook (ECB): In ECB mode, each block of plaintext is encrypted independently using the block cipher algorithm with the same encryption key. This means identical plaintext blocks will result in identical ciphertext blocks, which can lead to certain security vulnerabilities. ECB mode is not recommended for encrypting large amounts of data or data with patterns.
- Cipher Block Chaining (CBC): In CBC mode, each plaintext block is combined with the previous ciphertext block before encryption. The initialization vector (IV) is used as the first “previous ciphertext block” for the first block. CBC provides confidentiality and introduces diffusion, as the encryption of each block depends on the previous ciphertext block. However, parallelization is limited due to the sequential nature of block processing.
- Cipher Feedback (CFB): CFB mode turns a block cipher into a stream cipher. It operates at the level of individual bits or bytes, encrypting a feedback register (initially loaded with the IV) and then XORing the result with the plaintext to produce the ciphertext. The feedback register is shifted for the next encryption, incorporating the previous ciphertext. CFB mode supports streaming encryption and allows error propagation isolation.
- Output Feedback (OFB): OFB mode is similar to CFB mode but operates at the level of the block size. It encrypts a feedback register (initially loaded with the IV) to produce a keystream, which is then XORed with the plaintext to generate the ciphertext. The feedback register is then shifted for the next encryption, and the process is repeated. OFB mode supports streaming encryption and error propagation isolation.
- Counter (CTR): CTR mode transforms a block cipher into a stream cipher. It uses a counter value (initially set to a nonce or IV) combined with a unique identifier for each block. The counter value is encrypted using the block cipher algorithm, and the resulting keystream is XORed with the plaintext to generate the ciphertext. CTR mode supports parallel encryption and is often used in high-speed encryption scenarios.
- Galois/Counter Mode (GCM): GCM mode combines the Counter (CTR) mode with additional authenticated encryption features. It provides confidentiality, integrity, and authenticity. GCM generates a keystream using CTR mode and then uses the Galois field multiplication to compute a message authentication code (MAC) for integrity and authenticity checks. GCM is commonly used in network security protocols like TLS.
These modes of operation address different requirements such as confidentiality, integrity, parallelization, and error propagation. It’s important to choose the appropriate mode based on the specific security needs and constraints of the application or system. Additionally, it’s essential to use a secure and well-implemented block cipher, such as AES, with the chosen mode of operation.
