CBC stands for “Cipher Block Chaining,” which is a block cipher mode of operation used in cryptography to provide confidentiality and integrity to data during encryption. CBC is commonly used with block cipher algorithms like AES (Advanced Encryption Standard) to encrypt larger pieces of data, such as messages or files. Here’s how CBC works:
- Block Cipher Encryption: Block ciphers operate on fixed-size blocks of data (e.g., 128 bits for AES). However, data to be encrypted can be of variable lengths. CBC mode helps address this issue.
- Initialization Vector (IV): CBC requires an Initialization Vector (IV) for the first block of data. The IV is combined with the first plaintext block before encryption.
- Chaining: In CBC, each plaintext block is XORed with the previous ciphertext block before encryption. This “chaining” of blocks ensures that each ciphertext block depends on the previous block, introducing an element of randomness and preventing identical plaintext blocks from producing the same ciphertext blocks.
- Ciphertext Feedback: After XORing the plaintext block with the previous ciphertext block, the result is then encrypted using the block cipher algorithm.
- Decryption: During decryption, the ciphertext is decrypted using the block cipher algorithm, and then XORed with the previous ciphertext block to obtain the plaintext block.
- Dependent Blocks: Each ciphertext block depends on both the plaintext and the ciphertext blocks that precede it. This interdependency makes it more difficult for an attacker to manipulate or alter individual blocks of ciphertext without detection.
- Secure Initialization: The IV is crucial for CBC security. It must be random and unique for each encryption operation to prevent patterns in the ciphertext.
- Randomizing Effects: CBC introduces a randomizing effect due to the chaining and XOR operations, which helps prevent attackers from identifying patterns in the encrypted data.
- Padding: If the plaintext data is not a multiple of the block size, padding is often added to the last block to ensure that it matches the block size. Padding schemes like PKCS#7 are commonly used.
- Parallelization: One drawback of CBC is that blocks must be processed sequentially since each block’s encryption depends on the previous block’s ciphertext. This limits parallelization of encryption.
- Initialization Vector Size: The IV size should match the block size of the cipher being used (e.g., 128 bits for AES).
While CBC mode provides confidentiality and some integrity to encrypted data, it does not provide authentication on its own. For secure communications, a combination of encryption and message authentication techniques (e.g., HMAC) is recommended. It’s also important to use a secure IV generation process to ensure the effectiveness of the CBC mode.