/ˌsiː-biː-ˈsiː/
n. “Chaining blocks like a linked chain of trust.”
CBC, or Cipher Block Chaining, is a mode of operation for block ciphers used in cryptography. It was designed to improve the security of block cipher encryption by ensuring that each block of plaintext is combined with the previous ciphertext block before being encrypted. This creates a “chain” effect where the encryption of each block depends on all previous blocks, making patterns in the plaintext less discernible in the ciphertext.
In practice, CBC requires an initialization vector (IV) for the first block, which is combined with the first plaintext block to prevent identical plaintexts from producing identical ciphertexts across different messages. Each subsequent block is XORed with the previous ciphertext block before encryption. This design increases security but also introduces sensitivity to certain attacks if not implemented properly.
CBC has been widely used in protocols like SSL and TLS as part of encrypting network traffic, disk encryption, and secure file storage. However, it has also been the target of attacks like BEAST and padding oracle attacks, which exploit predictable patterns or improper padding handling. These vulnerabilities highlighted the importance of secure protocol design and eventually contributed to the adoption of more robust modes such as Galois/Counter Mode (GCM) in modern TLS deployments.
Example usage: In a file encryption system, plaintext data is divided into fixed-size blocks. CBC encryption ensures that changing a single bit in one block affects all subsequent ciphertext blocks, enhancing security. Conversely, decryption requires processing blocks in sequence, as each block relies on the previous block’s ciphertext.
Despite being superseded in many contexts by authenticated encryption modes, CBC remains a foundational concept in cryptography education. Understanding CBC illuminates the challenges of chaining dependencies, handling IVs correctly, and mitigating known vulnerabilities. It also connects to related terms such as BEAST, POODLE, and other cipher modes, showing the evolution of secure encryption practices.