Cipher Block Chaining (CBC) is a method for encrypting data using a block cipher where the data is divided into fixed-size chunks known as blocks, and each block is transformed into unreadable ciphertext. To achieve this, CBC relies on an initialization vector (IV), which adds randomness and ensures that identical plaintext blocks encrypt differently.
When encrypting using CBC, each plaintext block combines with the previous ciphertext block through a process called XOR. This means that the output of the encryption of one block becomes part of the input for the next block. We start by XORing the first plaintext block with the IV, encrypting the result with a cipher key to produce the first ciphertext block. The process continues, with each subsequent plaintext block XORing with the previous ciphertext block and then encrypting the result.
One important aspect of CBC is its chaining characteristic. If a single bit in a ciphertext block gets corrupted, it affects the decryption of that block and all blocks that follow. This makes CBC robust but also means that care must be taken to avoid losing or damaging any ciphertext blocks during transmission.
CBC has gained popularity because it enhances security compared to simpler methods like Electronic Codebook (ECB). While ECB encrypts blocks independently, allowing identical plaintext blocks to produce identical ciphertext blocks and creating potential vulnerabilities, CBC’s chaining process disrupts this pattern. It hides relationships between plaintext and ciphertext, making attacks more difficult.
However, CBC comes with its own challenges. Errors during encryption might propagate down the chain, and if any ciphertext block gets lost or corrupted, you might not be able to fully decrypt the remaining data. Additionally, because each block depends on the last, CBC cannot perform parallel encryption, which can slow down the process for large amounts of data.
In short, CBC is a widely used method that effectively secures large messages by chaining blocks of text together and utilizing both an encryption key and an IV, lending it a degree of unpredictability that improves over its predecessors in many scenarios, especially in terms of security against hackers.