What does the process of Cipher-block chaining (CBC) involve?

Cipher-block chaining (CBC) is a mode of operation for a block cipher that enhances security by ensuring that each block of plaintext is dependent on both itself and the preceding ciphertext block. This process involves XORing each block of plaintext with the previous ciphertext block prior to encryption. By doing this, even if two identical plaintext blocks appear in a message, the resulting ciphertext will be different because they are affected by different preceding blocks. This chaining effect provides a significant increase in security compared to simpler modes of operation, such as electronic codebook (ECB), where identical plaintext blocks are encrypted into identical ciphertext blocks.

Furthermore, in CBC mode, the first block of plaintext has a unique initialization vector (IV) that ensures that even if the same plaintext is encrypted multiple times, the output will differ with each encryption session. This reliance on the previous ciphertext block makes CBC resistant to certain types of attacks, as it prevents an attacker from altering one block without affecting subsequent blocks.