What encryption mode involves XORing each block of plaintext with the previous ciphertext block before encryption?

The encryption mode that involves XORing each block of plaintext with the previous ciphertext block before encryption is the Cipher-block chaining (CBC) mode.

In CBC mode, each block of plaintext is combined with the previous ciphertext block using the XOR operation, which creates a dependency between successive blocks. This means that the encryption of the current block relies on the ciphertext of the prior block, ensuring that identical plaintext blocks will encrypt to different ciphertext blocks, thereby enhancing security by preventing patterns. This chaining mechanism allows CBC to produce a unique ciphertext even when the same plaintext is repeatedly encrypted, which is a significant advantage over simpler modes that do not have this characteristic.

In contrast, other modes such as Electronic Codebook (ECB) process each block independently, leading to potential vulnerabilities as identical plaintext blocks will produce identical ciphertexts. Output Feedback (OFB) and Cipher Feedback (CFB) modes use a different approach to handling plaintext blocks, relying on previous outputs or processed ciphertext but do not specifically require XORing the plaintext with the preceding ciphertext block for encryption.