Which mode of encryption uses an initial chaining vector for the first round and combines previous outputs as input for subsequent rounds?

The mode of encryption that utilizes an initial chaining vector for the first round and combines previous outputs as input for subsequent rounds is Cipher Block Chaining (CBC). CBC works by taking the plaintext and encrypting it with the previous ciphertext block, combined with an initialization vector (IV) for the first block. This chaining mechanism enhances security by ensuring that identical plaintext blocks produce different ciphertext blocks, depending on the IV and previous ciphertext.

In CBC, the first block of plaintext is XORed with the IV before encryption. For each subsequent block, the previous ciphertext block is used as input, making the encryption de facto dependent on all prior blocks. This interdependence introduces diffusion, which is a vital aspect of secure encryption. If even a single bit in the plaintext changes, the change will propagate through the subsequent ciphertext blocks, thus altering the entire ciphertext output.

This chaining characteristic is crucial for maintaining the integrity and confidentiality of the data being encrypted, as it prevents patterns from emerging in the ciphertext that could be exploited by attackers.