What describes a mode where the message is divided into blocks, and each block is encrypted separately?

The mode where the message is divided into blocks, and each block is encrypted separately is known as Electronic Codebook (ECB). In the ECB mode, each block of plaintext is encrypted independently, which means that identical plaintext blocks will yield identical ciphertext blocks. This characteristic makes ECB straightforward and efficient for parallel processing, as each block can be encrypted without waiting for the previous block.

However, it is important to note that while ECB can be simple to implement, it has vulnerabilities. For instance, it does not provide sufficient security against pattern attacks because repeated blocks of plaintext will result in repeated blocks of ciphertext, revealing information about the structure of the original data.

In contrast, other options involve different ways of handling encryption on data blocks. Cipher feedback (CFB) and Output feedback (OFB) modes turn block ciphers into stream ciphers, allowing them to process data streams of varying lengths. Cipher-block chaining (CBC) encrypts each block of plaintext using the previous ciphertext block, thereby creating a dependency that enhances security by ensuring that identical plaintext blocks do not produce identical ciphertext blocks.