Which of the following is generally true about block ciphers?

The statement that the plaintext and ciphertext are always the same size is generally true for block ciphers. Block ciphers operate by taking a fixed-size block of plaintext, typically 64, 128, or 256 bits, and transforming it into an equally sized block of ciphertext. This one-to-one relationship in size between the plaintext and ciphertext is fundamental to how block ciphers function.

For instance, when encrypting data with a block cipher, if the plaintext is 128 bits long, the resulting ciphertext will also be 128 bits long. This consistency in size is crucial for maintaining data integrity during encryption and decryption processes, as the structure of the data remains uniform.

In contrast, other options either describe properties that are not universally true about block ciphers or do not pertain to size relationships. For instance, block ciphers do not require the key size to match the block size, as there are various configurations where these can differ. Additionally, block ciphers are not designed to operate in a stream of bits; that is characteristic of stream ciphers. Lastly, while securely exchanging keys is a best practice for any encryption system, it is not a defining characteristic of block ciphers specifically.