Which of the following describes a block cipher?

A block cipher is characterized as an algorithm that operates on fixed-size blocks of plaintext data and transforms each block into an equal-sized block of ciphertext. The description of it being a public algorithm is somewhat misleading, as block ciphers can be both symmetric and asymmetric, but the core function remains the same: to work on blocks of data.

When using a block cipher, input plaintext is divided into fixed-length blocks (for example, 64-bit or 128-bit blocks), and the cipher processes each block independently. This method allows for secure encryption of larger data sets rather than encrypting data one bit at a time. The distinction of operating on blocks, rather than individual bits or entire streams of data, is what fundamentally defines the functionality of block ciphers.

The other descriptions do not accurately describe the main characteristics or functions of a block cipher. For example, encrypting data as single bits suggests a stream cipher, and the focus solely on symmetric encryption does not encompass the potential applications of block ciphers, which also include public key systems in a broader sense. The reference to biometric verification is unrelated to the concept of ciphers altogether, as it pertains to methods of identifying individuals based on their biological characteristics rather than encrypting data.