Which of the following best defines a block cipher?

A block cipher is best defined as a form of symmetric key encryption that operates on fixed-size blocks of data. This means that it takes data input of a specific size (often 64 or 128 bits) and transforms it into an encrypted block of the same size, using a symmetric key for both encryption and decryption. The symmetry refers to the use of the same key for both processes, which is a hallmark of symmetric key algorithms.

Block ciphers, such as AES (Advanced Encryption Standard) or DES (Data Encryption Standard), divide the plaintext into blocks, and each block is processed separately, applying various transformations and permutations based on the specific algorithm. This method enhances security and efficiency, as it allows for structured data processing.

In contrast, other options do not accurately represent the characteristics of a block cipher. For instance, asymmetric key encryption utilizes a pair of keys (public and private) and typically processes data differently than a block cipher. Additionally, a bit-by-bit processing method refers to stream ciphers, which encrypt one bit at a time rather than in fixed-size blocks. Lastly, a standard using a single round does not provide adequate security, as modern block ciphers typically involve multiple rounds of permutation and substitution to enhance data protection.