Final answer:
The correct statement about block ciphers is that they permute the bits of the input plaintext to produce ciphertext, typically of the same size. Block ciphers work with fixed-size blocks rather than variable sizes. This is exemplified by the transposition cipher technique used in historical encryption methods like the Zimmerman Telegram.
Step-by-step explanation:
The question asks which of the following is generally true about block ciphers. The correct statement among the given options is that block ciphers permute the bits of the input plaintext. A block cipher takes a fixed-size block and encrypts it to a block of ciphertext, often of the same size, using a symmetric key. This method contrasts with stream ciphers, which encrypt data bit by bit. Therefore, it is not true that a block cipher is an encryption function for variable-size blocks of data, as they work with predefined block sizes.
A historical example that illustrates this concept is the Zimmerman Telegram, which was encrypted using a transposition cipher. In this type of cipher, the plaintext is divided into blocks and the positions of the letters within each block are shuffled according to a set rule, which can be aided by a keyword. This is a form of permutation of the bits or letters of the plaintext.