Which of the following BEST describes a block cipher?

A.
An asymmetric key algorithm that operates on a variable-length block of plaintext and transforms
it into a fixed-length block of ciphertext

B.
A symmetric key algorithm that operates on a fixed-length block of plaintext and transforms it
into a fixed-length block of ciphertext

C.
An asymmetric key algorithm that operates on a fixed-length block of plaintext and transforms it
into a fixed-length block of ciphertext

D.
A symmetric key algorithm that operates on a variable-length block of plaintext and transforms it
into a fixed-length block of ciphertext

Explanation:
A block cipher breaks the plaintext into fixed-length blocks, commonly 64-bits, and encrypts the
blocks into fixed-length blocks of ciphertext. Another characteristic of the block cipher is that, if the
same key is used, a particular plaintext block will be transformed into the same ciphertext block.
Examples of block ciphers are DES, Skipjack, IDEA, RC5 and AES. An example of a block cipher in a
symmetric key cryptosystem is the Electronic Code Book (ECB) mode of operation. In the ECB mode,
a plaintext block is transformed into a ciphertext block as shown in Figure. If the same key is used for
each transformation, then a Code Book can be compiled for each plaintext block and corresponding
ciphertext block. Answer a is incorrect since it refers to a variable-length block of plaintext being
transformed into a fixed-length block of ciphertext. Recall that this operation has some similarity to
a hash function, which takes a message of arbitrary length and converts it into a fixedlength message
digest. * Answers “An asymmetric key algorithm that operates on a variable-length block of plaintext
and transforms it into a fixed-length block of ciphertext” and “An asymmetric key algorithm that
operates on a fixed-length block of plaintext and transforms it into a fixed-length block of
ciphertext”are incorrect because they involve asymmetric key algorithms, and the block cipher is
used with symmetric key algorithms. Exhibit:

In other cryptographic modes of operation, such as Cipher Block Chaining (CBC), the result of the
encryption of the plaintext block, Pn, is fed into the encryption process of plaintext block Pn+1. Thus,
the result of the encryption of one block affects the result of the encryption of the next block in the
sequence.