An iterated block cipher encrypts by breaking the plaintext block into two halves and, with a subkey,
applying a round transformation to one of the halves. Then, the output of this transformation is
XORed with the remaining half. The round is completed by swapping the two halves. This type of
cipher is known as:

A.
RC6

B.
Diffie-Hellman

C.
RC4

D.
Feistel

Explanation:

The question stem describes one round of a Feistel cipher. This algorithm was developed by an IBM
team led by Horst Feistel. (h. Feistel, Cryptography and Computer Privacy, Scientific American, v.228,
n.5, May 1973) The algorithm was called Lucifer and was the basis for the Data Encryption Standard
(DES). In answer a, RC4 is a variable keysize stream cipher developed by Ronald Rivest. In this type of
cipher, a sequence of bits that are the key is bit-wise XORed with the plaintext. * Diffie-Hellman
describes the first public key algorithm and is based on the difficulty of calculating discrete
logarithms in a finite field. (W. Diffie and M.e. Hellman, New Directions in Cryptography, IEEE
Transactions on Information Theory, v. IT-22, n. 6, Nov 1976). It is used for exchanging keys. RC6, in
answer c, is a fast block cipher designed by Rivest, Sidney and Yin. In RC6, the block size, the key size
and the number of rounds are variable. The key size can be no larger than 2040 bits. RC6 was one of
the five finalists in the Advanced Encryption Standard (AES) competition.