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.