Which of the following is NOT an advantage of a stream cipher?

A.
The same equipment can be used for encryption and decryption.

B.
It is amenable to hardware implementations that result in higher speeds.

C.
Since encryption takes place bit by bit, there is no error propagation.

D.
The receiver and transmitter must be synchronized.

Explanation:
The transmitter and receiver must be synchronized since they must use the same keystream bits for
the same bits of the text that are to be enciphered and deciphered. Usually, synchronizing frames
must be sent to effect the synchronization and, thus, additional overhead is required for the
transmissions. * Answer “The same equipment can be used for encryption and decryption” describes
an advantage since stream ciphers commonly use Linear Feedback Shift Registers (LFSRs) to
generate the keystream and use XORs to operate on the plaintext input stream. Because of the
characteristics of the XOR, the same XOR gates and LFSRs can also decrypt the message. Since LFSRs
and XORs are used in a stream cipher to encrypt and decrypt, these components are amenable to
hardware implementation, which means higher speeds of operation. Thus, answer “It is amenable to
hardware implementations that result in higher speeds” describes an advantage. For answer “Since
encryption takes place bit by bit, there is no error propagation”, stream ciphers encrypt individual
bits with no feedback of the generated ciphertext bits and, therefore, errors do not propagate.