Which of the following correctly describes a drawback of symmetric key systems?

A.
Computationally less intensive than asymmetric systems

B.
Work much more slowly than asymmetric systems

C.
Carry out mathematically intensive tasks

D.
Key must be delivered via secure courier

Explanation:
D: In order for two users to exchange messages encrypted with a symmetric algorithm, they must first figure out how to distribute the key. If a key is compromised, then all
messages encrypted with that key can be decrypted and read by an intruder. It is not safe to simply send the key in an e-mail message, because the key is not protected and can be
easily intercepted and used by attackers. Thus, one user must send the key to the other using an out-of-band method. The user can save the key on a thumb drive and walk it over to
the other person’s desk, or have a secure courier deliver it. This is a disadvantage of symmetric cryptography because distribution is a hassle, as well as clumsy and insecure.
A is incorrect because it describes an advantage of symmetric algorithms. Because they are less computationally intensive than asymmetric algorithms, symmetric algorithms tend
to be much faster. They can encrypt and decrypt relatively quickly large amounts of data that would take an unacceptable amount of time to encrypt and decrypt with an asymmetric
algorithm.
B is incorrect because asymmetric systems work much more slowly than symmetric systems. The speed with which symmetric algorithms work is an advantage. Asymmetric
algorithms are slower than symmetric algorithms because they use much more complex mathematics to carry out their functions, which requires more processing time. However,
asymmetric algorithms can provide authentication and nonrepudiation, whereas symmetric algorithms cannot. Because both users employ the same key to encrypt and decrypt
messages, symmetric cryptosystems can provide confidentiality but they cannot provide authentication or nonrepudiation. There is no way to prove through cryptography who actually
sent a message if two people are using the same key.
C is incorrect because asymmetric algorithms carry out mathematically intensive tasks. Symmetric algorithms, on the other hand, carry out relatively simplistic mathematical
functions on the bits during the encryption and decryption processes. They substitute and scramble (transpose) bits, which is not overly difficult or processor-intensive. The reason it is
hard to break this type of encryption is that the symmetric algorithms carry out this type of functionality over and over again. So a set of bits will go through a long series of being
substituted and transposed.