Which of the following works similarly to stream ciphers?

A.
One-time pad

B.
AES

C.
Block

D.
RSA

Explanation:
A: Stream ciphers were developed to provide the same type of protection one-time pads do, which is why they work in such a similar manner. In reality, stream ciphers cannot
provide the level of protection one-time pads do, but because stream ciphers are implemented through software and automated means, they are much more practical. A one-time pad
is a perfect encryption scheme because it is considered unbreakable if implemented properly. This cipher uses a pad made up of random values. A plaintext message that needs to be
encrypted is converted into bits, and a one-time pad is made up of random bits. This encryption process uses a binary mathematical function called exclusive-OR, usually abbreviated
as XOR. XOR is an operation that is applied to two bits and is a function commonly used in binary mathematics and encryption methods. Stream ciphers also encrypt at the bit level,
which is how they are similar to one-time pad encryption schemes.
B is incorrect because AES is a symmetric block cipher. When a block cipher is used for encryption and decryption purposes, the message is divided into blocks of bits. These
blocks are then put through mathematical functions, one block at a time. Stream ciphers encrypt data one bit at a time whereas a block cipher encrypts data one block of bits at a time.
Suppose you need to encrypt a message you are sending to your friend and you are using a block cipher that uses 64 bits block size. Your message of 640 bits is chopped up into 10
individual blocks of 64 bits. Each block is put through a succession of mathematical formulas, and what you end up with is 10 blocks of encrypted text. You send this encrypted
message to your friend. He has to have the same block cipher and key, and those 10 ciphertext blocks go back through the algorithm in the reverse sequence and end up in your
plaintext message.
C is incorrect because as stated in the preceding answer, when a block cipher is used for encryption and decryption purposes, the message is divided into blocks of bits. These
blocks are then put through mathematical functions, one block at a time.
D is incorrect because RSA is a public key algorithm that is the most popular when it comes to asymmetric algorithms. Asymmetric algorithms use a different type of mathematics
than symmetric and are nothing similar to one-time pad encryption schemes. The security of this algorithm comes from the difficulty of factoring large numbers into their original prime
numbers.