What is the output of a pseudorandom bit generator when combined with plaintext using the XOR operation?

• A. Pseudocode
• B. Hash value
• C. Keystream
• D. Encryption key

Answer: (C)

The output of a pseudorandom bit generator when combined with plaintext using the XOR operation is referred to as a keystream. In cryptography, a keystream is a sequence of bits used in conjunction with plaintext to produce ciphertext or to obscure the plaintext itself. The key aspect of the XOR (exclusive OR) operation in this context is that it combines the bits of the plaintext with corresponding bits from the pseudorandom keystream. This combination results in the output that retains the statistical randomness needed to achieve confidentiality.

Because the pseudorandom bit generator produces a sequence of bits that appears random but is generated deterministically from a shorter seed value, when this sequence (the keystream) is XOR'd with the plaintext, it results in an output that can conceal the original message effectively. This mechanism is prevalent in stream ciphers, where the security of the encryption relies on the unpredictability of the keystream.

The other options are distinctly different in function: pseudocode is a method of representing algorithms in a structured way, a hash value is a fixed-size result derived from input data often used for integrity verification, and an encryption key refers to a specific value used to encrypt or decrypt messages but does not directly refer to the ke