A ________ is a random string of 40 to 4,000 bits used to encrypt messages.

• A. Key
• B. Cipher
• C. Plaintext
• D. Code

Answer: (A)

The main idea is that encryption relies on a key, which is a random string of bits that acts as the secret input to the encryption algorithm. This key, whose length can vary from shorter to quite long, determines how plaintext is turned into ciphertext. The security rests on keeping this key secret and using it with the chosen algorithm. The cipher itself is the algorithm that performs the transformation, not the secret material. Plaintext is the readable message before encryption, and code is a general term that doesn’t specifically refer to the secret used to encrypt.