Which term best describes the method of converting plaintext to ciphertext using a secret key?

• A. Substitution
• B. Cipher
• C. Cryptography
• D. Code

Answer: (B)

The method of turning readable data into unreadable data using a secret key is described by a cipher. A cipher is the algorithm that performs encryption and decryption, with the key guiding the transformation so that the intended recipient can recover the original plaintext. Substitution is a specific technique a cipher might use—replacing symbols with other symbols—but it’s just one approach inside the broader concept of a cipher. Cryptography is the overall field that studies encryption and security, not the particular method. Code refers to a system that maps words or phrases to other words or symbols, often without a cryptographic key. So the term that best fits the described process is cipher.