In cryptography, which term denotes the secret value used to encrypt or decrypt data?

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

Answer: (C)

The key is the secret value used to encrypt or decrypt data. The encryption algorithm combines the plaintext with the key to produce ciphertext, and the key is required again to reverse that process during decryption. In symmetric cryptography, the same key handles both directions of the transformation. In asymmetric cryptography, you have a public key used to encrypt and a private key used to decrypt, but the private key remains a secret piece of key material. The other terms—cipher refers to the algorithm, plaintext is the original readable message, and ciphertext is the encrypted output. So the secret value that enables both encryption and decryption is the key.