What is a shared (symmetric) key in cryptography?

• A. An encryption key used by two parties. One party uses the key to encrypt data and the other to decrypt data.
• B. A public key used in asymmetric encryption.
• C. A digital signature.
• D. A hardware security token.

Answer: (A)

A shared (symmetric) key is a single secret key that both parties use to perform both encryption and decryption. Because the same key does the job for both directions, secrecy of that key is essential: if it’s exposed, anyone who has it can both read and alter the messages. This approach is typically fast and efficient for handling large amounts of data, which is why symmetric keys are common for bulk encryption after a secure key exchange.

This concept is different from public-key cryptography, where encryption uses a public key and decryption uses a separate private key. It also differs from digital signatures, which use a private key to create a signature and a public key to verify it. A hardware security token, meanwhile, is just a device that protects or performs cryptographic operations; it isn’t the definition of a shared key itself.

So the idea described is the one where two parties share one secret key that is used to both encrypt and decrypt messages.