In cryptography, which of the following is NOT a type of key used?

A.  Symmetric Key
B.  Public Key
C.  Asymmetric Key
D.  Elliptic Key

Answer: D
Explanation:

A. Symmetric Key

Symmetric cryptography (also known as Secret Key or Private Key cryptography) uses a single, shared key.  

•  The Mechanism: The same key is used to both encrypt and decrypt the data.  Both the sender and the receiver must have an identical copy of the key.  

   

•  The Pros/Cons: It is extremely fast and efficient for large amounts of data, but it suffers from Key Distribution problems (how do you get the key to the other person securely?).  

•  Examples: AES, 3DES, Blowfish.  

B. & C. Public and Asymmetric Keys

Asymmetric cryptography uses a mathematically related pair of keys: a Public Key and a Private Key.  

• The Public Key: Is made available to everyone.  It is used to encrypt data or verify a digital signature. 

• The Private Key: Is kept secret by the owner.  It is used to decrypt data that was encrypted with the matching public key or to create a digital signature. 

• The Advantage: It solves the key distribution problem because you never have to share your secret (private) key.

D. Elliptic Key

This is a "distractor" answer. While Elliptic Curve Cryptography (ECC) is a very popular and powerful branch of asymmetric cryptography, the keys it uses are still simply called Asymmetric Keys (or Public/Private keys).

• Why it's the Answer: There is no such technical term as an "Elliptic Key." ECC uses the algebraic structure of elliptic curves over finite fields to create keys that are much smaller than traditional RSA keys while providing the same level of security.  

•  The CISSP Fact: A 256-bit ECC key provides roughly the same security as a 3072-bit RSA key.  This makes ECC ideal for mobile devices and IoT where processing power and battery life are limited.