Davos Has Been Reading About Encryption Recently

Multiple Choice

Davos has been reading about encryption recently. He begins to wonder how anything can be secure if everyone is using the same set of algorithms. After all, anyone using the same algorithm would be able to decrypt anything that had been encrypted using that algorithm. Which of the following helps make the data unusable by anyone else using that same encryption scheme without having this information?

A) Algorithm
B) Cipher
C) Key
D) Block