At a high level, I want to explain the difference between symmetric and asymmetric encryption keys as this will help you to understand how KMS works going forward, which uses both symmetric and asymmetric encryption.
Symmetric encryption uses a single key to encrypt and decrypt data. So if you were to encrypt a document using a symmetric key, in order to decrypt that same document, the user would have to have access to that very same key that performed the encryption to decrypt it. Examples of some common symmetric encryption algorithms include Advanced Encryption Standard (AES), Digital Encryption Standard (DES), and Triple DES:
Asymmetric encryption keys differ from symmetric encryption in that they use two keys to perform the encryption. The keys themselves are linked via a mathematical algorithm during their creation, where one of the keys can then be used to encrypt data (public key), and the second key (private key) is used in combination...