Customer master keys

The CMK is the main building block of the KMS service as it contains the key material used for both encrypting and decrypting data. 

KMS supports both symmetric and asymmetric CMKs. Any symmetric keys created and stored within KMS will be a 256-bit key that will, of course, be used for both encryption and decryption and will never leave the KMS service. The "private" key of any asymmetric key pairs that are created will be retained within the KMS service. Asymmetric keys were introduced into KMS in November 2019 and there are a few differences between the two, including the following:

• Symmetric keys can be used to generate symmetric data keys in addition to asymmetric data key pairs.
• Importing your own key material is only supported for symmetric CMKs.
• Automatic key rotation is only supported for symmetric CMKs.
• When using a custom key store, you can only store symmetric CMKs.
• Asymmetric keys can either be used for encryption and decryption...