Other than the excellent GenerateRandom API call (which you should check out for seeding your PRNGs), KMS is composed of a set of API operations for creating, managing, and using a relatively small set of encryption keys, called Customer Master Keys (here, "master keys"). There are a bunch of operations for managing grants and policy around who can use which keys for what operations, but the fundamental operations in KMS are CreateKey, Encrypt, and Decrypt. CreateKey will generate a key in the KMS service that will never leave the KMS service. Once you create a key in KMS, you can disable it, you can set permissions on who can use it, you can alias it, but you cannot export it. In order to use the keys for cryptography, you use the Encrypt and Decrypt API calls. This is the core security value proposition in KMS: no one can run off with the keys.