ArrayBuffer and Uint8Array
The methods of navigator.credentials can take 2 arguments type of ArrayBuffer and Uint8Array.
2 js files are not used.
I want explanation about how to use MDS AccessKey.
I think it is better to add an interface of storage that will be inherited by DevelopmentInMemoryStore. This is like the UserStore class of ASP.NET Identity. But, In this case, I think that the storage API only needs parameter of the StoredCredential poco class, User poco class is not needed. because, User poco class is already provided in almost cases. If you pass User poco class to storage API, I think that it is better to make it a Generic type parameter.