Implementing the Amazon Cognito User Pool Admin Authentication Flow with AWS SDK For .NET offers a path to implement user authentication without management of a host components otherwise needed to signup, verify, store and authenticate a user. Though Cognito is largely framed as a mobile service, it is well suited to support web applications. In order to implement this process you would use the Admin Auth Flow outlined in the AWS produced slide below. This example assumes that you have already configured both a Cognito User Pool w/ an App, ensuring the "Enable sign-in API for server-based authentication (ADMIN_NO_SRP_AUTH)" is checked for that app on the App tab and that no App client secret is defined for that App. App client secrets are not supported in the .NET SDK. It is also assumed that a Federated Identity Pool is configured to point to the before mentioned User Pool.
This auth flow bypasses the use of Secure Remote Password (SRP) protocol protections heavily used by AWS to prevent passwords from even been sent over the wire. As a result, when used in a client server web application, your users passwords would be transmitted to the server and that communication must be encrypted with strong encryption to prevent compromise of user credentials. The below code implements a CognitoAdminAuthenticationProvider with Authenticate and GetCredentials members. The Authenticate method returns a wrapped ChallengeNameType and AuthenticationResultType set of responses. A challenge will only be returned if additional details are needed for authentication, in which case you would simply ensure those details are included in the UserCredentials provided to the authenticate method and call Authenticate again. Once authenticated, a AuthenticationResultType will be included in the result and can be used to call the GetCredentials method and obtain temporary AWS Credentials.