Integrating AWS Cognito with API for authentication

Amazon Cognito was not designed to secure developer built APIs and I would caution you from using only the Amazon Cognito token to secure your API.

That being said, the vended Amazon Cognito token is a normal JWT signed using asymmetric encryption. This thread on the AWS forums has some example code in C# that another customer was able to use to verify the token.

Update 2015-07-09 AWS has announced Amazon API Gateway. Using API Gateway you can build a REST interface to your existing API (or to AWS Lamdba functions) secured with credentials retrieved via an Amazon Cognito authflow. See this blog post for additional announcement details.

You can retrieve the JWT tokens after authenticating users using Cognito. Pass the Access or ID token (depending on usecase) to your backend app and decode the token using any standard JWT decoder libraries. Here is an article with sample code for reference explaining the process.