I have to host an AMI in the Amazon Marketplace. i need to get the type of instance, whenever some user launches the AMI., like if its small medium or large. based on that i need to make some changes in the AMI when its created. I can do this with Amazon API call, to get the instance type, but the problem is that the instances created with the AMI will be started by other users, and i cannot use my AWS Credentials in the Amazon API.

Is there any way that i can create an anonymous readonly user to make only specific type of EC2 API Calls? Or can i encrypt my EC2 API credentials, so no one can use it?