If there is a particular file in an S3 bucket that is available to access, then there are three things that may want to download it:
- An AWS user - This request can be granted via IAM roles
- An AD user, who doesn't have an AWS account - this user might want temporary access, in which this can be done via a token that allows them to temporarily assume a role.
- An Application that's running on top of a resource (e.g. ec2 instance) - this request can be granted via tokens that have expiry dates
- Resources - e.g. an ec2 instance itself might want to download the file. In which case this can be done by assigning the necessary permissions to the EC2 instance's IAM role.
You can set up IAM roles that are specify permissions on specific resources, .e.g. EC2 resources.