A water utility company uses a number of Amazon EC2 instances to manage updates to a fleet of 2,000 Internet of Things (IoT) field devices that monitor water qu...

A water utility company uses a number of Amazon EC2 instances to manage updates to a fleet of 2,000 Internet of Things (IoT) field devices that monitor water quality. These devices each have unique access credentials.
An operational safety policy requires that access to specific credentials is independently auditable.
What is the MOST cost-effective way to manage the storage of credentials?