Ask HN: How to write sensitive data to S3 from thousands of untrusted devices?

I am trying to figure out how to write sensitive data to a private S3 folder from thousands of windows and Mac laptops. No matter how much I read about the topic I can’t figure out how to grant systems access to write to S3 without having to store some authentication secret on the system that a sophisticated user can find in the registry, environment variable or in the binary if they bother to look for it.