Recently I've been diving into some light embedded systems research, specifically in routers from various manufacturers running different OSes (Linux, eCos, etc).

I am wondering if anyone has studied the subject and knows what the sources of entropy for random number generation are.

I've found a few good reads online, but most of them are 5+ years outdated.

• sources of "uniqueness"/entropy on embedded systems

A lot of people point to RSSI or network access. Unless there are more sources, I'm confident that this would be extremely insecure as RSSI to both the AP and connected STAs are indicated in plaintext packets and if a network is not active there would be no entropy generated (correct me if I'm wrong).

So I'll rephrase my question:

What are sources of entropy in common embedded systems (wireless routers/gateways/access points) and where can I learn more about them?