Flume Water Monitor 915 MHz Security Is Pretty Good

> I reached out to Flume. They were very responsive, and their CTO shared an overview of their already-in-progress plans to further improve the product’s privacy and security.

Read: There will be a firmware update that further "secures" the device from owners who would like to use it without the "cloud" and the "app".

So if I understand correctly, they used hard coded constants to generate keys that only have 44 bits of entropy, which is brute forcible for $10, but you have to do it per device. They presumably did this to make pairing the bridge and sensor easier while not using a static key for all devices. Not the worst compromise I guess.

I've been working on an open source solution on and off for a number of years called http://y-drip.com. It uses Wi-Fi so battery life is only a few months and range is limited. My next design is probably going to use LoRa, but this complicates things a lot because now I have two devices (bridge and sensor) that need to be paired, support over-the-air updates, etc.

What's the best way to have them share keys without adding expensive hardware like Bluetooth or NFC. The sensor also needs to be waterproof so there's no exposed ports. Increasing from 44 bits to 128 would help, but if the key generation is done over the air it can be sniffed right?

It didn't really factor into the break here but, it's amazing people still use ECB even though the famous penguin image is well over a decade old now.

Interesting timing, I have spent the last week with Claude reverse engineering the on-air protocol and I'm getting pretty close to building a secondary "listener" receiver that will collect measurements from my bridge + sensor pair locally.

To Flume's credit, it required me to dump the firmware from the bridge device's ESP8266 in order to extract my key. I didn't consider an approach like this article took, given I have direct access to the HW and there's no flash protection on the 8266.

As usual, the weak point is key generation and management.

One thing I appreciate about this kind of research is that it doesn't assume every wireless protocol must end with a catastrophic vulnerability.

A result of "the security is actually pretty good" is arguably just as valuable as finding a critical flaw. It helps distinguish between systems that are merely proprietary and those that were designed with a reasonable threat model in mind.

I'd be interested to know where the remaining weaknesses lie. Are they primarily in the RF protocol itself, key management, device provisioning, or the surrounding cloud infrastructure? In many IoT systems, the radio protocol ends up being the strongest component while the weakest link is somewhere else entirely.