Chip-On-Board epoxy to protect pins from probing security attacks? - Page 1
Forum section: electronics / general

author: cdevidal
author_rank: Regular Contributor
timestamp: April 16, 2022, 07:33:43 pm
content: Can Chip-On-Board epoxy be used to prevent physical access to the UART pins of an ESP32 to resist probing as a security attack? Would need to hold to the board even if a knife pried at it. What type of epoxy do they use?I'm making a chicken coop door opener based on ESP32 which is an outdoor device that it holds Wi-Fi credentials. I want to resist casual walk-up probing attacks of the UART pins to collect the credentials; Someone walks up, unscrews the cover, and presses a serial-to-USB adapter against it to extract credentials.I realize that with enough time and physical access, eventually any coating could be removed, and that's acceptable. Someone working for a while at your coop may be noticed, or if the device were stolen it would definitely be noticed--there is a twice-daily heartbeat. So this defense need not be bulletproof; Just need to be able to resist casual walk-by attacks. Not trying to protect the Coca-Cola secret formula, just Wi-Fi credentials.I would not protect all components, just the UART pins.I am also taking steps to protect the data electronically.Also I plan to spray the entire board with a conformal coating, something that's compatible with the epoxy, for moisture resistance.

author: Ian.M
author_rank: Super Contributor
timestamp: April 16, 2022, 08:23:11 pm
content: Unless you are on the watch list of various three letter agencies, you are vastly overthinking this. The chances of a ne'er-do-well wandering around your yard having the skills to identify the UART header, and the parts to connect to it and dump the credentials are minimal. Personally, I'd use security screws to frustrate the lower grade of morons, and a lid switch that triggers a wakeup, sends an alarm message to a monitoring server if you have one, then wipes the credentials.

author: cdevidal
author_rank: Regular Contributor
timestamp: April 16, 2022, 08:55:41 pm
content: I like the approach of a lid switch, but I don't think I'm overthinking this. Ring doorbells made national news foran attack that was approximately the same difficulty. It required some foreknowledge of the vulnerability, just like it would for my device. And since my device is calledSecureCoop, adding a 10 cent dollop of epoxy is an inexpensive way to keep the promise of the name.My only question now is: What kind of epoxy is typically used?

author: ataradov
author_rank: Super Contributor
timestamp: April 16, 2022, 09:07:02 pm
content: An attack on the Ring door bell is vastly more valuable than an attack on a random one-off ESP32 device.Just use regular 5 minute epoxy if you are really worried, but it is truly an overkill.

author: cdevidal
author_rank: Regular Contributor
timestamp: April 16, 2022, 09:15:13 pm
content: Quote from: ataradov on April 16, 2022, 09:07:02 pmOf any kind? It'll adhere well enough to resist prying with a knife and not craze/etch/corrode the copper? I don't have much experience with epoxies and never on PCBs.

author: ataradov
author_rank: Super Contributor
timestamp: April 16, 2022, 09:20:17 pm
content: I'd assume any kind of two part epoxy would work. It adheres pretty well to solder resist. You would mangle the board quite a bit if you try to remove it mechanically.Applying some heat or chemicals would make the process easier, but that goes beyond a simple casual attack. Just make sure to cover tracks and pins of the module, not just test points.And cover the flash IC, of course. Otherwise it would be easier to dump that instead.

author: cdevidal
author_rank: Regular Contributor
timestamp: April 16, 2022, 09:28:54 pm
content: Any concern about compatibility with acrylic conformal coating? I assume I'd spray it on after pouring the epoxy, not the other way 'round.

author: ataradov
author_rank: Super Contributor
timestamp: April 16, 2022, 09:33:44 pm
content: No idea, I have not researched it at all. It does not feel like there would be any issues, but I don't know for sure.

author: ConKbot
author_rank: Super Contributor
timestamp: April 26, 2022, 11:57:05 am
content: I certainly wouldn't say any 5-minute epoxy is good to go. My co-workers and I generally consider devcon 5 minute to be temporary. Pcb soldermask is surprisingly hard to stick to. Just the tiniest bit of heat and the devcon comes off in a sheet.I'd generally say more heat resistance is better. JB weld is removable but not so easily. Loctite 9340 (still reasonably available and a long pot life makes it good for applying to a small batch of boards) is harder to get off than JB weld in my experience.The king of being a pain to get off I've played with is Henkel FP4531, but id imagine any of the nonreworkable capillary flow underfills would be similar. And 4531 at least in my experience bonds to soldermask. I've done a test, glueing an object to a board, prying it off, and in some places it took the soldermask, the first layer an a few via barrels with it.https://dm.henkel-dam.com/is/content/henkel/lt-8332-brochure-board-level-underfills-and-encapsulantsBe warned that the underfills will be a pain in your ass too use though. "1-part" aka premixed and shipped on dry ice. Want -40c storage, 160c cure temp (100c for longer works, but still nothing a battery can survive), and like a $2k MOQ.Not knowing how many you want to do, I'd suggest loctite 9340, as the most resistant while remaining reasonable.

author: cdevidal
author_rank: Regular Contributor
timestamp: April 26, 2022, 08:19:03 pm
content: In the low 1000s of boards, so Loctite it isThanks, this is why I keep coming back to this forum.

author: ConKbot
author_rank: Super Contributor
timestamp: April 26, 2022, 10:18:00 pm
content: If you end up with 9340, I'll add that it is dispensable at a reasonable flow rate with a 20awg or larger taper tip, and 50-100psi on a syringe. It is a paste that stays put in thin (sub 1.5 mm or so) layers, even inverted in a cure oven. Oven cure smooths it out and it comes out semi-glossy, room temp is less smoothing, and less glossy.I'd also consider your threat model a bit more in-depth. If no epoxy was 5 minutes of security, 5-min epoxy would be 6 minutes, but even using the underfill I'd consider it a 30 minute job with a butane hot air pen and scraping. Twice daily reporting is pretty far out from that, so I'd also be looking at intrusion detection as potential more valuable than sealing.

author: ataradov
author_rank: Super Contributor
timestamp: April 26, 2022, 10:46:11 pm
content: I use epoxy for mechanical connection to the PCBs a lot, and I'm not sure how easy it is to actually remove it. I just tried pocking at one of such connection between a PCB with a plastic part, and I would dread actually having to remove it without too much damage.5 minute epoxy is cheap and widely available - get some and try for yourself.I also suppose there are different types, so just for reference, I'm using thishttps://www.amazon.com/Bob-Smith-Industries-BSI-201-Quik-Cure/dp/B0166FFFD4

author: free_electron
author_rank: Super Contributor
timestamp: April 26, 2022, 11:08:00 pm
content: qfn package, short rx to ground under the chip itself. game over.