USB potting.

Anyone have any experiencing with potting PCBs? I've got some USB devices that I'm using as GPG keycards, and want to pot them with some kind of solvent resistant epoxy to ensure someone can't tamper with them and re-flash the firmware. I either suck at searching for this stuff or the information isn't very well organized, because I'm having a hard time finding anything real useful on the matter. I'd like if it were tamper-resistant, erring on the side of device destruction in the event someone tries to mess with it. All I can find is stuff on cutesy resin-molds and generic epoxy stuff, nothing particularly with a security focus.

High thermal conductivity + melting point above solder? That would be perfect.

Just an idea from someone who knows nothing of this sort of things : Use a transparent epoxy + a colored epoxy for the coating, forming a random pattern that you can learn to recognize. If someone manage to melt/rasp away the epoxy, tamper and then recoat the device, they won't be able to reproduce the pattern. That only warns you of tampering though, doesn't prevent it. >>3142 What about mechanical removal ? Or solvent ?

https://www.acc-silicones.com/products/encapsulants.ashx this may help

>>3142 >>3143 another option would be to place a destructive compound in it (if certain chemicals are used, then the special compound would chemically react, destroying the card) This could be a fire hazard though depending on what chemicals are mixed. alternative, what about coating it in some form of acrylic or plexiglass, since thy have higher melting points than solder? I have seen someone try to physically access a circuit board covered in a hard resin, and his solution was to literally use a dremel tool to cut away the resin, I don't know what it was, but it was pretty tough and chemically resistant http://blog.paryleneconformalcoating.com/blog/potting-protects-against-reverse-engineering https://arstechnica.com/civis/viewtopic.php?f=23&t=137245 http://www.avrfreaks.net/forum/how-can-one-removedissolve-potting-compound https://www.youtube.com/watch?v=Oiyg6jIZg_w https://www.youtube.com/watch?v=gDzKTbNA9EE http://www.dac-industries.com/products/dac-resin-removers/dac-111-extreme-resin-remover >HAS AN SDS

Remove original chip package with acid. No PCB, all traces, wires are suspended in this compound Highly adhesive, so mechanical removal would probably break the chip and wiring. A system where cutting certain wires erases flash memory and attacker can't guess which wire it should be from datasheets. >>3146 Dremel is a bit too much, but a precise hand with scalpel and enough time would make it look as clean as it was before potting.

You will want to do more than pot the PCB. These would be my steps, but you might as well just buy a fips level 3 or 4 flash drive if your going to do this. 1. circuit requires battery, if disconnected, master key is erased, rendering device useless. 2. connected tamper wrapper but do not wrap. 3. half pot both sides of PCB in one type of epoxy. 4. wrap (this tamper wrap is connected to key chip, tampering wrapper causes RAM to lose master key) 5. pot with different epoxy. 6. before pot is finished drying surround in steel. ALL of this has to be done to a chip that cant be flashed through the USB port. Just grab any of the fips level 3 or 4 devices. Rolling your own is hard, and the fips labs aren't on a federal pay check.