Signet High-capacity

An all-in-one encrypted USB flash drive, two-factor authentication token, and password manager

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May 13, 2019

Project update 1 of 20

Production Part Design

We now have complete designs for Signet’s production PCBA and enclosure. There are still certainly problems to solve this puts us much closer to being ready to launch our campaign. The enclosure has been physically prototyped while the PCBA is currently only designed.

PCBA design

You can see renders of the production board design below. It’s a six layer board with a very similar parts list to our development prototype. The main differences are

  1. The microcontroller is the same except we use a smaller BGA version of the part
  2. The micro SD slot has been replaced with a smaller and faster eMMC chip
  3. Many of the resistors and capacitors have been changed from the 0402 footprint size to the smaller 0201 size

The board features some expandability with three GPIO lines. These can be converted to act as a serial port or I2C bus. We also expose the microcontrollers boot pin to allow users to enable the bootloader to write their own firmware to the device.

Enclosure prototype

When we set out to develop Signet HC we set two major physical design goals for ourselves

  1. That it not be more than 20 mm wide or 52 mm long

    If it’s more than 20 mm wide it wont fit alongside some other devices in a USB hub. If it’s more than 52 mm long it will be longer than some common keys and key fobs.

  2. That it is easy to assemble and disassemble the case many times without damaging it

    There are some one time only case assembly methods that might be cheaper to manufacture. We value the ability for users to repair, extend, or re-purpose the hardware however and don’t want to get in the way of users wanting to own their devices.

    Some users may prefer a tamper resistant case but there are significant challenges to getting that truly right in a small form factor. For most threat models needing physical access to the device to execute an attack is a sufficient deterrent. Users who have concerns about tampering should store the device only in secured locations.

We created a number of different designs that meet these requirements and eventually settled on one to have CNC milled for real world testing. The halves connect together using “bumps” on three sides of the inner lip of the enclosure with corresponding grooves on the other side of the enclosure. The best way to think of it is as a “bottle cap” snap. We are happy with its performance and will likely not make many changes before having the design injection molded. You can see some photos and a video of it below. Note that this prototype is made from "Natural" ABS which has an off white color. The production enclosure will be black.

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