Duncan Wither

P-Stick

Written: 29-May-2021

A USB Password Stick.

Idea

The idea for this project was to simulate a keyboard and type in a master password (for a password manager or something that is waaay to long to remember). So there needed to be a button. But to improve the functionality I decided to add a screen and use an encoder to navigate a basic menu.

Why? Because:

• 1. I’m terrible at using a password manager because I can never remember the strong password.
• 2. This is a pretty simple project and would - hopefully - be a quick project / win to to distract me from another project.
  • As such this is - in part - pretty quick and dirty in terms of design.

If you want to provide any feedback, in terms of digital security, or on anything for that matter please feel free to do so on the github page.

As an overview the hardware is:

• Arduino compatible board with native USB.
  • I went with a Pro Micro.
• Small case, USB connector.
  • Custom case, and I just have the board plugged in
• Small Screen
  • Little 128x32 OLED seemed small enough to prevent mission creep with the menu.
• Rotary Encoder for Input.
  • I hadn’t used one to this date so seemed like fun.

The software was pretty simple, basically relying on pre-built libraries. After dealing with pure C and plain ol’ microcontrollers for the bigger project coming back to Arduino felt so easy, and I guess that’s the point.

The Brain

I had a Sparkfun Pro Micro lying around, and with it’s native USB support this seemed like an ideal project for it.

Using the Pro Micro wasn’t without its flaws though. I’ve found (after research) that Pro Micros brick¹ relatively easily. The cause is flashing the board 3V board with 5V firmware or vice versa². What didn’t help is the knockoff I had³ didn’t have any markings, and so the first flash was a gamble⁴… which didn’t pay off⁵.

This is potentially fixed by using the 8 second long boot loader by shorting resent to GND in quick succession, however I’ve not found this any use as it’s never long enough to re-program the device. Alternatively the method that did work was re-flashing the Arduino firmware using the Arduino as a programmer method. The pin-out I used was as follows:

Keyboard Functionality

The first code I wrote was to get the typing/keyboard functionality working. Initially I was going to work out how to Use the board as a HID but I found a simpler way. This involved (first testing with then) stripping out the useful parts of the KeyboardMessage example to give:

#include <Keyboard.h>

int buttonPin = 9;  // Set a button to any pin

void setup()
{
  pinMode(buttonPin, INPUT);  // Set the button as an input
  digitalWrite(buttonPin, HIGH);  // Pull the button high
}

void loop()
{
  if (digitalRead(buttonPin) == 0)  // if the button goes low
  {
    Keyboard.print("who-hoo");  // Send the String
    delay(1000);  // delay so there aren't a kajillion messages
  }
}

Dealing with the OLED

Using the OLED was another joyously simple task. All I had to do was following a tutorial… this tutorial in fact. A couple of things to note: - Make sure you match the voltage of the OLED with the voltage of the Pro Micro Board you use (i.e. 5V OLED for 5V Pro Micro etc.). - Ensure you install Adafruit_SSD1306 library and all its dependencies. - Remember to add a display.display(); after each text change.

Pin-out on the pro micro is:

Adding the Encoder

Adding the encoder was a little harder, but only because I had to find the interrupt pins, and then start implementing some form of menu. Regarding reading the encoder, all the hard work has been done already. Thank you very much Matthias Hertel⁷. The pinout is as follows:

Copying out the backbone of the example sketches just leaves the program logic to contend with.

This included deciding on how the menu structure⁸ would work in the code etc. My flow is as such:

This appeared to work perfectly using the test code!

Adding the Passwords

I wanted to open source this project, and thought it poor security to upload all my passwords to the internet. So I created a second file - holder-file.h - to house all the passwords.

This also included some variables for the “lost” screen which shows at boot up. I’d not recommend putting your whole name and your phone number, so I’ve just put “Dunc” and my flatmates phone number.

I created a dummy version for reference which is cloned with the repo⁹. Remember to update the list_max to the number of passwords you have.

Assembled Code:

All the files are on the github repo for this project, but it’s also below for reference:

Code as of Writing

holder-file.h as of Release

Final Pin-out:

The combined pinouts from above:

Le Coque

aka. the shell. For the case I used Heartman’s ultimate box maker¹⁰, with dimensions to allow the Pro Micro + Cable (which I was planning on gluing in place) rotary encoder and some wiggle room. There were several modifications made: - A hole was cut for the Encoder in the top shell - Another was cut for the screen - A cut was made in one of the end plates to allow a cable to be slid in. - Finally several dimensions were changed in the shell scripts so M2 bolts are reasonably self tapping.

For the self tapping holes an exterior hole of 2.4mm and an interior hole of 1.7mm seemed to work. The quality of your printer may help or hinder. For me a rough print seemed to result oblong hole allowing the screw to smoosh the plastic around without splitting the print.

Files can be downloaded here or from the github repo.

Encoder Knob

My encoder didn’t come with a knob, this meant I could create my own. Luckily Steve Cooley had already created a parametric knob using OpenSCAD so I toyed with the parameters to create this:

Only one modification was required, which was adding more cutout for the shaft without flat spot on it. This meant adding the following line within a difference bracket in the make_the_knob module: openscad translate([0,0,-0.1])cylinder(shaft_height-7,shaft_radius+0.1,shaft_radius+0.1); Where the 7 is the length of the flat spot along the shaft.

Final Result: