NetBSD SysInst Entropy Fill Device

Caution

USE AT YOUR OWN RISK: This Arduino sketch is experimental and has only seen limited testing

Important

INDEPENDENT: The NetBSD Foundation or developers do NOT endorse this project

Note

ADC NOISE: This device uses (LSB) ADC noise as it's main source of entropy

Requirements

• An Arduino Leonardo (ATmega32U4) compatible device (like the LilyGO F308)

Disadvantages

• No tamper proofing whatsoever (Arduino bootloader present)
• No device self verification implemented
• Limited entropy health tests implemented
  • NIST SP 800-90B (4.4.1) Repetition Count Test
  • NIST SP 800-90B (4.4.2) Adaptive Proportion Test (without sliding window)
• Generates raw entropy of limited quality (mitigated through conditioning in NetBSD SysInst)

Advantages

• Code is very easily understood
• Cheap and readily available hardware
• The entropy input can be visually inspected while being input

Theory of operation

1. The device reads an analog value from an unconnected floating pin
2. All bits except for the Least Significant Bit are discarded
3. Two Least Significant Bits are collected to be debiased
   • The two bits are discarded if they're the same
   • The two bits are different, the first is passed as raw entropy
4. Six debiased bits are shifted into a near byte of raw entropy
5. Limited health test are performed
6. The six bits of raw debiased entropy are encoded into a modified Base64
7. The encoded raw entropy is entered through a virtual keyboard function
8. NetBSD SysInst conditions (see src/usr.sbin/sysinst/util.c#entropy_add_manual) the raw entropy through SHA256
9. NetBSD SysInst injects the conditioned entropy into the kernel entropy pool

Technical notes

The Base64 encoding is modified to limit the virtual keyboard from accidentally causing unwanted behavior for sysinst or the bourne shell if the device is inserted untimely.

The device generates 3072 bits of debiased raw entropy, exceeding the required 256 bits by a factor of 12 while NIST SP 800-90C (4.2) recommends a factor of 2, with some caveats.

How to use

If the system you're installing NetBSD onto does not have an entropy source that NetBSD readily supports, you may encounter the following screen:

At this point plug in the NetBSD SysInst Entropy Fill Device, which will automatically proceed to the following screen, and start entering the collected raw entropy through the virtual keyboard function:

When the device is finished, it should exit the above screen automatically.