REDOUBLER is a Random Number Generator based on Modular Entropy Multiplication. It is Open Source Hardware. The device plugs into a host's USB port, and enumerates as a standard microphone. When the microphone is sampled it produces white noise which contains a high proportion of random entropy. The digital data behind that white noise is random numbers, and they can be recorded and used for any purpose that requires a non-deterministic entropy source.

By enumerating as a standard USB microphone (sound card with one input) REDOUBLER does not require drivers on any major operating system. This simplifies development, and reduces the "attack surface" that an adversary can use to compromise security. Moreover, the data paths used by microphones are highly optimized for efficiency.

Non-deterministic random number generators use various methods to gather entropy. Avalanche noise from reverse-biased semiconductor junctions is popular, but others include video of Lava Lamps, radios tuned away from stations, quantum effects, digital logic race conditions, and radioactive decay events. Modular Entropy Multiplication (MEM) is a new method.

Invented by the author, Peter Allan, in the late 1990's and independently by Bill Cox in the early 2010's, MEM repeatedly compares a voltage with the midpoint of its range, then doubles either the voltage or excess over the midpoint as appropriate, and stores the result. Depending on the comparison, either a '0' or a '1' bit is output.

As a circuit, the analog part of MEM looks like this

The Track and Holds store the voltages, one for the source voltage and one for the result. The Comparators compare the stored voltages with the midpoint of their range, and produce a digital output based on the result. The Opamps double the input voltage and, depending on the result of the comparison, shift it by the voltage range. The clocks q and qp are symmetric and non-overlapping and control the pace of the circuit, while the output bits are collected at c0 and c1. When recorded over time, the analog voltages have characteristic time traces like this.

In the REDOUBLER, all of the components except the resistors are reconfigurable analog blocks on a Cypress Semiconductor PSoC 5LP chip. It has two such generators operating simultaneously at 768,000 bps, for a combined output of 1,536,000 bps. The bit-stream has about 7.9 bits/byte of entropy, and is not whitened, leaving the host to apply a whitening filter if required. Here is one second worth of data as an image (click twice to see it full-scale).

Modular Entropy Multiplication has advantages over all other methods because it is simultaneously simple, efficient, fast, and robust. It uses common components, used within their specified ranges. It is amenable to implementation on silicon, and has proven resistant to external electrical influence, even when unshielded. Bill and I hope that REDOUBLER and his Infinite Noise TRNG are just the beginning for this method, and we have disclaimed any patent rights to the technology.

The REDOUBLER circuit is designed in the latest free version of CadSoft Eagle and files are in the circuit branch of this repository.

The PSoC chip contains both reconfigurable analog components and digital logic, so the only components unique to REDOUBLER are the eight resistors at top-right.

A Bill Of Materials is also disclosed. Boards are available from OSH Park.

The firmware for REDOUBLER is written in the highly integrated and free-to-use Cypress PSoC Creator development environment. The source code is in the psoc_creator_firmware branch of this repository. I highly recommend this toolchain and microcontroller product line. The current state of the firmware source is efficient and reliable, but a little rough looking. The USB audio functions were culled from an example project, and much of that code remains in comments.

Once the firmware is built, it can be programmed onto a REDOUBLER using the $90 MiniProg3 or the left portion of the $10 CY8CKIT-059. Pogo Pins are useful for connecting to the 5 pads on the REDOUBLER board.

REDOUBLER uses a USB product ID granted by openmoko.org.

As open source software, users are asked to contribute their developments back to this repository. For now support is sparse. Anything that can be done with a USB microphone can be done with REDOUBLER. Linux is likely to be best supported, but Windows and Mac OS-X will have branches here too. Audacity runs on all three platforms, and can be used to record, view, manipulate and save data from REDOUBLER, so we know everything's possible.

Ultimately the goal is for REDOUBLER to be plugged into any flavor of machine, and have the operating system's random number generator immediately enhanced with copious high-quality entropy.