True-Random Number Generator

Modern security and cryptography call for a source of true-random numbers. This design creates them from thermal noise in a resistor.

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This design creates a random bitstream from the analog random fluctuations that are inherent in resistors. This requires a lot of gain, and typically would be sensitive to interfering signals that would overwhelm the noise and ruin the randomness of the output. This design entry is based on the circuit in my US patent 6,070,178, which has expired. This project adds a new interface to the random number generator to make it compatible with modern computers. This practical device provides anyone who needs it with access to cryptographically-secure true-random numbers. It is simple enough that many people will be able to build versions the device using a wide variety of generic components. The purpose of this contest entry is to spread knowledge of this circuit widely, in the hope that its use will make it more difficult for malicious actors to create a single point of failure in privacy and security.

Other applications of true-random numbers are gaming and engineering simulation.

"Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin."

Von Neumann

This project provides an analog means to create true-random numbers from the thermal noise of resistors. The circuit is easy to measure and verify.

The recent unpleasantness in privacy rights and security from eavesdropping emphasizes the importance of secure cryptography. Hard crypto, the cypherpunks would call it. Random numbers are a necessary ingredient in the larger problem of cryptography.

This project is somewhat similar to an existing product, the TrueRNG – Hardware Random Number Generator, which used to be available as a $50 USB stick. I don't think there is really much of a market for this type of device. The benefit from this design will come from it being incorporated into other products to make them more secure. For example, IoT devices should be running HTTPS, and yet they may not have a ready source of entropy from their random number generator. These numbers are (slowly) generated by the Linux Kernel and put into a 4k buffer. Sources of entropy in Linux for these devices is mostly limited to network traffic. This is not always a great way to do this, and is not done in all cases. Here example random number generators used to seed SSL from the CyaSSL library. It has #ifdefs for the various operating systems, since they vary with the available hardware:

Not good. Sinful, actually. Not as bad as this one, though:

Some processors have built-in random number generators that might be pretty good. Who knows? They can't be verified by the application designer.

I designed this random number generator that uses the thermal noise in a resistor:

My original prototype was hand-soldered to a 68HC11 board that had a prototyping area, using components from a local surplus store. I don't know where the board is now, but I might be able to find it and post a picture. I wrote code to send the random bits out the RS-232 port, which I connected to an HP-735 workstation. I wrote test code for verifying that the output data is random. These days, there is more code available for that purpose.

The patent was assigned to a startup, who embedded it into a crypto product. The startup dissolved and they did not pay the renewal fee. This irrevocably ended their rights to it, and the circuit can be freely copied. This will take care of the analog section of generating the numbers. For this project, I am going to have to create a new digital interface, since apparently the 68HC11 is no longer in vogue. I checked, and there are a few parts left on Digi-Key! But I have thrown out my paper copy of the Pink Book.

I got a discount offer from AutoDesk for the Standard Version of EAGLE, and paid $50 for a 1 year subscription. The offer was because I have a free account on I like the EAGLE XML file formats for schematic and layout, so I had wanted it anyway. I took an EAGLE class last year from Matt Berggren, and today I watched some refreshers on YouTube (part1) (part2) (part3) by Jeremy Blum. If I don't like EAGLE, I'll probably try KiCAD again. If anyone wants to help with the project and has strong feelings about which tools to use, I am open to suggestions.

I created a project github repo. I will be keeping the EAGLE files there under revision control, along with any firmware and application software. The name of the repo is true-rng. The license will be BSD.

"Any sufficiently advanced cluelessness is indistinguishable from malice." - J. Porter Clark.

I honestly don't know if the current state of random number generation is cluelessness or malice. In either case, please help me to help others fix it!


Schematic in EAGLE 8.1.1

sch - 267.95 kB - 04/28/2017 at 08:18

See BOM Download


Layout, not complete yet, in EAGLE 8.1.1

brd - 125.49 kB - 04/28/2017 at 08:18



Design equations and BOM

sheet - 17.97 kB - 04/28/2017 at 08:14


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  • RNG Layout in progress: using EAGLE 8.1.1

    Tom Anderson6 days ago 0 comments

    I have been learning EAGLE layout. My review so far is "Good, not bad. Could be better."

    I like the interactive routing, and the MOVE command works well for moving existing routing. Not all tools are good at this. I need to learn more to do a real comparison. After using PCB layout for about 12 hours, I think I am better at it than I was after using gEDA/PCB for 12 hours. I am much better than I was with much more expensive CAD tools after 12 hours. I haven't tried KiCAD for that long, so I have no comparison for it.

    There are some things that bother me about it: The polygons are painted, and not very well. This means that sharp-cornered polygons don't seem to be possible. After a while of using gEDA/PDB, I got to the point where I could make it do anything that I wanted. With EAGLE, I have to make the usual tradeoff: I can't draw exactly what I want, but what I can draw is more productive.

    There has been one crash, but I lost very little work by manually renaming an auto-saved file. It did not offer to restore this file for me. The program briefly flashed an error message about some missing object as it went down during an UNDO. After launching it again, it didn't offer me an auto-saved file, and it had been 30 minutes since my last SAVE. Weird. I found the tip that the autosave files exist from Google search results. Google results are one of the best parts about EAGLE. There is a lot written about it online. EAGLE has more good advice online than the Open Source tools I have used. It also has much more online than the 100X and 1000X more expensive tools. If you are using the Guess-Google-Guess-Google... learning cycle, it works fine.

    There are some delightful things about EAGLE layout. One is the icons. The downside of my 4k monitor is that many applications have tiny icons and no way to make them larger. Some applications let you scale the Icons. The EAGLE 2X icons have high resolution, and they are just the right size on my 4k display. They are crisp! This is a true retina application. Nice! This is my first PC layout design with a 4k monitor. There is no way I am going back, because 4k layout is amazing. Applications without scaling capability have fallen behind where they need to be.

    In the example screenshot, there is a lot of label clutter because I have the component values turned on. It looks better with just reference designators and net names. The net names help. For this type of layout, I am not just making connections. Layout is an important part of the design, and I have to understand exactly what I am hooking up, and how. There is cross-highlighting to the schematic to help with this. Also, I have named all the nets, and now I am getting the payoff for this extra work.

    The proof of the CAD tool is in the Gerber output. I won't really know what I have until I look at the design in an independent tool.

  • Work-in-progress design uploaded to Github

    Tom Anderson04/20/2017 at 07:17 0 comments

    The design spreadsheet, schematic, and layout are now at . The README has been updated with the license, which is MIT.

    If anyone is interested in the analog design technique that I am using, please let me know. I think there is a lot written about how to analyze circuits, but not so much about how to design them. It would be good to hear what people want to read about. I could take many different directions with the writing.

    The schematic is not too bad but the layout is really rough. I have the first cut at a placement done, and a little bit of routing. I can see that I should rip up all the routing, refine the placement, and route again.

    One of the things that engineers often wonder is, "When is the design done?"

    When I build a board, my intention is to make it as good as I know how. I have ordered production quantity boards on a new PC board layout first cut, and shipped them with no modifications needed. The reason for this is that when ordering boards, it is not that much more expensive to get more, as long as you don't mind waiting a few weeks for the boards. It is just too tempting to have more.

    For this project, I don't intend to sell anything, although I might if there is sufficient interested. However, I have discovered that I enjoy improving designs until I can find any ways to make them better. That is when I know the design is finished: I can't find anything to improve.

    This is my first EAGLE board. In my experience, the first board in a new CAD tool is disappointing. How will EAGLE work out? Checking will help. I should look at every part footprint and pinout. I intend to simulate the design. To do this, I will hook up a Spice simulator and bring in my simulation models. This design is about random noise, and Spice can calculate noise spectral density versus frequency. I can integrate the noise data across frequency to find the RMS value of the noise. There is also more to simulate such as a switched-capacitor power supply, voltage regulators, and filtering.

    The design spreadsheet is an xlsx file. I wish there was a great spreadsheet application for electronic engineering! I used to use an open source spreadsheet called sc written by James Gosling. I contributed to sc back in the day. While writing this, I checked and found sc on github. If you are into code archeology, try it out!

  • Designing a PC Board for the RNG

    Tom Anderson04/19/2017 at 17:50 0 comments

    Enter constraints, derive component values, choose components, draw schematic, draw layout. Welcome to analog land!

    The constraints are in Excel. To really check to make sure that everything will work, I can use a simulator. More time in the simulator increases the odds of first-pass success.

    Read more »

  • The True-Random Number Generator schematic

    Tom Anderson04/10/2017 at 20:04 0 comments

    I have been learning EAGLE while drawing the schematic. I just bought a subscription using a 50% discount from being a (free) registered user. $50 a year. Not bad, worth a try.

    Here is what I have been working on from the point of view of the analog design. The reference designators correspond to the schematic in the project description.

    In my next iteration (coming soon!) the values for R16 and R17 are 100k. The noise from R16 and R17 is amplified by U1A and U1D. These two amplified signals are then subtracted from each other by U1C. This subtraction operation cancels out any external interference that might be picked up by R16 and R17. After subtraction, there is another gain stage, and then a comparator to make a bitstream. For more detail, the patent explains every component, if I recall correctly.

    The op amps U1A and U1D need to be low-noise in order to prevent the op amp noise from overwhelming the resistor noise. I am using the OPA4134, which I have used before. The resistors are 0.1% thin film 0805 25ppm/degC. Good resistors are important for good cancellation. They also have nearly the same noise as an ideal resistor. This is unlike thick-film resistors, which have more noise than that.

    This is an instrument-grade design that is not optimized for cost. The idea is to have a well-controlled circuit that is easy to measure. The goal is to be easy to show that the source of numbers is truly random thermal noise from the resistors. If I were to make a lot of these, perhaps there will be a reduced-cost version.

    I have a spreadsheet with all the component value calculations and signal levels, and I will post something about that after I clean it up. I might turn it into a Python program or something.

    The changes I am working on will enable the circuit to run off of a USB supply. At some point I will provide a USB interface to send the output to a computer. I could do this with an Arduino, but I might use something else.

    The power supply design is coming out a little weird, because it turns out that people don't seem to use negative supplies very much these days. Negative-voltage low-dropout voltage regulators are expensive and uncommon. I am changing the design to use all positive supplies. Analog circuits without negative supplies are much harder to understand. No wonder EEs have trouble learning analog these days! I think I have figured out a way to draw the schematic so that it looks as simple as a bipolar design, but uses all positive supplies.

    This has been a good way to learn EAGLE, because I am spending a lot of time editing the schematic and learning how things work. I am quite particular about both schematics and layout. They are finished when I can't find anything more that can be done to improve them, either electrically or otherwise. The main problem with EAGLE is that I think the libraries are ugly. The file format is beautiful XML, though, so I might port a better-looking library to it. Would that be procrastination? I you don't hear from me for a while, that would be 'yes!'

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Tom Anderson wrote 04/10/2017 at 19:40 point

R16 and R17 are both thermal noise sources. More explanations will be posted soon.

For me, this sort of thing is simple, and digital circuits are complex!

  Are you sure? yes | no

antti.lukats wrote 04/10/2017 at 07:43 point

very very complicated schematic, it uses thermal noise from which one of the resistors??

  Are you sure? yes | no

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