Fourmilab HotBits Radioactive Random Number Generator Retirement

Since May of 1996, Fourmilab has made true random numbers generated from radioactive decay events, “HotBits”, available for free on the Web to all who wanted them, subject only to a daily quota to avoid exhausting the limited capacity of the generator machines and, later, an API key to defend the site against the denial of service attacks any resource on the Web inevitably attracts.

Migration of the Fourmilab site to cloud hosting on Amazon Web Services has made it increasingly difficult to support HotBits, especially on the existing hardware, some of which is two decades old. Consequently, the existing radioactively-generated HotBits service will shut down at the end of 2022 (more precisely, at 00:00 UTC on 2023-01-01). If you have an existing HotBits API Key, you can continue to use it without any changes until that date.

Starting on 2022-07-18 and for the foreseeable future, including after retirement of the radioactive generator, HotBits API key holders may request data from a “secondary hardware random number” generator which uses the Intel CPU built-in RDSEED [PDF] instruction to generate random data. To use the secondary generator, use the same API key you have been using, but with the initial “H” replaced by an “R”. Other than the source of the random data, requests using the secondary generator work exactly like those using the radioactive generator. You can, of course, continue to use the pseudorandom generator by specifying an API key of “Pseudorandom”. Pseudorandom requests are not subject to any quota limitations. After the radioactive generators are retired, the pseudorandom generator will be seeded by the secondary hardware generator instead of the radioactive source.

I have no direct knowledge of the internal operation of the Intel RDSEED generator and do not certify its suitability for any purpose. Access to it is provided purely as a convenience for users of HotBits who wish an alternative when the radioactive generator shuts down. After the end of the radioactive HotBits service, the HotBits document collection will remain accessible on Fourmilab as an archive of the project and guide to those interested in setting up their own servers.

On 2022-08-01, a notice of the retirement of the radioactive generator will appear on the main HotBits page, the HotBits request form, and in all HotBits results produced from the radioactive generator with the exception of binary file downloads. These notices will link to a document explaining how to access the secondary generator.

Starting on 2022-09-01, new HotBits API keys will be issued only in the “RB1” form which request data from the secondary (RDSEED) generator.

As of 2023-01-01, requests for radioactively-generated HotBits with “HB1” API keys will be rejected with a notice of retirement of the generators and link to instructions for those who wish to switch to the secondary generator.

Timeline summary:

  • 2022-07-18: HotBits server introduces secondary hardware generator, accepts “RB1” form of all existing API keys to access secondary generator.
  • 2022-08-01: Notice of retirement of radioactive generator posted on HotBits main, request, and result pages.
  • 2022-09-01: Issuance of new “HB1” API keys ceases. Keys issued after that date work only with the secondary generator.
  • 2023-01-01: Radioactively-generated service discontinued, generators shut down. Subsequently, only secondary generator requests are accepted.


The Fourmilab HotBits service was actually my second foray into radioactive random number generation. In 1986, I designed and wire-wrapped a prototype HotBits hardware interface as a plug-in card for the IBM PC bus architecture. Because I didn’t consider a 6 MHz IBM PC/AT fast enough to reliably time decay events in software, I did the whole thing in hardware using an Intel 8254 counter/timer chip. (And even if the PC were fast enough, locking out interrupts to obtain reliable timing would have rendered the machine unresponsive to user input.)

This generator was used to produce the encryption keys we used to secure source code of Autodesk software on the dial-up FILETRAN machine programmers working from home would use to download and upload code.

The schematic was, of course, drawn with AutoCAD.


What was the radioactive decay element used in the generation of the random numbers? Just curious.


In the current (third generation) generators, Cesium-137, in a 5 microcurie check source from Oxford Nuclear. Details are in the “HotBits Hardware” page.

The first generation hardware, used from 1996 through 2006, used a Jordan Nuclear 60 microcurie Krypton-85 check source scavenged from a 1980s survey meter. Because the half-life of 85Kr is just 10.7 years, the activity of the source had dropped to around half its original value by the time I started using it.

In the U.S. you can order radioactive check sources through the post from United Nuclear. No permit is required.


Thank you, Mr. W. Curiosity satisfied.

It is truly astonishing that one can acquire anything with the word “radioactive” in its name in the US without a background check and a permit (or two, or three). Doubtless there is a 20-man (woman?) panel somewhere holding regular meetings to devise some new regulations to put an end to this current dangerous practice.


As announced here on 2022-07-18 in the main post, at 00:00 UTC on 2022-08-01 notices were posted on all of the HotBits entry pages (see the main page for an example) of the planned retirement of the HotBits radioactive random number generator service. All HotBits queries that use the radioactive generator with the exception of binary file downloads will now contain a message warning of the retirement and directing users to the “HotBits Radioactive Generator Retirement” page for further information and how to use the secondary generator after the radioactive generators are retired.

The secondary generator has been available since 2022-07-18 and may be accessed by any holder of a HotBits API key simply by replacing the “HB1” at the start of the key with “RB1”. After 2022-09-01, only “RB1” API keys will be issued.