Bitcoin wallet keys emerge from radioactive decay

The open source Bitcoin seed generator uses radioactive decay events to generate BIP39 recovery phrases

Entropy32 Plus is an offline hardware device that converts the timing of radioactive decay into a 12-word or 24-word Bitcoin BIP39 recovery phrase. The device's firmware collects 512-bit comparison bits in a local offline environment and conditions the data pool through the SHA-256 algorithm. The generated words are temporarily stored in random access memory (RAM), and the user can then execute a manual clear command to delete the data.

Its developers pointed out that the quality of the entropy source has not yet been verified through NIST SP800 - 90B testing and independent review. The open source code base covers firmware, board design, manufacturing data, casings, and the BIP39 vocabulary for inspection.

A physical entropy source based on radioactive decay

The project is called The Universe Bifurcator, which receives pulses from a Geiger counter and measures the time interval between detected events. Its firmware converts comparisons between consecutive intervals into bits: if the new interval is longer than the previous interval, a "1" is generated; if it is shorter, a "0" is generated. Intervals of equal length are discarded, and measurements below 200 microseconds are rejected as possible faults. When the device collects all 512 bits, it stops collecting.

Radioactive decay is unpredictable at the level of individual atoms. Entropy32 Plus uses the time of detected decay events as a source of physical randomness rather than relying entirely on software-generated values. The current design accepts pulses from the GQ Electronics GMC-320SG Geiger counter through a 3.5-mm interface. The document states that the counter can detect emissions from radioactive sources or background radiation.

After receiving the pulse, the LM393 comparator converts the counter's analog signal into a digital input. The custom circuit board passes this signal to the ATmega328P microcontroller. The project firmware uses a microsecond timer to record each accepted event.

SHA-256 processes raw seed data

Once the 512-bit data pool is built, Entropy32 Plus processes all 64 bytes through the built-in SHA-256 code. The firmware takes the first 16 bytes of the result hash to generate a 12-word phrase, or takes all 32 bytes to generate a 24-word phrase.

According to the project, hashing is designed to reduce visible patterns related to detector dead time, count rate changes, or inter-measurement correlations. Although hashing can spread the available entropy into the output, it does not prove how much unpredictability exists in the raw data.

Firmware includes a SHA-256 boot test: Calculate the hash of the string "abc" and compare it to a known value. If the results are incorrect, the device will stop running before collecting entropy. The self-test only confirms that the realization returns the expected test vector results, and does not test the Geiger counter, evaluate pulse independence, or measure the minimum entropy into the hash.

The project collects twice the number of comparison bits as the maximum 256-bit BIP39 input. Before physical sources are formally measured, the code base does not establish how much available entropy each comparison contains.

Mapping from conditional data to BIP39 words

After SHA-256 processing, the device applies the process defined in the BIP39 specification. BIP39 converts the approved entropy length into mnemonic sentences selected from a standardized 2,048-word list.

• 12-word phrase: Use 128-bit conditioned data and calculate a 4-bit checksum. The combined 132 digits were divided into 12 groups of 11 digits, and each group selected one word.
• 24-word phrase: uses 256-bit data and 8-bit checksum. The resulting 264-bit sequence is segmented into 24 word indexes.

The restore phrase contains the information needed to restore wallet access. Anyone who gets the correct order of words can rebuild the wallet key. It should be noted that a seed phrase is not a collection of independently generated words. Modern Bitcoin wallets derive many private keys from a master seed, so weaknesses in raw entropy can affect every address generated thereby.

Entropy32 Plus compiles the complete BIP39 English vocabulary into firmware. According to code base information, the vocabulary occupies 13,117 bytes, accounting for approximately 43% of the microcontroller's flash memory capacity. Coupled with the OLED interface, SHA-256 implementation and acquisition logic, the compiled program used 30,006 bytes of the 30,720 bytes available on the chip, leaving only 714 bytes unused.

Offline design keeps seeds away from connected devices

The published hardware document does not list any wireless communication hardware. The user operates the generator with two buttons and reads the phrase from the 0.91-inch OLED screen.

The generated word index is temporarily retained in the microcontroller's RAM. Once the user records the phrase, the press of two buttons starts the confirmation process, overrides the word array and restarts entropy collection. Firmware purges the raw data pool after processing the copy. After calculating the BIP39 index, the temporary hash, checksum and entropy buffer are also overwritten.

According to the developer, the device will not save recovery phrases in persistent storage. However, open source designs do not include secure elements, and there is no mechanism to prove that installed firmware matches public code. Physical isolation eliminates multiple network-based attack routes, but cannot verify that the device is correctly assembled or loaded with trusted software. Modified cells may display preset words or retain generated data without changing appearance.

Independent entropy testing has not yet been completed

Code base claims, and entropy sources based on Geiger counters have not yet been tested by NIST Special Publication 800- 90B. NIST's methodology covers the evaluation of physical entropy sources, including separate procedures for independent and dependent data.

Higgins called for long-term recording of raw arrival intervals and checking them using NIST's non-IID minimum entropy estimator. The firmware note warns that general statistical test results alone are not sufficient to establish sufficient entropy. "Please verify the entropy source you intend to use, otherwise please know that you will bear the risk of using the device." The README document wrote.

Developers recommend mixing Geiger-derived data with other independently verified sources before creating wallets for valuable funds. As of September 13, the codebase did not include external audits, NIST assessments, or public raw event datasets.

Past wallet failure cases show why entropy testing is more than just confirming that the final word conforms to BIP39 format. The technical review explains how weak entropy exposes Bitcoin wallets without revealing who used the affected device. Attackers can generate possible seeds, derive their public addresses and search for matches on the blockchain.

Entropy32 Plus is still in active development. Submission history shows that the initial submission date was September 4, and subsequent changes to firmware, board and enclosure were made. As of September 13, there were no marked production versions of the code base, and no timetable for completing independent security assessments was announced.