[cvaldess]

begin() started the CRACEN TRNG and end() stopped it, whoever was still drawing. Readers run on different tasks:

• Bluefruit seeds the SoftDevice with begin() + random() and never calls end().
• uECC (LESC key generation, on the BLE task) starts it on demand through default_CSPRNG and leaves it on.
• An application that pairs begin()/random()/end() stops it as soon as it is done, even under another task's draw, which then spins until RNG_TIMEOUT and fails. Meshtastic's HardwareRNG::fill() does exactly that.

Meanwhile, after BLE comes up nothing stops the TRNG until the application's first begin()/end() pair. On an nRF54LM20, whose CRACEN has no rings-off standby, that was 56 s after every boot at +0.5 mA (INA228 on the 3.3 V rail). The nRF54L15 parks a full FIFO in IDLE_STANDBY, so there it should cost less; I have not measured it.

Now random() and default_CSPRNG start the TRNG, fill and stop it, one draw at a time under a FreeRTOS mutex (created on first use; before the scheduler starts there is only one caller). begin() only registers the generator with uECC and end() has nothing left to do; both stay for callers that pair them.

The cost is a TRNG start per draw: 32 bytes through HardwareRNG::fill() take ~0.53 ms on an nRF54L15, as before, since its end() already stopped the TRNG each time.

Testing

• nRF54L15-DK, Meshtastic 2.8.2: HardwareRNG::fill() 20 times and TinyCrypt P-256 keygen/ECDH through default_CSPRNG, all correct; CRACEN ENABLE = 0 twenty seconds after boot. A new LESC pairing with an iPhone (passkey) worked, and ENABLE was back to 0 after it and after 9 h 19 min running without a reset.
• nRF54LM20-DK: the same change, plus the critical sections its PKE port needs, in production since 2026-10-11 (24 h soak, no reset). Measured there: TRNG on 0 of 81 s after boot (56 s before), 10.9 mA at idle.

Summary by CodeRabbit

• Bug Fixes
  • Random number generation now runs for each request and is coordinated when multiple tasks request random data, including during key generation.
  • Crypto initialization no longer starts the random number generator; it starts when random data is requested.
  • Random-data requests now report failure when generation cannot proceed. Calls must be made from a task or before the scheduler starts, not from an interrupt or critical section.
  • Bluetooth pairing now reports failures when key generation or shared-secret calculation fails.

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• libraries/Bluefruit54Lib/src/BLESecurity.cpp
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• libraries/nRF54Crypto/src/nRF54Crypto.cpp
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• libraries/nRF54Crypto/src/nRF54Crypto.h

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📝 Walkthrough

The crypto library now manages RNG startup and shutdown for each draw. LESC key generation retries when needed, and pairing replies report key-generation and ECDH failures.

Changes: RNG and LESC security

Layer / File(s)	Summary
Manage RNG for each draw libraries/nRF54Crypto/src/nRF54Crypto.cpp, libraries/nRF54Crypto/src/nRF54Crypto.h, libraries/Bluefruit54Lib/src/bluefruit.cpp	The RNG draw path uses scheduler-aware locking, fills the requested buffer, and disables the RNG module. default_CSPRNG and nRF54CryptoClass::random use this path. begin() and end() no longer start or stop the RNG. The API documentation describes supported call contexts.
Track LESC key generation libraries/Bluefruit54Lib/src/BLESecurity.h, libraries/Bluefruit54Lib/src/BLESecurity.cpp	BLESecurity tracks whether LESC keys are available. It attempts key generation during begin() and retries when security parameters are requested. It refuses the request if key generation fails.
Report ECDH failure libraries/Bluefruit54Lib/src/BLESecurity.cpp	The DH-key reply now uses a computed status. CRACEN shared-secret failure sets that status to BLE_GAP_SEC_STATUS_DHKEY_FAILURE.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: caveman99

Merge Risk: ⚪ Minimal · up to 0635d

The change makes the random number generator run only during each draw and adds failure handling for BLE LESC pairing. No unresolved merge-blocking risk is evident from the supplied review.

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Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 43.48% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: limiting TRNG operation to the duration of each draw.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @libraries/nRF54Crypto/src/nRF54Crypto.cpp:
- Around line 175-176: Update the RNG draw flow around cracen_rng_start() so
startup failure explicitly disables the RNG module without relying on
_rng_started or cracen_rng_stop(); retain the existing stop cleanup after
successful startup.

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[caveman99]

Thanks, running the TRNG per draw fixes the begin()/end() race and the idle TRNG current. Two things to address before merge (inline: the CRACEN ENABLE update and rng_lock()), plus some optional points.

Outside the diff:

• BLESecurity.cpp:121 ignores the result of genKeyPair(), and the DHKEY handler at BLESecurity.cpp:376 only logs a failed sharedSecret() and still replies. This predates the PR, but with a TRNG cold start on every draw there are more ways for keygen or ECDH to fail. Fine as a follow-up.

Cleanups:

• _rng_started is always false outside rng_draw(), so the guards in cracen_rng_start(), cracen_rng_stop() and cracen_rng_fill() no longer do anything; rng_draw() could call start, fill and disable directly.
• With draws serialized under _rng_lock, the critical section around each FIFO poll in cracen_rng_word() and the comments about readers on other tasks (there and above cracen_rng_reset_if_halted()) are out of date.
• The mutex can be created once up front with xSemaphoreCreateMutexStatic(), which is safe before the scheduler starts. That removes the lazy init and the NULL checks.

[cvaldess]

Thanks for the review. 0bf3a70 has the two fixes (inline replies above) and the cleanups:

• the mutex is created up front with xSemaphoreCreateMutexStatic(); this port's critical-section count starts at 0, so the enter/exit around it before the scheduler starts is balanced
• _rng_started is gone: rng_draw() starts, fills and disables directly, and a failed start has already turned the TRNG off
• the critical section around each FIFO poll and the comments about readers on other tasks are gone

The BLESecurity.cpp points (the ignored genKeyPair() result, and the DHKEY reply after a failed sharedSecret()) I'll take as a follow-up PR.

Tested on an nRF54L15-DK (bootloader v0.3.0, s145 9.0.0), Meshtastic built against master + this PR:

• boots normally; CRACEN->ENABLE reads 0 between draws; the mutex is free with no owner or waiters
• an already-bonded iPhone reconnected on its own, encrypted with LESC + MITM, and the node sent its NodeInfo on the mesh 23 s after the reset
• a bench sketch on the same board: a draw with the scheduler running and one with it suspended both return true, with the TRNG off afterwards

[caveman99]

Thanks, 0bf3a70 covers both points and the cleanups. I checked the static mutex creation against the port: uxCriticalNesting starts at 0 in port_cmsis.c, so the critical section before the scheduler starts is balanced, and an early random() from another file's global constructor never touches the mutex.

Rather than follow-ups, please fold the remaining items into this PR so they land together:

1. Suspended-scheduler corner, which comes from combining my two earlier suggestions: if task A is preempted between the FIFO level check and the read in cracen_rng_word(), and task B then draws with the scheduler suspended, B bypasses the mutex and turns the TRNG off, and A reads the FIFO with the module off. If that is A's last word, the draw returns true with a word that is not random. Having rng_draw() return false when the scheduler is suspended and uxSemaphoreGetCount(_rng_lock) == 0 closes it.
2. The conditioning key: soft reset and load the key from the first FIFO words on each start, as nrfx_cracen_entropy_get() does, keeping a timeout around the wait (either by calling nrfx in FIFO-sized chunks with a bounded wait, or by doing the same steps in cracen_rng_start()).
3. BLESecurity.cpp: check the result of genKeyPair() before using the key pair, and fail the pairing instead of replying with success to the DHKEY request when sharedSecret() fails.

[cvaldess]

Folded in, as two commits:

1. ff5a19c: a draw with the scheduler suspended now fails while the mutex is held; with the mutex free it still goes through. I reproduced your case on the L15-DK first. A lower-priority task drew 1 KB at a time, and every tick a higher-priority task preempted it, suspended the scheduler and drew 4 bytes. With 0bf3a70 the first such draw went through and turned the TRNG off under the other task, and that task's next FIFO level read took a HardFault (precise bus fault, BFAR 0x51801004 = RNGCONTROL.FIFOLEVEL). So on this chip it ends in a fault rather than a bad word. With ff5a19c all 300 suspended draws were refused, and the 281 background draws all succeeded.
2. ff5a19c: each draw starts from a soft reset and loads the conditioning key from the first four FIFO words, as nrfx_cracen_entropy_get() does. A generator halted on an error is reset and gets a new key the same way. The wait for those words counts against the per-word timeout of the fill, so cracen_rng_start() no longer has a failure path of its own. The rest of the configuration is unchanged (one 128-bit block, default clock divider). A 16-byte draw takes ~535 µs, against 533 µs with 0bf3a70: the startup dominates, and the reset and key add nothing measurable.
3. 0635d51: begin() keeps the result of genKeyPair(). A pairing that finds no key pair tries again, and is refused with BLE_GAP_SEC_STATUS_UNSPECIFIED if that fails too. A failed sharedSecret() is answered with BLE_GAP_SEC_STATUS_DHKEY_FAILURE.

Meshtastic on the L15-DK with these commits and #7: it boots, the TRNG is off between draws (CRACEN->ENABLE = 0), and a fresh LESC pairing from an iPhone with the passkey went through.