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50 lines
3.7 KiB
ReStructuredText
50 lines
3.7 KiB
ReStructuredText
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Random Number Generation
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========================
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{IDF_TARGET_RF_NAME: default="Wi-Fi or Bluetooth", esp32s2="Wi-Fi"}
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{IDF_TARGET_RF_IS: default="are", esp32s2="is"}
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{IDF_TARGET_BOOTLOADER_RANDOM_INCOMPATIBLE: default="", esp32="I2S, "}
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{IDF_TARGET_NAME} contains a hardware random number generator, values from it can be obtained using the APIs :cpp:func:`esp_random` and :cpp:func:`esp_fill_random`.
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The hardware RNG produces true random numbers under any of the following conditions:
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- RF subsystem is enabled (i.e. {IDF_TARGET_RF_NAME} {IDF_TARGET_RF_IS} enabled).
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- An internal entropy source has been enabled by calling :cpp:func:`bootloader_random_enable` and not yet disabled by calling :cpp:func:`bootloader_random_disable`.
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- While the ESP-IDF :ref:`second-stage-bootloader` is running. This is because the default ESP-IDF bootloader implementation calls :cpp:func:`bootloader_random_enable` when the bootloader starts, and :cpp:func:`bootloader_random_disable` before executing the app.
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When any of these conditions are true, samples of physical noise are continuously mixed into the internal hardware RNG state to provide entropy. Consult the *{IDF_TARGET_NAME} Technical Reference Manual* > *Random Number Generator (RNG)* [`PDF <{IDF_TARGET_TRM_EN_URL}#rng>`__] chapter for more details.
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If none of the above conditions are true, the output of the RNG should be considered pseudo-random only.
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Startup
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-------
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During startup, ESP-IDF bootloader temporarily enables a non-RF entropy source (internal reference voltage noise) that provides entropy for any first boot key generation. However, after the app starts executing then normally only pseudo-random numbers are available until {IDF_TARGET_RF_NAME} {IDF_TARGET_RF_IS} initialized.
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To re-enable the entropy source temporarily during app startup, or for an application that does not use {IDF_TARGET_RF_NAME}, call the function :cpp:func:`bootloader_random_enable` to re-enable the internal entropy source. The function :cpp:func:`bootloader_random_disable` must be called to disable the entropy source again before using ADC, {IDF_TARGET_BOOTLOADER_RANDOM_INCOMPATIBLE}{IDF_TARGET_RF_NAME}.
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.. note::
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The entropy source enabled during the boot process by the ESP-IDF Second Stage Bootloader will seed the internal RNG state with some entropy. However, the internal hardware RNG state is not large enough to provide a continuous stream of true random numbers. This is why a continuous entropy source must be enabled whenever true random numbers are required.
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.. note::
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If an application requires a source of true random numbers but it is not possible to permanently enable a hardware entropy source, consider using a strong software DRBG implementation such as the mbedTLS CTR-DRBG or HMAC-DRBG, with an initial seed of entropy from hardware RNG true random numbers.
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.. only:: not esp32
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Secondary Entropy
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-----------------
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{IDF_TARGET_NAME} RNG contains a secondary entropy source, based on sampling an asynchronous 8MHz internal oscillator (see the Technical Reference Manual for details). This entropy source is always enabled in ESP-IDF and continuously mixed into the RNG state by hardware. In testing, this secondary entropy source was sufficient to pass the `Dieharder`_ random number test suite without the main entropy source enabled (test input was created by concatenating short samples from a continuously resetting {IDF_TARGET_NAME}). However, it is currently only guaranteed that true random numbers will be produced when the main entropy source is also enabled as described above.
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API Reference
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-------------
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.. include-build-file:: inc/esp_random.inc
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.. include-build-file:: inc/bootloader_random.inc
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.. _Dieharder: https://webhome.phy.duke.edu/~rgb/General/dieharder.php
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