docs: Add random number generation to the API Reference System section

components/bootloader_support/include/bootloader_random.h

@@ -21,33 +21,38 @@ extern "C" {
 #endif
 
 /**
- * @brief Enable early entropy source for RNG
+ * @brief Enable an entropy source for RNG if RF is disabled
  *
- * Uses the SAR ADC to feed entropy into the HWRNG. The ADC is put
- * into a test mode that reads an internal reference voltage and
- * constantly feeds the LSB of data into the HWRNG. Consult the
- * SoC Technical Reference Manual for more information.
+ * The exact internal entropy source mechanism depends on the chip in use but
+ * all SoCs use the SAR ADC to continuously mix random bits (an internal
+ * noise reading) into the HWRNG. Consult the SoC Technical Reference
+ * Manual for more information.
  *
  * Can also be used from app code early during operation, if true
- * random numbers are required before WiFi stack is initialised.
- * Call this function from app code only if WiFi/BT are not yet
- * enabled and I2S and ADC are not in use.
- *
- * Call bootloader_random_disable() when done.
+ * random numbers are required before RF is initialised. Consult
+ * ESP-IDF Programming Guide "Random Number Generation" section for
+ * details.
  */
 void bootloader_random_enable(void);
 
 /**
- * @brief Disable early entropy source for RNG
+ * @brief Disable entropy source for RNG
  *
- * Disables SAR ADC source and resets the I2S hardware.
+ * Disables internal entropy source. Must be called after
+ * bootloader_random_enable() and before RF features, ADC, or
+ * I2S (ESP32 only) are initialized.
  *
+ * Consult the ESP-IDF Programming Guide "Random Number Generation"
+ * section for details.
  */
 void bootloader_random_disable(void);
 
 /**
  * @brief Fill buffer with 'length' random bytes
  *
+ * @note If this function is being called from app code only, and never
+ * from the bootloader, then it's better to call esp_fill_random().
+ *
  * @param buffer Pointer to buffer
  * @param length This many bytes of random data will be copied to buffer
  */

components/esp_hw_support/include/esp_random.h

@@ -24,28 +24,13 @@ extern "C" {
 /**
  * @brief  Get one random 32-bit word from hardware RNG
  *
- * The hardware RNG produces true random numbers under any of the following conditions:
- *
- * - An RF subsystem is running (i.e. Bluetooth or WiFi is enabled)
- * - An internal entropy source has been enabled by calling bootloader_random_enable()
- *   and not yet disabled by calling bootloader_random_disable()
- * - While the ESP-IDF bootloader is running (due to the internal entropy source being enabled
- *   for the duration of bootloader execution).
- *
- * If none of the above conditions are true, the hardware RNG will produce pseudo-random numbers only.
- *
- * When the hardware RNG is producing true random numbers, external entropy (noise samples) are
- * continuously mixed into the internal hardware RNG state. Consult the SoC Technical Reference Manual
- * for more details.
+ * If Wi-Fi or Bluetooth are enabled, this function returns true random numbers. In other
+ * situations, if true random numbers are required then consult the ESP-IDF Programming
+ * Guide "Random Number Generation" section for necessary prerequisites.
  *
  * This function automatically busy-waits to ensure enough external entropy has been
  * introduced into the hardware RNG state, before returning a new random number.
  *
- * If generating random numbers from an app which has not yet enabled Bluetooth or Wi-Fi, call the
- * API function bootloader_random_enable() before generating random numbers and then call
- * bootloader_random_disable() before using any APIs for Bluetooth, Wi-Fi, ADC, or I2S. Consult the
- * bootloader_random.h header for more details.
- *
  * @return Random value between 0 and UINT32_MAX
  */
 uint32_t esp_random(void);
@@ -53,7 +38,8 @@ uint32_t esp_random(void);
 /**
  * @brief Fill a buffer with random bytes from hardware RNG
  *
- * @note This function has the same restrictions regarding available entropy as esp_random()
+ * @note This function is implemented via calls to esp_random(), so the same
+ * constraints apply.
  *
  * @param buf Pointer to buffer to fill with random numbers.
  * @param len Length of buffer in bytes

docs/doxygen/Doxyfile_common

@@ -142,6 +142,7 @@ INPUT = \
     $(IDF_PATH)/components/spi_flash/include/esp_flash.h \
     $(IDF_PATH)/components/spi_flash/include/esp_partition.h \
     $(IDF_PATH)/components/bootloader_support/include/esp_flash_encrypt.h \
+    $(IDF_PATH)/components/bootloader_support/include/bootloader_random.h \
     $(IDF_PATH)/components/spiffs/include/esp_spiffs.h \
     $(IDF_PATH)/components/driver/include/driver/sdmmc_types.h \
     $(IDF_PATH)/components/sdmmc/include/sdmmc_cmd.h \
@@ -179,8 +180,9 @@ INPUT = \
     $(IDF_PATH)/components/esp_system/include/esp_expression_with_stack.h \
     $(IDF_PATH)/components/app_update/include/esp_ota_ops.h \
     $(IDF_PATH)/components/esp_https_ota/include/esp_https_ota.h \
-    $(IDF_PATH)/components/esp_hw_support/include/esp_sleep.h \
     $(IDF_PATH)/components/esp_hw_support/include/esp_async_memcpy.h \
+    $(IDF_PATH)/components/esp_hw_support/include/esp_random.h \
+    $(IDF_PATH)/components/esp_hw_support/include/esp_sleep.h \
     $(IDF_PATH)/components/log/include/esp_log.h \
     $(IDF_PATH)/components/esp_rom/include/esp_rom_sys.h \
     $(IDF_PATH)/components/esp_system/include/esp_system.h \

docs/en/api-reference/system/index.rst

@@ -27,6 +27,7 @@ System API
     Over The Air Updates (OTA) <ota>
     :CONFIG_IDF_TARGET_ARCH_XTENSA: Performance Monitor <perfmon>
     Power Management <power_management>
+    Random Number Generation <random>
     Sleep Modes <sleep_modes>
     Watchdogs <wdts>
     System Time <system_time>

docs/en/api-reference/system/random.rst

@@ -0,0 +1,49 @@
+Random Number Generation
+========================
+
+{IDF_TARGET_RF_NAME: default="Wi-Fi or Bluetooth", esp32s2="Wi-Fi"}
+{IDF_TARGET_RF_IS: default="are", esp32s2="is"}
+{IDF_TARGET_BOOTLOADER_RANDOM_INCOMPATIBLE: default="", esp32="I2S, "}
+
+{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`.
+
+The hardware RNG produces true random numbers under any of the following conditions:
+
+- RF subsystem is enabled (i.e. {IDF_TARGET_RF_NAME} {IDF_TARGET_RF_IS} enabled).
+- 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`.
+- 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.
+
+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.
+
+If none of the above conditions are true, the output of the RNG should be considered pseudo-random only.
+
+Startup
+-------
+
+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.
+
+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}.
+
+.. note::
+
+   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.
+
+.. note::
+
+   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.
+
+.. only:: not esp32
+
+    Secondary Entropy
+    -----------------
+
+    {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.
+
+API Reference
+-------------
+
+.. include-build-file:: inc/esp_random.inc
+.. include-build-file:: inc/bootloader_random.inc
+
+.. _Dieharder: https://webhome.phy.duke.edu/~rgb/General/dieharder.php
+

docs/en/api-reference/system/system.rst

@@ -25,13 +25,6 @@ Two heap memory related functions are provided:
 
 Note that ESP-IDF supports multiple heaps with different capabilities. Functions mentioned in this section return the size of heap memory which can be allocated using ``malloc`` family of functions. For further information about heap memory see :doc:`Heap Memory Allocation <mem_alloc>`.
 
-Random number generation
-------------------------
-
-{IDF_TARGET_NAME} contains a hardware random number generator, values from it can be obtained using :cpp:func:`esp_random`.
-
-When Wi-Fi or Bluetooth are enabled, numbers returned by hardware random number generator (RNG) can be considered true random numbers. Without Wi-Fi or Bluetooth enabled, hardware RNG is a pseudo-random number generator. At startup, ESP-IDF bootloader seeds the hardware RNG with entropy, but care must be taken when reading random values between the start of ``app_main`` and initialization of Wi-Fi or Bluetooth drivers.
-
 .. _MAC-Address-Allocation:
 
 MAC Address
@@ -189,12 +182,9 @@ To set version in your project manually you need to set ``PROJECT_VER`` variable
 
 If :ref:`CONFIG_APP_PROJECT_VER_FROM_CONFIG` option is set, the value of :ref:`CONFIG_APP_PROJECT_VER` will be used. Otherwise if ``PROJECT_VER`` variable is not set in the project then it will be retrieved from either ``$(PROJECT_PATH)/version.txt`` file (if present) else using git command ``git describe``. If neither is available then ``PROJECT_VER`` will be set to "1". Application can make use of this by calling :cpp:func:`esp_ota_get_app_description` or :cpp:func:`esp_ota_get_partition_description` functions.
 
-
-
 API Reference
 -------------
 
 .. include-build-file:: inc/esp_system.inc
 .. include-build-file:: inc/esp_idf_version.inc
 
-

docs/zh_CN/api-guides/startup.rst

@@ -2,13 +2,15 @@
 ===================
 :link_to_translation:`en:[English]`
 
+{IDF_TARGET_BOOTLOADER_OFFSET:default="0x0", esp32="0x1000", esp32s2="0x1000"}
+
 本文将会介绍 {IDF_TARGET_NAME} 从上电到运行 ``app_main``
 函数中间所经历的步骤（即启动流程）。
 
 宏观上，该启动流程可以分为如下 3 个步骤：
 
 1. 一级引导程序被固化在了 {IDF_TARGET_NAME} 内部的 ROM 中，它会从 Flash 的
-   ``0x1000`` 偏移地址处加载二级引导程序至 RAM(IRAM & DRAM) 中。
+   {IDF_TARGET_BOOTLOADER_OFFSET} 偏移地址处加载二级引导程序至 RAM(IRAM & DRAM) 中。
 
 2. 二级引导程序从 Flash 中加载分区表和主程序镜像至内存中，主程序中包含了
    RAM 段和通过 Flash 高速缓存映射的只读段。
@@ -53,20 +55,22 @@ SoC 复位后，PRO CPU 会立即开始运行，执行复位向量代码，而 A
    SoC，重复整个过程。如果解析器收到了来自 UART
    的输入，程序会关闭看门狗。
 
-应用程序的二进制镜像会从 Flash 的 ``0x1000`` 地址处加载。Flash 的第一个
+应用程序的二进制镜像会从 Flash 的 {IDF_TARGET_BOOTLOADER_OFFSET} 地址处加载。Flash 的第一个
 4kB
 扇区用于存储安全引导程序和应用程序镜像的签名。有关详细信息，请查看安全启动文档。
 
 .. TODO: describe application binary image format, describe optional flash configuration commands.
 
+.. _second-stage-bootloader:
+
 二级引导程序
 ~~~~~~~~~~~~
 
-在 ESP-IDF 中，存放在 Flash 的 ``0x1000``
+在 ESP-IDF 中，存放在 Flash 的 {IDF_TARGET_BOOTLOADER_OFFSET}
 偏移地址处的二进制镜像就是二级引导程序。二级引导程序的源码可以在 ESP-IDF
 的 components/bootloader 目录下找到。请注意，对于 {IDF_TARGET_NAME}
 芯片来说，这并不是唯一的安排程序镜像的方式。事实上用户完全可以把一个功能齐全的应用程序烧写到
-Flash 的 ``0x1000`` 偏移地址处运行，但这超出本文档的范围。ESP-IDF
+Flash 的 {IDF_TARGET_BOOTLOADER_OFFSET} 偏移地址处运行，但这超出本文档的范围。ESP-IDF
 使用二级引导程序可以增加 Flash 分区的灵活性（使用分区表），并且方便实现
 Flash 加密，安全引导和空中升级（OTA）等功能。
 

docs/zh_CN/api-reference/system/random.rst

@@ -0,0 +1 @@
+.. include:: ../../../en/api-reference/system/random.rst