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esp_random: Add some documentation about using before RF subsystem is on

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Angus Gratton 2017-07-11 11:23:33 +08:00 committed by Angus Gratton
parent 9b30f66fac
commit 3bb90bab97
2 changed files with 14 additions and 6 deletions
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10
components/esp32/hw_random.c
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@ -24,11 +24,11 @@
uint32_t IRAM_ATTR esp_random(void)
{
/* The PRNG which implements WDEV_RANDOM register gets 2 bits
* of extra entropy from a hardware randomness source every APB clock cycle.
* To make sure entropy is not drained faster than it is added,
* this function needs to wait for at least 16 APB clock cycles after reading
* previous word. This implementation may actually wait a bit longer
* due to extra time spent in arithmetic and branch statements.
* of extra entropy from a hardware randomness source every APB clock cycle
* (provided WiFi or BT are enabled). To make sure entropy is not drained
* faster than it is added, this function needs to wait for at least 16 APB
* clock cycles after reading previous word. This implementation may actually
* wait a bit longer due to extra time spent in arithmetic and branch statements.
*
* As a (probably unncessary) precaution to avoid returning the
* RNG state as-is, the result is XORed with additional

10
components/esp32/include/esp_system.h
View file

@ -98,7 +98,15 @@ uint32_t system_get_free_heap_size(void) __attribute__ ((deprecated));
/**
* @brief Get one random 32-bit word from hardware RNG
*
* @return random value between 0 and UINT32_MAX
* The hardware RNG is fully functional whenever an RF subsystem is running (ie Bluetooth or WiFi is enabled). For secure
* random values, call this function after WiFi or Bluetooth are started.
*
* When the app is running without an RF subsystem enabled, it should be considered a PRNG. To help improve this
* situation, the RNG is pre-seeded with entropy while the IDF bootloader is running. However no new entropy is
* available during the window of time between when the bootloader exits and an RF subsystem starts. It may be possible
* to discern a non-random pattern in a very large amount of output captured during this window of time.
*
* @return Random value between 0 and UINT32_MAX
*/
uint32_t esp_random(void);