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Merge branch 'doc/esp_random' into 'master'

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

See merge request !976
This commit is contained in:
Angus Gratton
2017-07-12 16:18:43 +08:00
parent 2243bb00f6 3bb90bab97
commit cfbb1a652b
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) uint32_t IRAM_ATTR esp_random(void)
{ {
/* The PRNG which implements WDEV_RANDOM register gets 2 bits /* The PRNG which implements WDEV_RANDOM register gets 2 bits
* of extra entropy from a hardware randomness source every APB clock cycle. * of extra entropy from a hardware randomness source every APB clock cycle
* To make sure entropy is not drained faster than it is added, * (provided WiFi or BT are enabled). To make sure entropy is not drained
* this function needs to wait for at least 16 APB clock cycles after reading * faster than it is added, this function needs to wait for at least 16 APB
* previous word. This implementation may actually wait a bit longer * clock cycles after reading previous word. This implementation may actually
* due to extra time spent in arithmetic and branch statements. * wait a bit longer due to extra time spent in arithmetic and branch statements.
* *
* As a (probably unncessary) precaution to avoid returning the * As a (probably unncessary) precaution to avoid returning the
* RNG state as-is, the result is XORed with additional * RNG state as-is, the result is XORed with additional

10
components/esp32/include/esp_system.h
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@@ -105,7 +105,15 @@ uint32_t esp_get_minimum_free_heap_size( void );
/** /**
* @brief Get one random 32-bit word from hardware RNG * @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); uint32_t esp_random(void);