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esp_random: XOR the RNG register value several times before returning it

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Probably unnecessary, but avoids returning internal RNG state as-is.
This commit is contained in:
Angus Gratton 2017-01-04 16:30:07 +11:00
parent 3922ce47b2
commit 63e9806d85
1 changed files with 7 additions and 1 deletions
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8
components/esp32/hw_random.c
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@ -29,13 +29,19 @@ uint32_t IRAM_ATTR esp_random(void)
* 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
* WDEV_RND_REG reads while waiting.
*/
static uint32_t last_ccount = 0;
uint32_t ccount;
uint32_t result = 0;
do {
ccount = XTHAL_GET_CCOUNT();
result ^= REG_READ(WDEV_RND_REG);
} while (ccount - last_ccount < XT_CLOCK_FREQ / APB_CLK_FREQ * 16);
last_ccount = ccount;
return REG_READ(WDEV_RND_REG);
return result ^ REG_READ(WDEV_RND_REG);
}