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Use CRC-8 to validate the random seed in EEPROM/Flash

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This commit is contained in:
Rhys Weatherley
2017-11-26 10:06:29 +10:00
parent 8400d51420
commit 06987988be
4 changed files with 55 additions and 11 deletions
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27
libraries/Crypto/RNG.cpp
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@@ -154,6 +154,9 @@ RNGClass RNG;
/** @cond */
// Imported from Crypto.cpp.
extern uint8_t crypto_crc8(uint8_t tag, const void *data, unsigned size);
// Tag for 256-bit ChaCha20 keys. This will always appear in the
// first 16 bytes of the block. The remaining 48 bytes are the seed.
static const char tagRNG[16] PROGMEM = {
@@ -361,17 +364,19 @@ void RNGClass::begin(const char *tag)
memcpy_P(block + 4, initRNG, sizeof(initRNG));
#if defined(RNG_EEPROM)
int address = RNG_EEPROM_ADDRESS;
if (eeprom_read_byte((const uint8_t *)address) == 'S') {
eeprom_read_block(stream, (const void *)address, SEED_SIZE);
if (crypto_crc8('S', stream, SEED_SIZE - 1) ==
((const uint8_t *)stream)[SEED_SIZE - 1]) {
// We have a saved seed: XOR it with the initialization block.
for (int posn = 0; posn < 12; ++posn) {
block[posn + 4] ^=
eeprom_read_dword((const uint32_t *)(address + posn * 4 + 1));
}
// Note: the CRC-8 value is included. No point throwing it away.
for (int posn = 0; posn < 12; ++posn)
block[posn + 4] ^= stream[posn];
}
#elif defined(RNG_DUE_TRNG)
// Do we have a seed saved in the last page of flash memory on the Due?
int posn, counter;
if (((const uint32_t *)RNG_SEED_ADDR)[0] == 'S') {
if (crypto_crc8('S', ((const uint32_t *)RNG_SEED_ADDR) + 1, SEED_SIZE)
== ((const uint32_t *)RNG_SEED_ADDR)[0]) {
// XOR the saved seed with the initialization block.
for (posn = 0; posn < 12; ++posn)
block[posn + 4] ^= ((const uint32_t *)RNG_SEED_ADDR)[posn + 1];
@@ -694,12 +699,16 @@ void RNGClass::save()
++(block[12]);
ChaCha::hashCore(stream, block, RNG_ROUNDS);
#if defined(RNG_EEPROM)
// We shorten the seed from 48 bytes to 47 to leave room for
// the CRC-8 value. We do this to align the data on an 8-byte
// boundary in EERPOM.
int address = RNG_EEPROM_ADDRESS;
eeprom_write_block(stream, (void *)(address + 1), 48);
eeprom_update_byte((uint8_t *)address, 'S');
eeprom_write_block(stream, (void *)address, SEED_SIZE - 1);
eeprom_write_byte((uint8_t *)(address + SEED_SIZE - 1),
crypto_crc8('S', stream, SEED_SIZE - 1));
#elif defined(RNG_DUE_TRNG)
unsigned posn;
((uint32_t *)(RNG_SEED_ADDR))[0] = 'S';
((uint32_t *)(RNG_SEED_ADDR))[0] = crypto_crc8('S', stream, SEED_SIZE);
for (posn = 0; posn < 12; ++posn)
((uint32_t *)(RNG_SEED_ADDR))[posn + 1] = stream[posn];
for (posn = 13; posn < (RNG_FLASH_PAGE_SIZE / 4); ++posn)