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[aes/sw] Load all-zero vector into PRNG buffer stage for masking off #22930

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May 2, 2024
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[aes/sw] Load all-zero vector into PRNG buffer stage for masking off #22930

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5 changes: 5 additions & 0 deletions sw/device/sca/aes_serial.c
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Original file line number Diff line number Diff line change
Expand Up @@ -776,8 +776,13 @@ bool test_main(void) {
LOG_INFO("Initializing entropy complex.");
CHECK_STATUS_OK(aes_testutils_masking_prng_zero_output_seed());
CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerPrngReseed));
bool idle = false;
do {
CHECK_DIF_OK(dif_aes_get_status(&aes, kDifAesStatusIdle, &idle));
} while (!idle);
}
#endif
CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerDataOutClear));
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I notice that the equivalent line in aes_sca.c is skipped for EnglishBreakfast. Is putting this line outside of the if/endif intentional?

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@vogelpi vogelpi May 2, 2024 •
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Yes it is. Only aes_serial.c runs on English Breakfast (where we use a different way to 0 masks because there is no CSRNG). aes_sca.c only runs on the actual chip and the CW310/340 (because English Breakfast has just 1 UART whereas the uJSON communication needs 2 UARTs).


LOG_INFO("Starting simple serial packet handling.");
while (true) {
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5 changes: 5 additions & 0 deletions sw/device/tests/aes_masking_off_test.c
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Expand Up @@ -93,6 +93,11 @@ status_t execute_test(void) {
CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerPrngReseed));
AES_TESTUTILS_WAIT_FOR_STATUS(&aes, kDifAesStatusIdle, true, kTestTimeout);

// Trigger the clearing of the output data registers. After this point, also
// the PRNG buffer stage will output an all-zero vector.
CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerDataOutClear));
AES_TESTUTILS_WAIT_FOR_STATUS(&aes, kDifAesStatusIdle, true, kTestTimeout);

// "Convert" plain data byte arrays to `dif_aes_data_t` array.
enum {
kAesNumBlocks = 4,
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8 changes: 8 additions & 0 deletions sw/device/tests/crypto/cryptotest/firmware/aes_sca.c
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Expand Up @@ -791,7 +791,15 @@ status_t handle_aes_sca_seed_lfsr(ujson_t *uj) {
if (res.value != 0) {
return ABORTED();
}
// Load the magic seed into the PRNG. After this, the PRNG outputs
// an all-zero vector.
UJSON_CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerPrngReseed));
bool idle = false;
do {
TRY(dif_aes_get_status(&aes, kDifAesStatusIdle, &idle));
} while (!idle);
// Load the PRNG output into the buffer stage.
UJSON_CHECK_DIF_OK(dif_aes_trigger(&aes, kDifAesTriggerDataOutClear));
}
#endif

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