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fix: validate config CRC with CRC-16/CCITT (advisory) to match toolbox/nRF/Silabs #83

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fix: validate config CRC with CRC-16/CCITT (advisory) to match toolbox/nRF/Silabs #83

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37 changes: 35 additions & 2 deletions src/config_parser.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -213,6 +213,38 @@ bool hasValidStoredConfig(void) {
return loadConfig(buf, &len);
}

static uint16_t crc16_ccitt_feed(uint16_t crc, uint8_t b) {
crc ^= (uint16_t)((uint16_t)b << 8);
for (int j = 0; j < 8; j++) {
if ((crc & 0x8000U) != 0U) {
crc = (uint16_t)(((uint32_t)crc << 1) ^ 0x1021U);
} else {
crc = (uint16_t)((uint32_t)crc << 1);
}
}
return crc;
}

// CRC-16/CCITT-FALSE over the toolbox-outer config container body (excludes the
// trailing 2 CRC bytes), with the first two (length) bytes forced to zero so the
// value is independent of the container length. Matches the canonical helper in
// OpenDisplay-Firmware_NRF/config_parser.c and OpenDisplay-Firmware_Silabs.
static uint16_t config_toolbox_outer_crc16(const uint8_t* data, uint32_t body_len) {
uint16_t crc = 0xFFFFU;
if (body_len < 2U) {
for (uint32_t i = 0; i < body_len; i++) {
crc = crc16_ccitt_feed(crc, data[i]);
}
return crc;
}
crc = crc16_ccitt_feed(crc, 0);
crc = crc16_ccitt_feed(crc, 0);
for (uint32_t i = 2U; i < body_len; i++) {
crc = crc16_ccitt_feed(crc, data[i]);
}
return crc;
}

uint32_t calculateConfigCRC(uint8_t* data, uint32_t len){
uint32_t crc = 0xFFFFFFFF;
for (uint32_t i = 0; i < len; i++) {
Expand Down Expand Up @@ -578,8 +610,9 @@ bool loadGlobalConfig(){
}
if (offset < configLen - 2) {
uint16_t crcGiven = configData[configLen - 2] | (configData[configLen - 1] << 8);
uint32_t crcCalculated32 = calculateConfigCRC(configData, configLen - 2);
uint16_t crcCalculated = (uint16_t)(crcCalculated32 & 0xFFFF); // Use lower 16 bits for backwards compatibility
// Advisory (warn-only) validation using CRC-16/CCITT to match the toolbox,
// nRF and Silabs firmware. Not enforced: a mismatch logs a warning only.
uint16_t crcCalculated = config_toolbox_outer_crc16(configData, configLen - 2);
if (crcGiven != crcCalculated) {
writeSerial("WARNING: Config CRC mismatch (given: 0x" + String(crcGiven, HEX) +
", calculated: 0x" + String(crcCalculated, HEX) + ")");
Expand Down