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Add CRC-16/X-25 transport-frame verification to SML decoder #15

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74 changes: 72 additions & 2 deletions esp8266-egy-counter/sml_decoder.h
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,61 @@ static const uint8_t OBIS_POWER_L3[6] = { 0x01, 0x00, 0x3D, 0x07, 0x00, 0xFF
/* ---- SML framing ------------------------------------------------------- */
static const uint8_t SML_ESCAPE_SEQ[4] = { 0x1B, 0x1B, 0x1B, 0x1B };
static const uint8_t SML_START_SEQ[4] = { 0x01, 0x01, 0x01, 0x01 };
static const uint8_t SML_END_MARKER = 0x1A;

/* ---- CRC-16/X-25 lookup table (reflected polynomial 0x8408) ----------- */
static const uint16_t SML_CRC16_TABLE[256] = {
0x0000, 0x1189, 0x2312, 0x329B, 0x4624, 0x57AD, 0x6536, 0x74BF,
0x8C48, 0x9DC1, 0xAF5A, 0xBED3, 0xCA6C, 0xDBE5, 0xE97E, 0xF8F7,
0x1081, 0x0108, 0x3393, 0x221A, 0x56A5, 0x472C, 0x75B7, 0x643E,
0x9CC9, 0x8D40, 0xBFDB, 0xAE52, 0xDAED, 0xCB64, 0xF9FF, 0xE876,
0x2102, 0x308B, 0x0210, 0x1399, 0x6726, 0x76AF, 0x4434, 0x55BD,
0xAD4A, 0xBCC3, 0x8E58, 0x9FD1, 0xEB6E, 0xFAE7, 0xC87C, 0xD9F5,
0x3183, 0x200A, 0x1291, 0x0318, 0x77A7, 0x662E, 0x54B5, 0x453C,
0xBDCB, 0xAC42, 0x9ED9, 0x8F50, 0xFBEF, 0xEA66, 0xD8FD, 0xC974,
0x4204, 0x538D, 0x6116, 0x709F, 0x0420, 0x15A9, 0x2732, 0x36BB,
0xCE4C, 0xDFC5, 0xED5E, 0xFCD7, 0x8868, 0x99E1, 0xAB7A, 0xBAF3,
0x5285, 0x430C, 0x7197, 0x601E, 0x14A1, 0x0528, 0x37B3, 0x263A,
0xDECD, 0xCF44, 0xFDDF, 0xEC56, 0x98E9, 0x8960, 0xBBFB, 0xAA72,
0x6306, 0x728F, 0x4014, 0x519D, 0x2522, 0x34AB, 0x0630, 0x17B9,
0xEF4E, 0xFEC7, 0xCC5C, 0xDDD5, 0xA96A, 0xB8E3, 0x8A78, 0x9BF1,
0x7387, 0x620E, 0x5095, 0x411C, 0x35A3, 0x242A, 0x16B1, 0x0738,
0xFFCF, 0xEE46, 0xDCDD, 0xCD54, 0xB9EB, 0xA862, 0x9AF9, 0x8B70,
0x8408, 0x9581, 0xA71A, 0xB693, 0xC22C, 0xD3A5, 0xE13E, 0xF0B7,
0x0840, 0x19C9, 0x2B52, 0x3ADB, 0x4E64, 0x5FED, 0x6D76, 0x7CFF,
0x9489, 0x8500, 0xB79B, 0xA612, 0xD2AD, 0xC324, 0xF1BF, 0xE036,
0x18C1, 0x0948, 0x3BD3, 0x2A5A, 0x5EE5, 0x4F6C, 0x7DF7, 0x6C7E,
0xA50A, 0xB483, 0x8618, 0x9791, 0xE32E, 0xF2A7, 0xC03C, 0xD1B5,
0x2942, 0x38CB, 0x0A50, 0x1BD9, 0x6F66, 0x7EEF, 0x4C74, 0x5DFD,
0xB58B, 0xA402, 0x9699, 0x8710, 0xF3AF, 0xE226, 0xD0BD, 0xC134,
0x39C3, 0x284A, 0x1AD1, 0x0B58, 0x7FE7, 0x6E6E, 0x5CF5, 0x4D7C,
0xC60C, 0xD785, 0xE51E, 0xF497, 0x8028, 0x91A1, 0xA33A, 0xB2B3,
0x4A44, 0x5BCD, 0x6956, 0x78DF, 0x0C60, 0x1DE9, 0x2F72, 0x3EFB,
0xD68D, 0xC704, 0xF59F, 0xE416, 0x90A9, 0x8120, 0xB3BB, 0xA232,
0x5AC5, 0x4B4C, 0x79D7, 0x685E, 0x1CE1, 0x0D68, 0x3FF3, 0x2E7A,
0xE70E, 0xF687, 0xC41C, 0xD595, 0xA12A, 0xB0A3, 0x8238, 0x93B1,
0x6B46, 0x7ACF, 0x4854, 0x59DD, 0x2D62, 0x3CEB, 0x0E70, 0x1FF9,
0xF78F, 0xE606, 0xD49D, 0xC514, 0xB1AB, 0xA022, 0x92B9, 0x8330,
0x7BC7, 0x6A4E, 0x58D5, 0x495C, 0x3DE3, 0x2C6A, 0x1EF1, 0x0F78
};

/* ======================================================================== */
/* CRC-16/X-25 */
/* ======================================================================== */

/*
* Compute CRC-16/X-25 (init=0xFFFF, poly=0x1021 reflected, xorout=0xFFFF)
* over buf[0..len-1]. The SML end escape stores the result little-endian:
* buf[endPos+6] = crc & 0xFF (low byte)
* buf[endPos+7] = crc >> 8 (high byte)
*/
static uint16_t smlCrc16(const uint8_t *buf, uint16_t len)
{
uint16_t crc = 0xFFFF;
for (uint16_t i = 0; i < len; i++)
crc = SML_CRC16_TABLE[(buf[i] ^ crc) & 0xFF] ^ (crc >> 8);
return crc ^ 0xFFFF;
}

/* ======================================================================== */
/* Low-level TLV helpers */
Expand Down Expand Up @@ -349,19 +404,21 @@ static void smlParseGetListRes(const uint8_t *buf, uint16_t bufLen, uint16_t &po
* val.pphase[1] ← OBIS 1.0.41.7.0.255 power L2 (W)
* val.pphase[2] ← OBIS 1.0.61.7.0.255 power L3 (W)
*
* Returns true if a valid SML start sequence was found.
* Returns true if a valid, CRC-verified SML telegram was found and decoded.
*/
static bool decodeSml(const uint8_t *buf, uint16_t len, CounterValues &val)
{
if (len < 8) return false;
if (len < 16) return false;

/* Locate start: escape sequence followed by version bytes */
uint16_t pos = 0;
uint16_t startPos = 0;
bool found = false;
while (pos + 8 <= len) {
if (memcmp(buf + pos, SML_ESCAPE_SEQ, 4) == 0 &&
memcmp(buf + pos + 4, SML_START_SEQ, 4) == 0)
{
startPos = pos;
pos += 8;
found = true;
break;
Expand Down Expand Up @@ -437,5 +494,18 @@ static bool decodeSml(const uint8_t *buf, uint16_t len, CounterValues &val)
for (uint16_t i = 6; i < count; i++) smlSkip(buf, len, pos);
}

/* Verify end escape and check CRC
* End escape layout: 1B1B1B1B 1A PP CRC_LO CRC_HI (8 bytes)
* CRC-16/X-25 covers everything from startPos up to and including PP.
* The two CRC bytes are stored little-endian (low byte first).
*/
if (pos + 8 > len) return false;
if (memcmp(buf + pos, SML_ESCAPE_SEQ, 4) != 0) return false;
if (buf[pos + 4] != SML_END_MARKER) return false;

uint16_t crcCalc = smlCrc16(buf + startPos, pos + 6 - startPos);
uint16_t crcStored = (uint16_t)buf[pos + 6] | ((uint16_t)buf[pos + 7] << 8);
if (crcCalc != crcStored) return false;

return true;
}