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| Register Value | Index | Value | Value=1? | Calculation | New Register Value (xor with LSFR Sequence starting at the 5th (0xF1) |
|---|---|---|---|---|---|
| 00 | 23 -MSB | 1 | Yes | 00 xor f1 | f1 |
| f1 | 22 | 1 | Yes | f1 xor e0 | 11 |
| 11 | 21 | 1 | Yes | 11 xor 70 | 61 |
| ... | |||||
| 13 | 0 | 0 | No | - | 13 |
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Checksum Source Code
| Code Block |
|---|
#define MESSAGE_SIZE 32
#define OFFSET 4
uint8_t LSFR_sequence[MESSAGE_SIZE] = {0};
void calculateLSFR() {
int i;
uint8_t reg = 0x7C;
uint8_t temp_reg = 0;
for (i = 0; i < MESSAGE_SIZE; i++) {
temp_reg = reg & 0x01;
reg >>= 1;
reg |= (temp_reg << 7);
if (temp_reg) {
reg ^= 0x18;
}
LSFR_sequence[i] = reg;
//printf("%02x\n", LSFR_sequence[i]);
}
}
uint8_t combineLSFR(uint8_t len, uint8_t *data) {
uint8_t hash_reg = 0; // not 0x64
int byte_idx, bit_idx;
uint8_t byte, bit;
//printf("***COMBINE\n");
for (byte_idx = 0; byte_idx < len; byte_idx++) {
for (bit_idx = 7; bit_idx >= 0; bit_idx--) {
bit = (data[byte_idx] & (1 << bit_idx)) >> bit_idx;
if (bit) {
hash_reg ^= LSFR_sequence[byte_idx * 8 + (7 - bit_idx) + OFFSET];
//printf("[%d]: %02x\n", byte_idx * 8 + (7 - bit_idx), hash_reg);
}
bit = 0;
}
}
return hash_reg;
}
uint8_t Checksum(int length, uint8_t *buff) {
calculateLSFR();
return combineLSFR(length, buff);
} |
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