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@andresv
Created January 21, 2013 20:44
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#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <time.h>
uint32_t d_shift_register = 0;
uint32_t d_taps[32];
uint32_t d_tap_count;
void ax25_g3ruh_scrambler_init(uint32_t tap_mask) {
uint8_t i;
d_tap_count = 0;
for (i=0; i<32; i++) {
if (tap_mask & 0x01 == 1) {
d_taps[d_tap_count] = i;
d_tap_count++;
}
tap_mask = tap_mask >> 1;
}
d_shift_register = 0;
}
void ax25_g3ruh_scrambler(uint8_t* unscrambled, uint8_t* scrambled, uint16_t len) {
uint8_t unscrambled_bit;
uint8_t scrambled_bit;
uint32_t tap_bit;
uint16_t i, j, t;
for (i=0; i<len; i++) {
for (j=0; j<8; j++) {
unscrambled_bit = unscrambled[i]>>j & 0x01;
d_shift_register <<= 1;
scrambled_bit = unscrambled_bit;
for (t=0; t<d_tap_count; t++) {
tap_bit = (d_shift_register >> d_taps[t]) & 0x01;
scrambled_bit = scrambled_bit ^ tap_bit;
}
d_shift_register |= scrambled_bit;
if (scrambled_bit)
scrambled[i] |= (1 << j);
else
scrambled[i] &= ~(1 << j);
}
}
}
void ax25_g3ruh_descrambler(uint8_t* scrambled, uint8_t* unscrambled, uint16_t len) {
uint8_t unscrambled_bit;
uint8_t scrambled_bit;
uint32_t tap_bit;
uint16_t i, j, t;
for (i=0; i<len; i++) {
for (j=0; j<8; j++) {
scrambled_bit = scrambled[i]>>j & 0x01;
d_shift_register <<= 1;
unscrambled_bit = scrambled_bit;
for (t=0; t<d_tap_count; t++) {
tap_bit = (d_shift_register >> d_taps[t]) & 0x01;
unscrambled_bit = unscrambled_bit ^ tap_bit;
}
d_shift_register |= scrambled_bit;
if (unscrambled_bit)
unscrambled[i] |= (1 << j);
else
unscrambled[i] &= ~(1 << j);
}
}
}
void ax25_nrzi_encode(uint8_t* buf, uint8_t* output, uint16_t len) {
uint8_t prev_nrzi_bit = 0;
uint8_t nrz_bit;
uint8_t nrzi_bit;
uint16_t i, j;
for (i=0; i<len; i++) {
for (j=0; j<8; j++) {
nrz_bit = buf[i]>>j & 0x01;
if (nrz_bit == 0)
nrzi_bit = prev_nrzi_bit ^ 1;
else
nrzi_bit = prev_nrzi_bit;
if (nrzi_bit)
output[i] |= (1 << j);
else
output[i] &= ~(1 << j);
prev_nrzi_bit = nrzi_bit;
}
}
}
void ax25_nrzi_decode(uint8_t* buf, uint8_t* output, uint16_t len) {
uint8_t prev_nrzi_bit = 0;
uint8_t nrz_bit;
uint8_t nrzi_bit;
uint16_t i, j;
for (i=0; i<len; i++) {
for (j=0; j<8; j++) {
nrzi_bit = buf[i]>>j & 0x01;
if (nrzi_bit != prev_nrzi_bit)
nrz_bit = 0;
else
nrz_bit = 1;
if (nrz_bit)
output[i] |= (1 << j);
else
output[i] &= ~(1 << j);
prev_nrzi_bit = nrzi_bit;
}
}
}
int main() {
uint8_t data[] = {0x7E, 0x7E, 0x7E, 0x7E, 0x7E, 0x00, 0x01, 0x02, 0x03};
uint8_t i;
printf("original data : ");
for (i=0; i<sizeof(data); i++) {
printf("%02X ", data[i]);
}
printf("\n");
ax25_nrzi_encode(data, data, sizeof(data));
printf("nrzi encoded : ");
for (i=0; i<sizeof(data); i++) {
printf("%02X ", data[i]);
}
printf("\n");
ax25_g3ruh_scrambler_init(0x21000UL);
ax25_g3ruh_scrambler(data, data, sizeof(data));
printf("g3ruh scrambled : ");
for (i=0; i<sizeof(data); i++) {
printf("%02X ", data[i]);
}
printf("\n\n");
srand(time(NULL));
// simulate out of sync descrambler, it may take 3 bytes to resync
// that is why punch of 7E is sent before real data
d_shift_register = rand() % 0xFFFFFFFF + 1;
ax25_g3ruh_descrambler(data, data, sizeof(data));
printf("g3ruh descrambled : ");
for (i=0; i<sizeof(data); i++) {
printf("%02X ", data[i]);
}
printf("\n");
ax25_nrzi_decode(data, data, sizeof(data));
printf("nrzi decoded : ");
for (i=0; i<sizeof(data); i++) {
printf("%02X ", data[i]);
}
printf("\n");
return 0;
}
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