dg_decryptor

#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>

void tea_decrypt (uint32_t *v, uint32_t *k) {
    uint32_t v0=0, v1=0, sum=0;
    uint32_t delta=0x61C88647;
    uint32_t k0=k[0], k1=k[1], k2=k[2], k3=k[3];
    do {
        sum -= delta;
        v0 += ((v1<<4) + k0) ^ (v1 + sum) ^ ((v1>>5) + k1);
        v1 += ((v0<<4) + k2) ^ (v0 + sum) ^ ((v0>>5) + k3);
    } while (sum != 0x78DDE6E4);
    v[0] ^= v0;
    v[1] ^= v1;
}

int main (int argc, char **argv) {
    if (argc != 3) {
        printf("usage: %s gateway.dg decrypted.dg\n", argv[0]);
        return 0;
    }
    
    uint32_t base[2];
    uint32_t tea_key[4] = { 0x6E6629A0, 0x6F6AC2A1, 0x12B1908A, 0x1A31FDB};

    FILE *gateway = fopen(argv[1], "rb");
    FILE *decrypted = fopen(argv[2], "wb");
    
    while (fread(base, 8, 1, gateway)) {
        tea_decrypt(base, tea_key);
        tea_key[0] += 8;
        fwrite(base, 8, 1, decrypted);
    }
    
    // done
    fclose(gateway);
    fclose(decrypted);
    
    return EXIT_SUCCESS;
}