A C program to calculate LoRaWAN's Network Session Key (NwSKey) using mbed TLS

nwkskey.c

/*
 * This C program calculates LoRaWAN's network session key (NwkSKey) using
 * mbed TLS.
 * To use it, you should provide the following parameters in this order when 
 * executing the program:
 *  AppKey: An AES-128 key
 *  AppNonce: A random 3 byte value
 *  NetID: A network identifier
 *  DevNonce: A 2 byte nonce 
 *  Example:
 *      ./nwkskey 0x4F34BF40BFFF839D07FC62DEFA24D185 0x15AE9F 0x8A34DD 0x5519
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <inttypes.h>
#include "mbedtls/aes.h"

#define ARGC     5
#define KEY_LEN  16
#define KEY_SIZE 128
#define USAGE   \
    "\nUsage: ./nwkskey <AppKey> <AppNonce> <NetID> <DevNonce>\n\n"

static mbedtls_aes_context aes_ctx;

int main(int argc, char* argv[]){
    if (argc != ARGC){
        fprintf(stderr, USAGE);
        exit(EXIT_FAILURE);
    }
    assert(argc == ARGC);

    // Define this loop internal variable to avoid c99/gnu99 compatibility errors 
    int i;

    unsigned char app_key[KEY_LEN];
    unsigned char input[KEY_LEN];
    unsigned char nwkskey[KEY_LEN];

    // Cast the parameters to void* in order to avoid memset argument 
    // type warnings
    memset(input, 0, sizeof(input));
    memset(nwkskey, 0, sizeof(nwkskey));
    memset(app_key, 0, sizeof(app_key));

    // Parse AppKey from the first command line argument
    unsigned char* p;
    p = &argv[1][0]; 
    // shift the pointer if the hex number starts with a "0[X|x]" pattern
    if ((strncmp(argv[1], "0x", 2) == 0) || (strncmp(argv[1], "0X", 2)))
        p += 2;

    i = 0;
    unsigned char tmp;
    while ((sscanf(p, "%X", &tmp) > 0) && (i < KEY_LEN)){
        app_key[KEY_LEN - i++] = tmp; // data stored in little endian mode
        p++;
    }

    // Parse input parameters from the command line arguments
    uint32_t app_nonce = 0, net_id = 0;
    uint16_t dev_nonce = 0;

    sscanf(argv[2], "%X", &app_nonce);
    sscanf(argv[3], "%X", &net_id);
    sscanf(argv[4], "%X", &dev_nonce);
    // An alternative may be using an array of pointers (void* p[]) 
    // and sscanf'ing in a loop

    // Build up the input (data are stored in little endian (network byte order)
    input[0] = 0x1;
    input[1] = (unsigned char) (app_nonce & 0xFF);
    input[2] = (unsigned char) ((app_nonce >> 8) & 0xFF);
    input[3] = (unsigned char) ((app_nonce >> 16) & 0xFF);

    input[4] = (unsigned char) (net_id & 0xFF);
    input[5] = (unsigned char) ((net_id >> 8) & 0xFF);
    input[6] = (unsigned char) ((net_id >> 16) & 0xFF);

    input[7] = (unsigned char) (dev_nonce & 0xFF);
    input[8] = (unsigned char) ((dev_nonce >> 8) & 0xFF);

    mbedtls_aes_init(&aes_ctx);
    mbedtls_aes_setkey_enc(&aes_ctx, app_key, KEY_SIZE);

    mbedtls_aes_encrypt(&aes_ctx, input, nwkskey);

    printf("Network Session Key: ");
    for (i = 0; i < KEY_LEN; ++i)
        printf("%c", nwkskey[i]);
    printf("\n");

    return(EXIT_SUCCESS);
}