/* compute secret - the ->peer field of context must have been set before calling this function */
void bctbx_DHMComputeSecret(bctbx_DHMContext_t *context, int (*rngFunction)(void *, uint8_t *, size_t), void *rngContext) {
size_t keyLength;
/* import the peer public value G^Y mod P in the polar ssl context */
dhm_read_public((dhm_context *)(context->cryptoModuleData), context->peer, context->primeLength);
/* compute the secret key */
keyLength = context->primeLength; /* undocumented but this value seems to be in/out, so we must set it to the expected key length */
context->key = (uint8_t *)malloc(keyLength*sizeof(uint8_t)); /* allocate key buffer */
dhm_calc_secret((dhm_context *)(context->cryptoModuleData), context->key, &keyLength);
}
int main( int argc, char *argv[] )
{
FILE *f;
int ret;
size_t n, buflen;
int server_fd = -1;
unsigned char *p, *end;
unsigned char buf[2048];
unsigned char hash[20];
const char *pers = "dh_client";
entropy_context entropy;
ctr_drbg_context ctr_drbg;
rsa_context rsa;
dhm_context dhm;
aes_context aes;
((void) argc);
((void) argv);
memset( &rsa, 0, sizeof( rsa ) );
memset( &dhm, 0, sizeof( dhm ) );
/*
* 1. Setup the RNG
*/
printf( "\n . Seeding the random number generator" );
fflush( stdout );
entropy_init( &entropy );
if( ( ret = ctr_drbg_init( &ctr_drbg, entropy_func, &entropy,
(const unsigned char *) pers,
strlen( pers ) ) ) != 0 )
{
printf( " failed\n ! ctr_drbg_init returned %d\n", ret );
goto exit;
}
/*
* 2. Read the server's public RSA key
*/
printf( "\n . Reading public key from rsa_pub.txt" );
fflush( stdout );
if( ( f = fopen( "rsa_pub.txt", "rb" ) ) == NULL )
{
ret = 1;
printf( " failed\n ! Could not open rsa_pub.txt\n" \
" ! Please run rsa_genkey first\n\n" );
goto exit;
}
rsa_init( &rsa, RSA_PKCS_V15, 0 );
if( ( ret = mpi_read_file( &rsa.N, 16, f ) ) != 0 ||
( ret = mpi_read_file( &rsa.E, 16, f ) ) != 0 )
{
printf( " failed\n ! mpi_read_file returned %d\n\n", ret );
goto exit;
}
rsa.len = ( mpi_msb( &rsa.N ) + 7 ) >> 3;
fclose( f );
/*
* 3. Initiate the connection
*/
printf( "\n . Connecting to tcp/%s/%d", SERVER_NAME,
SERVER_PORT );
fflush( stdout );
if( ( ret = net_connect( &server_fd, SERVER_NAME,
SERVER_PORT ) ) != 0 )
{
printf( " failed\n ! net_connect returned %d\n\n", ret );
goto exit;
}
/*
* 4a. First get the buffer length
*/
printf( "\n . Receiving the server's DH parameters" );
fflush( stdout );
memset( buf, 0, sizeof( buf ) );
if( ( ret = net_recv( &server_fd, buf, 2 ) ) != 2 )
{
printf( " failed\n ! net_recv returned %d\n\n", ret );
goto exit;
}
n = buflen = ( buf[0] << 8 ) | buf[1];
if( buflen < 1 || buflen > sizeof( buf ) )
{
printf( " failed\n ! Got an invalid buffer length\n\n" );
goto exit;
}
/*
* 4b. Get the DHM parameters: P, G and Ys = G^Xs mod P
*/
memset( buf, 0, sizeof( buf ) );
if( ( ret = net_recv( &server_fd, buf, n ) ) != (int) n )
{
printf( " failed\n ! net_recv returned %d\n\n", ret );
goto exit;
}
p = buf, end = buf + buflen;
if( ( ret = dhm_read_params( &dhm, &p, end ) ) != 0 )
{
printf( " failed\n ! dhm_read_params returned %d\n\n", ret );
goto exit;
}
if( dhm.len < 64 || dhm.len > 512 )
{
ret = 1;
printf( " failed\n ! Invalid DHM modulus size\n\n" );
goto exit;
}
/*
* 5. Check that the server's RSA signature matches
* the SHA-1 hash of (P,G,Ys)
*/
printf( "\n . Verifying the server's RSA signature" );
fflush( stdout );
p += 2;
if( ( n = (size_t) ( end - p ) ) != rsa.len )
{
ret = 1;
printf( " failed\n ! Invalid RSA signature size\n\n" );
goto exit;
}
sha1( buf, (int)( p - 2 - buf ), hash );
if( ( ret = rsa_pkcs1_verify( &rsa, RSA_PUBLIC, SIG_RSA_SHA1,
0, hash, p ) ) != 0 )
{
printf( " failed\n ! rsa_pkcs1_verify returned %d\n\n", ret );
goto exit;
}
/*
* 6. Send our public value: Yc = G ^ Xc mod P
*/
printf( "\n . Sending own public value to server" );
fflush( stdout );
n = dhm.len;
if( ( ret = dhm_make_public( &dhm, dhm.len, buf, n,
ctr_drbg_random, &ctr_drbg ) ) != 0 )
{
printf( " failed\n ! dhm_make_public returned %d\n\n", ret );
goto exit;
}
if( ( ret = net_send( &server_fd, buf, n ) ) != (int) n )
{
printf( " failed\n ! net_send returned %d\n\n", ret );
goto exit;
}
/*
* 7. Derive the shared secret: K = Ys ^ Xc mod P
*/
printf( "\n . Shared secret: " );
fflush( stdout );
n = dhm.len;
if( ( ret = dhm_calc_secret( &dhm, buf, &n ) ) != 0 )
{
printf( " failed\n ! dhm_calc_secret returned %d\n\n", ret );
goto exit;
}
for( n = 0; n < 16; n++ )
printf( "%02x", buf[n] );
/*
* 8. Setup the AES-256 decryption key
*
* This is an overly simplified example; best practice is
* to hash the shared secret with a random value to derive
* the keying material for the encryption/decryption keys,
* IVs and MACs.
*/
printf( "...\n . Receiving and decrypting the ciphertext" );
fflush( stdout );
aes_setkey_dec( &aes, buf, 256 );
memset( buf, 0, sizeof( buf ) );
if( ( ret = net_recv( &server_fd, buf, 16 ) ) != 16 )
{
printf( " failed\n ! net_recv returned %d\n\n", ret );
goto exit;
}
aes_crypt_ecb( &aes, AES_DECRYPT, buf, buf );
buf[16] = '\0';
printf( "\n . Plaintext is \"%s\"\n\n", (char *) buf );
exit:
net_close( server_fd );
rsa_free( &rsa );
dhm_free( &dhm );
#if defined(_WIN32)
printf( " + Press Enter to exit this program.\n" );
fflush( stdout ); getchar();
#endif
return( ret );
}