/* dhtest - Diffie-Hellman key exchange
**
** To build:
** cc -Wall -I/usr/local/include dhtest.c -s -L/usr/local/lib -lcrypto -o dhtest
**
** To use:
**
** Choose a modulus p and a base g. p should be a large prime number,
** say 2048 bits. g should be a primitive root modulo p, which is
** a complicated topic but it's fine to use: 2. Distribute p and g
** to the client and the server.
**
** Do the first phase on the client side:
**   dhtest -1 p g
** This gives you two numbers, a and A. Keep a and send A to the
** server side.
**
** Do the second phase on the server side:
**   dhtest -2 p g A
** This gives you two numbers, B and s. s is the shared secret.
** Send B back to the client side.
**
** Do the third phase on the client side, using the number you
** saved from the first phase:
**   dhtest -3 p g a B
** This gives you s, the shared secret, on the client side too.
**
** This all runs pretty fast. On my system with a 2048-bit modulus,
** each phase takes about 0.05 seconds.
**
**
** Copyright © 2016 by Jef Poskanzer <jef@mail.acme.com>.
** All rights reserved.
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions
** are met:
** 1. Redistributions of source code must retain the above copyright
**    notice, this list of conditions and the following disclaimer.
** 2. Redistributions in binary form must reproduce the above copyright
**    notice, this list of conditions and the following disclaimer in the
**    documentation and/or other materials provided with the distribution.
**
** THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
** ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
** IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
** ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
** FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
** DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
** OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
** HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
** OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
** SUCH DAMAGE.
**
** For commentary on this license please see http://acme.com/license.html
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <openssl/bn.h>

static char* argv0;

static void usage( void );


int
main( int argc, char** argv )
    {
    int phase;
    BN_CTX* ctx;
    BIGNUM* p;
    BIGNUM* g;
    BIGNUM* littlea;
    BIGNUM* bigA;
    BIGNUM* littleb;
    BIGNUM* bigB;
    BIGNUM* s;

    /* Figure out the program's name. */
    argv0 = strrchr( argv[0], '/' );
    if ( argv0 != (char*) 0 )
	++argv0;
    else
	argv0 = argv[0];

    ctx = BN_CTX_new();
    p = BN_new();
    g = BN_new();
    littlea = BN_new();
    bigA = BN_new();
    littleb = BN_new();
    bigB = BN_new();
    s = BN_new();

    if ( argc < 4 )
	usage();

    phase = - atoi( argv[1] );
    BN_dec2bn( &p, argv[2] );
    BN_dec2bn( &g, argv[3] );
    switch ( phase )
	{
	case 1:
	if ( argc != 4 )
	    usage();
	BN_rand_range( littlea, p );
	(void) printf( "a = %s\n", BN_bn2dec( littlea ) );
	BN_mod_exp( bigA, g, littlea, p, ctx );
	(void) printf( "A = %s\n", BN_bn2dec( bigA ) );
	break;
	case 2:
	if ( argc != 5 )
	    usage();
	BN_dec2bn( &bigA, argv[4] );
	BN_rand_range( littleb, p );
	BN_mod_exp( bigB, g, littleb, p, ctx );
	(void) printf( "B = %s\n", BN_bn2dec( bigB ) );
	BN_mod_exp( s, bigA, littleb, p, ctx );
	(void) printf( "s = %s\n", BN_bn2dec( s ) );
	break;
	case 3:
	if ( argc != 6 )
	    usage();
	BN_dec2bn( &littlea, argv[4] );
	BN_dec2bn( &bigB, argv[5] );
	BN_mod_exp( s, bigB, littlea, p, ctx );
	(void) printf( "s = %s\n", BN_bn2dec( s ) );
	break;
	default:
	usage();
	}

    BN_free( p );
    BN_free( g );
    BN_free( littlea );
    BN_free( bigA );
    BN_free( littleb );
    BN_free( bigB );
    BN_free( s );
    BN_CTX_free(ctx);

    exit( EX_OK );
    }


static void
usage( void )
    {
    (void) fprintf( stderr, "usage:  %s -1 <p> <g>\n", argv0 );
    (void) fprintf( stderr, "        %s -2 <p> <g> <A>\n", argv0 );
    (void) fprintf( stderr, "        %s -3 <p> <g> <a> <B>\n", argv0 );
    exit( EX_USAGE );
    }