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/*
* Off-the-Record Messaging Toolkit
* Copyright (C) 2004-2014 Ian Goldberg, David Goulet, Chris Alexander,
* Nikita Borisov
* <otr@cypherpunks.ca>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of version 2 of the GNU General Public License as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
/* system headers */
#include <stdlib.h>
/* libgcrypt headers */
#include <gcrypt.h>
static const char* DH1536_MODULUS_S = "0x"
"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1"
"29024E088A67CC74020BBEA63B139B22514A08798E3404DD"
"EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245"
"E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED"
"EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D"
"C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F"
"83655D23DCA3AD961C62F356208552BB9ED529077096966D"
"670C354E4ABC9804F1746C08CA237327FFFFFFFFFFFFFFFF";
static const int DH1536_MOD_LEN_BITS = 1536;
static const char *DH1536_GENERATOR_S = "0x02";
/* Generate the session id and the two encryption keys from our private
* DH key and their public DH key. Also indicate in *high_endp if we
* are the "high" end of the key exchange (set to 1) or the "low" end
* (set to 0) */
void sesskeys_gen(unsigned char sessionid[20], unsigned char sendenc[16],
unsigned char rcvenc[16], int *high_endp, gcry_mpi_t *our_yp,
gcry_mpi_t our_x, gcry_mpi_t their_y)
{
gcry_mpi_t modulus, generator, secretv;
unsigned char *secret;
size_t secretlen;
unsigned char hash[20];
int is_high;
gcry_mpi_scan(&modulus, GCRYMPI_FMT_HEX,
(const unsigned char *)DH1536_MODULUS_S, 0, NULL);
gcry_mpi_scan(&generator, GCRYMPI_FMT_HEX,
(const unsigned char *)DH1536_GENERATOR_S, 0, NULL);
*our_yp = gcry_mpi_snew(DH1536_MOD_LEN_BITS);
gcry_mpi_powm(*our_yp, generator, our_x, modulus);
secretv = gcry_mpi_snew(DH1536_MOD_LEN_BITS);
gcry_mpi_powm(secretv, their_y, our_x, modulus);
gcry_mpi_release(generator);
gcry_mpi_release(modulus);
gcry_mpi_print(GCRYMPI_FMT_USG, NULL, 0, &secretlen, secretv);
secret = malloc(secretlen + 5);
secret[1] = (secretlen >> 24) & 0xff;
secret[2] = (secretlen >> 16) & 0xff;
secret[3] = (secretlen >> 8) & 0xff;
secret[4] = (secretlen) & 0xff;
gcry_mpi_print(GCRYMPI_FMT_USG, secret+5, secretlen, NULL, secretv);
gcry_mpi_release(secretv);
is_high = (gcry_mpi_cmp(*our_yp, their_y) > 0);
/* Calculate the session id */
secret[0] = 0x00;
gcry_md_hash_buffer(GCRY_MD_SHA1, hash, secret, secretlen+5);
memmove(sessionid, hash, 20);
/* Calculate the sending enc key */
secret[0] = is_high ? 0x01 : 0x02;
gcry_md_hash_buffer(GCRY_MD_SHA1, hash, secret, secretlen+5);
memmove(sendenc, hash, 16);
/* Calculate the receiving enc key */
secret[0] = is_high ? 0x02 : 0x01;
gcry_md_hash_buffer(GCRY_MD_SHA1, hash, secret, secretlen+5);
memmove(rcvenc, hash, 16);
*high_endp = is_high;
free(secret);
}
/* Generate a MAC key from the corresponding encryption key */
void sesskeys_make_mac(unsigned char mackey[20], unsigned char enckey[16])
{
gcry_md_hash_buffer(GCRY_MD_SHA1, mackey, enckey, 16);
}