/*

  *  Copyright (C) 2003 - 2004 Tomasz Kojm <tkojm@clamav.net>

  *
  *  Number encoding rutines are based on yyyRSA by Erik Thiele
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation; either version 2 of the License, or
  *  (at your option) any later version.
  *
  *  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., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 

 #if HAVE_CONFIG_H
 #include "clamav-config.h"
 #endif
 

 #ifdef HAVE_GMP
 
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <gmp.h>
 
 #include "clamav.h"
 #include "others.h"

 #include "dsig.h"

 #include "str.h"

 static const char *cli_nstr = "118640995551645342603070001658453189751527774412027743746599405743243142607464144767361060640655844749760788890022283424922762488917565551002467771109669598189410434699034532232228621591089508178591428456220796841621637175567590476666928698770143328137383952820383197532047771780196576957695822641224262693037"; /* 1024 bits */

 static const char *cli_estr = "100001027";

 static char cli_ndecode(char value)

 {
 	int i;
 	char ncodec[] = {
 	    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 
 	    'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 
 	    'y', 'z',
 	    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 
 	    'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 
 	    'Y', 'Z',
 	    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
 	    '+', '/'
 	};
 
 
     for(i = 0; i < 64; i++)
 	if(ncodec[i] == value)
 	    return i;
 
     cli_errmsg("cli_ndecode: value out of range\n");
     return -1;
 }
 

 static char *cli_decodesig(const char *sig, int plainlen, mpz_t e, mpz_t n)

 {
 	int i, siglen = strlen(sig), dec;
 	char *decoded;
 	mpz_t r, p, c;
 
 
     mpz_init(r);
     mpz_init(c);
 
     for(i = 0; i < siglen; i++) {
 	if((dec = cli_ndecode(sig[i])) < 0)
 	    return NULL;
 
 	mpz_set_ui(r, dec);
 	mpz_mul_2exp(r, r, 6 * i);
 	mpz_add(c, c, r);
     }
 
     mpz_init(p);
     decoded = (char *) calloc(plainlen + 1, sizeof(char));
 
     mpz_powm(p, c, e, n); /* plain = cipher^e mod n */
     mpz_clear(c);
 
     for(i = plainlen - 1; i >= 0; i--) { /* reverse */
 	mpz_tdiv_qr_ui(p, r, p, 256);
 	decoded[i] = mpz_get_ui(r);
     }
 
     mpz_clear(p);
     mpz_clear(r);
 
     return decoded;
 }
 
 int cli_versig(const char *md5, const char *dsig)
 {
 	mpz_t n, e;
 	char *pt, *pt2;
 
     if(strlen(md5) != 32 || !isalnum(md5[0])) {
 	/* someone is trying to fool us with empty/malformed MD5 ? */
 	cli_errmsg("SECURITY WARNING: MD5 basic test failure.\n");
 	return CL_EMD5;
     }
 
     mpz_init_set_str(n, cli_nstr, 10);
     mpz_init_set_str(e, cli_estr, 10);

     if(!(pt = cli_decodesig(dsig, 16, e, n))) {
 	mpz_clear(n);
 	mpz_clear(e);

 	return CL_EDSIG;

     }

     pt2 = cli_str2hex(pt, 16);

     free(pt);
 
     cli_dbgmsg("Decoded signature: %s\n", pt2);
 
     if(strncmp(md5, pt2, 32)) {
 	cli_dbgmsg("Signature doesn't match.\n");
 	free(pt2);

 	mpz_clear(n);
 	mpz_clear(e);

 	return CL_EDSIG;
     }
 
     free(pt2);

     mpz_clear(n);
     mpz_clear(e);

 
     cli_dbgmsg("Digital signature is correct.\n");
     return 0;
 }
 
 #endif