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+/* elgamal.c - ElGamal Public Key encryption
+ * Copyright (C) 1998 Free Software Foundation, Inc.
+ *
+ * For a description of the algorithm, see:
+ * Bruce Schneier: Applied Cryptography. John Wiley & Sons, 1996.
+ * ISBN 0-471-11709-9. Pages 476 ff.
+ *
+ * This file is part of GnuPG.
+ *
+ * GnuPG 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.
+ *
+ * GnuPG 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., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
+ */
+
+#ifdef PLUTO
+#include <gmp.h>
+#include <freeswan.h>
+#include "constants.h"
+#include "defs.h"
+#include "log.h"
+#include "rnd.h"
+#include "gcryptfix.h"
+#else /*! PLUTO */
+/* #include <config.h> */
+#endif /* !PLUTO */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#ifndef PLUTO
+/* #include "util.h" */
+/* #include "mpi.h" */
+/* #include "cipher.h" */
+#endif
+
+#include "elgamal.h"
+
+typedef struct {
+ MPI p; /* prime */
+ MPI g; /* group generator */
+ MPI y; /* g^x mod p */
+} ELG_public_key;
+
+
+typedef struct {
+ MPI p; /* prime */
+ MPI g; /* group generator */
+ MPI y; /* g^x mod p */
+ MPI x; /* secret exponent */
+} ELG_secret_key;
+
+
+static void test_keys( ELG_secret_key *sk, unsigned nbits );
+static MPI gen_k( MPI p );
+static void generate( ELG_secret_key *sk, unsigned nbits, MPI **factors );
+static int check_secret_key( ELG_secret_key *sk );
+static void encrypt(MPI a, MPI b, MPI input, ELG_public_key *pkey );
+static void decrypt(MPI output, MPI a, MPI b, ELG_secret_key *skey );
+static void sign(MPI a, MPI b, MPI input, ELG_secret_key *skey);
+static int verify(MPI a, MPI b, MPI input, ELG_public_key *pkey);
+
+
+static void
+progress( int c )
+{
+ fputc( c, stderr );
+}
+
+
+static void
+test_keys( ELG_secret_key *sk, unsigned nbits )
+{
+ ELG_public_key pk;
+ MPI test = mpi_alloc( 0 );
+ MPI out1_a = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
+ MPI out1_b = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
+ MPI out2 = mpi_alloc( nbits / BITS_PER_MPI_LIMB );
+
+ pk.p = sk->p;
+ pk.g = sk->g;
+ pk.y = sk->y;
+
+ /*mpi_set_bytes( test, nbits, get_random_byte, 0 );*/
+ { char *p = get_random_bits( nbits, 0, 0 );
+ mpi_set_buffer( test, p, (nbits+7)/8, 0 );
+ m_free(p);
+ }
+
+ encrypt( out1_a, out1_b, test, &pk );
+ decrypt( out2, out1_a, out1_b, sk );
+ if( mpi_cmp( test, out2 ) )
+ log_fatal("ElGamal operation: encrypt, decrypt failed\n");
+
+ sign( out1_a, out1_b, test, sk );
+ if( !verify( out1_a, out1_b, test, &pk ) )
+ log_fatal("ElGamal operation: sign, verify failed\n");
+
+ mpi_free( test );
+ mpi_free( out1_a );
+ mpi_free( out1_b );
+ mpi_free( out2 );
+}
+
+
+/****************
+ * generate a random secret exponent k from prime p, so
+ * that k is relatively prime to p-1
+ */
+static MPI
+gen_k( MPI p )
+{
+ MPI k = mpi_alloc_secure( 0 );
+ MPI temp = mpi_alloc( mpi_get_nlimbs(p) );
+ MPI p_1 = mpi_copy(p);
+ unsigned int nbits = mpi_get_nbits(p);
+ unsigned int nbytes = (nbits+7)/8;
+ char *rndbuf = NULL;
+
+ if( DBG_CIPHER )
+ log_debug("choosing a random k ");
+ mpi_sub_ui( p_1, p, 1);
+ for(;;) {
+ if( DBG_CIPHER )
+ progress('.');
+ if( !rndbuf || nbits < 32 ) {
+ m_free(rndbuf);
+ rndbuf = get_random_bits( nbits, 1, 1 );
+ }
+ else { /* change only some of the higher bits */
+ /* we could imporove this by directly requesting more memory
+ * at the first call to get_random_bits() and use this the here
+ * maybe it is easier to do this directly in random.c */
+ char *pp = get_random_bits( 32, 1, 1 );
+ memcpy( rndbuf,pp, 4 );
+ m_free(pp);
+ }
+ mpi_set_buffer( k, rndbuf, nbytes, 0 );
+
+ for(;;) {
+ /* make sure that the number is of the exact lenght */
+ if( mpi_test_bit( k, nbits-1 ) )
+ mpi_set_highbit( k, nbits-1 );
+ else {
+ mpi_set_highbit( k, nbits-1 );
+ mpi_clear_bit( k, nbits-1 );
+ }
+ if( !(mpi_cmp( k, p_1 ) < 0) ) { /* check: k < (p-1) */
+ if( DBG_CIPHER )
+ progress('+');
+ break; /* no */
+ }
+ if( !(mpi_cmp_ui( k, 0 ) > 0) ) { /* check: k > 0 */
+ if( DBG_CIPHER )
+ progress('-');
+ break; /* no */
+ }
+ if( mpi_gcd( temp, k, p_1 ) )
+ goto found; /* okay, k is relatively prime to (p-1) */
+ mpi_add_ui( k, k, 1 );
+ }
+ }
+ found:
+ m_free(rndbuf);
+ if( DBG_CIPHER )
+ progress('\n');
+ mpi_free(p_1);
+ mpi_free(temp);
+
+ return k;
+}
+
+/****************
+ * Generate a key pair with a key of size NBITS
+ * Returns: 2 structures filles with all needed values
+ * and an array with n-1 factors of (p-1)
+ */
+static void
+generate( ELG_secret_key *sk, unsigned nbits, MPI **ret_factors )
+{
+ MPI p; /* the prime */
+ MPI p_min1;
+ MPI g;
+ MPI x; /* the secret exponent */
+ MPI y;
+ MPI temp;
+ unsigned qbits;
+ byte *rndbuf;
+
+ p_min1 = mpi_alloc( (nbits+BITS_PER_MPI_LIMB-1)/BITS_PER_MPI_LIMB );
+ temp = mpi_alloc( (nbits+BITS_PER_MPI_LIMB-1)/BITS_PER_MPI_LIMB );
+ if( nbits < 512 )
+ qbits = 120;
+ else if( nbits <= 1024 )
+ qbits = 160;
+ else if( nbits <= 2048 )
+ qbits = 200;
+ else
+ qbits = 240;
+ g = mpi_alloc(1);
+ p = generate_elg_prime( 0, nbits, qbits, g, ret_factors );
+ mpi_sub_ui(p_min1, p, 1);
+
+
+ /* select a random number which has these properties:
+ * 0 < x < p-1
+ * This must be a very good random number because this is the
+ * secret part. The prime is public and may be shared anyway,
+ * so a random generator level of 1 is used for the prime.
+ */
+ x = mpi_alloc_secure( nbits/BITS_PER_MPI_LIMB );
+ if( DBG_CIPHER )
+ log_debug("choosing a random x ");
+ rndbuf = NULL;
+ do {
+ if( DBG_CIPHER )
+ progress('.');
+ if( rndbuf ) { /* change only some of the higher bits */
+ if( nbits < 16 ) {/* should never happen ... */
+ m_free(rndbuf);
+ rndbuf = get_random_bits( nbits, 2, 1 );
+ }
+ else {
+ char *r = get_random_bits( 16, 2, 1 );
+ memcpy(rndbuf, r, 16/8 );
+ m_free(r);
+ }
+ }
+ else
+ rndbuf = get_random_bits( nbits, 2, 1 );
+ mpi_set_buffer( x, rndbuf, (nbits+7)/8, 0 );
+ mpi_clear_highbit( x, nbits+1 );
+ } while( !( mpi_cmp_ui( x, 0 )>0 && mpi_cmp( x, p_min1 )<0 ) );
+ m_free(rndbuf);
+
+ y = mpi_alloc(nbits/BITS_PER_MPI_LIMB);
+ mpi_powm( y, g, x, p );
+
+ if( DBG_CIPHER ) {
+ progress('\n');
+ log_mpidump("elg p= ", p );
+ log_mpidump("elg g= ", g );
+ log_mpidump("elg y= ", y );
+ log_mpidump("elg x= ", x );
+ }
+
+ /* copy the stuff to the key structures */
+ sk->p = p;
+ sk->g = g;
+ sk->y = y;
+ sk->x = x;
+
+ /* now we can test our keys (this should never fail!) */
+ test_keys( sk, nbits - 64 );
+
+ mpi_free( p_min1 );
+ mpi_free( temp );
+}
+
+
+/****************
+ * Test whether the secret key is valid.
+ * Returns: if this is a valid key.
+ */
+static int
+check_secret_key( ELG_secret_key *sk )
+{
+ int rc;
+ MPI y = mpi_alloc( mpi_get_nlimbs(sk->y) );
+
+ mpi_powm( y, sk->g, sk->x, sk->p );
+ rc = !mpi_cmp( y, sk->y );
+ mpi_free( y );
+ return rc;
+}
+
+
+static void
+encrypt(MPI a, MPI b, MPI input, ELG_public_key *pkey )
+{
+ MPI k;
+
+ /* Note: maybe we should change the interface, so that it
+ * is possible to check that input is < p and return an
+ * error code.
+ */
+
+ k = gen_k( pkey->p );
+ mpi_powm( a, pkey->g, k, pkey->p );
+ /* b = (y^k * input) mod p
+ * = ((y^k mod p) * (input mod p)) mod p
+ * and because input is < p
+ * = ((y^k mod p) * input) mod p
+ */
+ mpi_powm( b, pkey->y, k, pkey->p );
+ mpi_mulm( b, b, input, pkey->p );
+ #if 0
+ if( DBG_CIPHER ) {
+ log_mpidump("elg encrypted y= ", pkey->y);
+ log_mpidump("elg encrypted p= ", pkey->p);
+ log_mpidump("elg encrypted k= ", k);
+ log_mpidump("elg encrypted M= ", input);
+ log_mpidump("elg encrypted a= ", a);
+ log_mpidump("elg encrypted b= ", b);
+ }
+ #endif
+ mpi_free(k);
+}
+
+
+
+
+static void
+decrypt(MPI output, MPI a, MPI b, ELG_secret_key *skey )
+{
+ MPI t1 = mpi_alloc_secure( mpi_get_nlimbs( skey->p ) );
+
+ /* output = b/(a^x) mod p */
+
+ mpi_powm( t1, a, skey->x, skey->p );
+ mpi_invm( t1, t1, skey->p );
+ mpi_mulm( output, b, t1, skey->p );
+ #if 0
+ if( DBG_CIPHER ) {
+ log_mpidump("elg decrypted x= ", skey->x);
+ log_mpidump("elg decrypted p= ", skey->p);
+ log_mpidump("elg decrypted a= ", a);
+ log_mpidump("elg decrypted b= ", b);
+ log_mpidump("elg decrypted M= ", output);
+ }
+ #endif
+ mpi_free(t1);
+}
+
+
+/****************
+ * Make an Elgamal signature out of INPUT
+ */
+
+static void
+sign(MPI a, MPI b, MPI input, ELG_secret_key *skey )
+{
+ MPI k;
+ MPI t = mpi_alloc( mpi_get_nlimbs(a) );
+ MPI inv = mpi_alloc( mpi_get_nlimbs(a) );
+ MPI p_1 = mpi_copy(skey->p);
+
+ /*
+ * b = (t * inv) mod (p-1)
+ * b = (t * inv(k,(p-1),(p-1)) mod (p-1)
+ * b = (((M-x*a) mod (p-1)) * inv(k,(p-1),(p-1))) mod (p-1)
+ *
+ */
+ mpi_sub_ui(p_1, p_1, 1);
+ k = gen_k( skey->p );
+ mpi_powm( a, skey->g, k, skey->p );
+ mpi_mul(t, skey->x, a );
+ mpi_subm(t, input, t, p_1 );
+ while( mpi_is_neg(t) )
+ mpi_add(t, t, p_1);
+ mpi_invm(inv, k, p_1 );
+ mpi_mulm(b, t, inv, p_1 );
+
+ #if 0
+ if( DBG_CIPHER ) {
+ log_mpidump("elg sign p= ", skey->p);
+ log_mpidump("elg sign g= ", skey->g);
+ log_mpidump("elg sign y= ", skey->y);
+ log_mpidump("elg sign x= ", skey->x);
+ log_mpidump("elg sign k= ", k);
+ log_mpidump("elg sign M= ", input);
+ log_mpidump("elg sign a= ", a);
+ log_mpidump("elg sign b= ", b);
+ }
+ #endif
+ mpi_free(k);
+ mpi_free(t);
+ mpi_free(inv);
+ mpi_free(p_1);
+}
+
+
+/****************
+ * Returns true if the signature composed of A and B is valid.
+ */
+static int
+verify(MPI a, MPI b, MPI input, ELG_public_key *pkey )
+{
+ int rc;
+ MPI t1;
+ MPI t2;
+ MPI base[4];
+ MPI exp[4];
+
+ if( !(mpi_cmp_ui( a, 0 ) > 0 && mpi_cmp( a, pkey->p ) < 0) )
+ return 0; /* assertion 0 < a < p failed */
+
+ t1 = mpi_alloc( mpi_get_nlimbs(a) );
+ t2 = mpi_alloc( mpi_get_nlimbs(a) );
+
+ #if 0
+ /* t1 = (y^a mod p) * (a^b mod p) mod p */
+ mpi_powm( t1, pkey->y, a, pkey->p );
+ mpi_powm( t2, a, b, pkey->p );
+ mpi_mulm( t1, t1, t2, pkey->p );
+
+ /* t2 = g ^ input mod p */
+ mpi_powm( t2, pkey->g, input, pkey->p );
+
+ rc = !mpi_cmp( t1, t2 );
+ #elif 0
+ /* t1 = (y^a mod p) * (a^b mod p) mod p */
+ base[0] = pkey->y; exp[0] = a;
+ base[1] = a; exp[1] = b;
+ base[2] = NULL; exp[2] = NULL;
+ mpi_mulpowm( t1, base, exp, pkey->p );
+
+ /* t2 = g ^ input mod p */
+ mpi_powm( t2, pkey->g, input, pkey->p );
+
+ rc = !mpi_cmp( t1, t2 );
+ #else
+ /* t1 = g ^ - input * y ^ a * a ^ b mod p */
+ mpi_invm(t2, pkey->g, pkey->p );
+ base[0] = t2 ; exp[0] = input;
+ base[1] = pkey->y; exp[1] = a;
+ base[2] = a; exp[2] = b;
+ base[3] = NULL; exp[3] = NULL;
+ mpi_mulpowm( t1, base, exp, pkey->p );
+ rc = !mpi_cmp_ui( t1, 1 );
+
+ #endif
+
+ mpi_free(t1);
+ mpi_free(t2);
+ return rc;
+}
+
+/*********************************************
+ ************** interface ******************
+ *********************************************/
+
+int
+elg_generate( int algo, unsigned nbits, MPI *skey, MPI **retfactors )
+{
+ ELG_secret_key sk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+
+ generate( &sk, nbits, retfactors );
+ skey[0] = sk.p;
+ skey[1] = sk.g;
+ skey[2] = sk.y;
+ skey[3] = sk.x;
+ return 0;
+}
+
+
+int
+elg_check_secret_key( int algo, MPI *skey )
+{
+ ELG_secret_key sk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+ if( !skey[0] || !skey[1] || !skey[2] || !skey[3] )
+ return G10ERR_BAD_MPI;
+
+ sk.p = skey[0];
+ sk.g = skey[1];
+ sk.y = skey[2];
+ sk.x = skey[3];
+ if( !check_secret_key( &sk ) )
+ return G10ERR_BAD_SECKEY;
+
+ return 0;
+}
+
+
+
+int
+elg_encrypt( int algo, MPI *resarr, MPI data, MPI *pkey )
+{
+ ELG_public_key pk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+ if( !data || !pkey[0] || !pkey[1] || !pkey[2] )
+ return G10ERR_BAD_MPI;
+
+ pk.p = pkey[0];
+ pk.g = pkey[1];
+ pk.y = pkey[2];
+ resarr[0] = mpi_alloc( mpi_get_nlimbs( pk.p ) );
+ resarr[1] = mpi_alloc( mpi_get_nlimbs( pk.p ) );
+ encrypt( resarr[0], resarr[1], data, &pk );
+ return 0;
+}
+
+int
+elg_decrypt( int algo, MPI *result, MPI *data, MPI *skey )
+{
+ ELG_secret_key sk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+ if( !data[0] || !data[1]
+ || !skey[0] || !skey[1] || !skey[2] || !skey[3] )
+ return G10ERR_BAD_MPI;
+
+ sk.p = skey[0];
+ sk.g = skey[1];
+ sk.y = skey[2];
+ sk.x = skey[3];
+ *result = mpi_alloc_secure( mpi_get_nlimbs( sk.p ) );
+ decrypt( *result, data[0], data[1], &sk );
+ return 0;
+}
+
+int
+elg_sign( int algo, MPI *resarr, MPI data, MPI *skey )
+{
+ ELG_secret_key sk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+ if( !data || !skey[0] || !skey[1] || !skey[2] || !skey[3] )
+ return G10ERR_BAD_MPI;
+
+ sk.p = skey[0];
+ sk.g = skey[1];
+ sk.y = skey[2];
+ sk.x = skey[3];
+ resarr[0] = mpi_alloc( mpi_get_nlimbs( sk.p ) );
+ resarr[1] = mpi_alloc( mpi_get_nlimbs( sk.p ) );
+ sign( resarr[0], resarr[1], data, &sk );
+ return 0;
+}
+
+int
+elg_verify( int algo, MPI hash, MPI *data, MPI *pkey,
+ int (*cmp)(void *, MPI) UNUSED, void *opaquev UNUSED)
+{
+ ELG_public_key pk;
+
+ if( !is_ELGAMAL(algo) )
+ return G10ERR_PUBKEY_ALGO;
+ if( !data[0] || !data[1] || !hash
+ || !pkey[0] || !pkey[1] || !pkey[2] )
+ return G10ERR_BAD_MPI;
+
+ pk.p = pkey[0];
+ pk.g = pkey[1];
+ pk.y = pkey[2];
+ if( !verify( data[0], data[1], hash, &pk ) )
+ return G10ERR_BAD_SIGN;
+ return 0;
+}
+
+
+
+unsigned
+elg_get_nbits( int algo, MPI *pkey )
+{
+ if( !is_ELGAMAL(algo) )
+ return 0;
+ return mpi_get_nbits( pkey[0] );
+}
+
+
+/****************
+ * Return some information about the algorithm. We need algo here to
+ * distinguish different flavors of the algorithm.
+ * Returns: A pointer to string describing the algorithm or NULL if
+ * the ALGO is invalid.
+ * Usage: Bit 0 set : allows signing
+ * 1 set : allows encryption
+ * NOTE: This function allows signing also for ELG-E, which is not
+ * okay but a bad hack to allow to work with old gpg keys. The real check
+ * is done in the gnupg ocde depending on the packet version.
+ */
+const char *
+elg_get_info( int algo, int *npkey, int *nskey, int *nenc, int *nsig,
+ int *use )
+{
+ *npkey = 3;
+ *nskey = 4;
+ *nenc = 2;
+ *nsig = 2;
+
+ switch( algo ) {
+ case PUBKEY_ALGO_ELGAMAL:
+ *use = PUBKEY_USAGE_SIG|PUBKEY_USAGE_ENC;
+ return "ELG";
+ case PUBKEY_ALGO_ELGAMAL_E:
+ *use = PUBKEY_USAGE_SIG|PUBKEY_USAGE_ENC;
+ return "ELG-E";
+ default: *use = 0; return NULL;
+ }
+}
+
+
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