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gnupg/cipher/dsa.c
Werner Koch d0d72d98f3 Normalize the MPIs used as input to secret key functions. 
* cipher/rsa.c (secret): Normalize the INPUT.
(rsa_decrypt): Pass reduced data to secret.
* cipher/elgamal.c (decrypt): Normalize A and B.
* cipher/dsa.c (sign): Normalize HASH.
--

mpi_normalize is in general not required because extra leading zeroes
do not harm the computation.  However, adding extra all zero limbs or
padding with multiples of N may be useful in side-channel attacks. In
particular they are used by the acoustic crypt-analysis.  This is an
extra pre-caution which alone would not be sufficient to mitigate the
described attack.

CVE-id: CVE-2013-4576

Signed-off-by: Werner Koch <wk@gnupg.org>
2013-12-03 09:26:04 +01:00

497 lines
12 KiB
C
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/* dsa.c - DSA signature algorithm
* Copyright (C) 1998, 1999, 2000, 2003, 2006 Free Software Foundation, Inc.
*
* 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 3 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, see <http://www.gnu.org/licenses/>.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include "util.h"
#include "mpi.h"
#include "cipher.h"
#include "dsa.h"
typedef struct {
MPI p; /* prime */
MPI q; /* group order */
MPI g; /* group generator */
MPI y; /* g^x mod p */
} DSA_public_key;
typedef struct {
MPI p; /* prime */
MPI q; /* group order */
MPI g; /* group generator */
MPI y; /* g^x mod p */
MPI x; /* secret exponent */
} DSA_secret_key;
static MPI gen_k( MPI q );
static void test_keys( DSA_secret_key *sk, unsigned qbits );
static int check_secret_key( DSA_secret_key *sk );
static void generate( DSA_secret_key *sk, unsigned nbits, unsigned qbits,
MPI **ret_factors );
static void sign(MPI r, MPI s, MPI input, DSA_secret_key *skey);
static int verify(MPI r, MPI s, MPI input, DSA_public_key *pkey);
static void (*progress_cb) ( void *, int );
static void *progress_cb_data;
void
register_pk_dsa_progress ( void (*cb)( void *, int), void *cb_data )
{
progress_cb = cb;
progress_cb_data = cb_data;
}
static void
progress( int c )
{
if ( progress_cb )
progress_cb ( progress_cb_data, c );
else
fputc( c, stderr );
}
/****************
* Generate a random secret exponent k less than q
*/
static MPI
gen_k( MPI q )
{
MPI k = mpi_alloc_secure( mpi_get_nlimbs(q) );
unsigned int nbits = mpi_get_nbits(q);
unsigned int nbytes = (nbits+7)/8;
char *rndbuf = NULL;
if( DBG_CIPHER )
log_debug("choosing a random k ");
for(;;) {
if( DBG_CIPHER )
progress('.');
if( !rndbuf || nbits < 32 ) {
xfree(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 );
xfree(pp);
}
mpi_set_buffer( k, rndbuf, nbytes, 0 );
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, q ) < 0) ) { /* check: k < q */
if( DBG_CIPHER )
progress('+');
continue; /* no */
}
if( !(mpi_cmp_ui( k, 0 ) > 0) ) { /* check: k > 0 */
if( DBG_CIPHER )
progress('-');
continue; /* no */
}
break; /* okay */
}
xfree(rndbuf);
if( DBG_CIPHER )
progress('\n');
return k;
}
static void
test_keys( DSA_secret_key *sk, unsigned qbits )
{
DSA_public_key pk;
MPI test = mpi_alloc ( mpi_nlimb_hint_from_nbits (qbits) );
MPI out1_a = mpi_alloc ( mpi_nlimb_hint_from_nbits (qbits) );
MPI out1_b = mpi_alloc( mpi_nlimb_hint_from_nbits (qbits) );
pk.p = sk->p;
pk.q = sk->q;
pk.g = sk->g;
pk.y = sk->y;
/*mpi_set_bytes( test, qbits, get_random_byte, 0 );*/
{ char *p = get_random_bits( qbits, 0, 0 );
mpi_set_buffer( test, p, (qbits+7)/8, 0 );
xfree(p);
}
sign( out1_a, out1_b, test, sk );
if( !verify( out1_a, out1_b, test, &pk ) )
log_fatal("DSA:: sign, verify failed\n");
mpi_free( test );
mpi_free( out1_a );
mpi_free( out1_b );
}
/****************
* Generate a DSA key pair with a key of size NBITS
* Returns: 2 structures filled with all needed values
* and an array with the n-1 factors of (p-1)
*/
static void
generate( DSA_secret_key *sk, unsigned nbits, unsigned qbits,
MPI **ret_factors )
{
MPI p; /* the prime */
MPI q; /* the prime factor */
MPI g; /* the generator */
MPI y; /* g^x mod p */
MPI x; /* the secret exponent */
MPI h, e; /* helper */
byte *rndbuf;
assert( nbits >= 512 );
assert( qbits >= 160 );
assert( qbits %8 == 0 );
p = generate_elg_prime( 1, nbits, qbits, NULL, ret_factors );
/* get q out of factors */
q = mpi_copy((*ret_factors)[0]);
if( mpi_get_nbits(q) != qbits )
BUG();
/* find a generator g (h and e are helpers)*/
/* e = (p-1)/q */
e = mpi_alloc( mpi_get_nlimbs(p) );
mpi_sub_ui( e, p, 1 );
mpi_fdiv_q( e, e, q );
g = mpi_alloc( mpi_get_nlimbs(p) );
h = mpi_alloc_set_ui( 1 ); /* we start with 2 */
do {
mpi_add_ui( h, h, 1 );
/* g = h^e mod p */
mpi_powm( g, h, e, p );
} while( !mpi_cmp_ui( g, 1 ) ); /* continue until g != 1 */
/* select a random number which has these properties:
* 0 < x < q-1
* This must be a very good random number because this
* is the secret part. */
if( DBG_CIPHER )
log_debug("choosing a random x ");
x = mpi_alloc_secure( mpi_get_nlimbs(q) );
mpi_sub_ui( h, q, 1 ); /* put q-1 into h */
rndbuf = NULL;
do {
if( DBG_CIPHER )
progress('.');
if( !rndbuf )
rndbuf = get_random_bits( qbits, 2, 1 );
else { /* change only some of the higher bits (= 2 bytes)*/
char *r = get_random_bits( 16, 2, 1 );
memcpy(rndbuf, r, 16/8 );
xfree(r);
}
mpi_set_buffer( x, rndbuf, (qbits+7)/8, 0 );
mpi_clear_highbit( x, qbits+1 );
} while( !( mpi_cmp_ui( x, 0 )>0 && mpi_cmp( x, h )<0 ) );
xfree(rndbuf);
mpi_free( e );
mpi_free( h );
/* y = g^x mod p */
y = mpi_alloc( mpi_get_nlimbs(p) );
mpi_powm( y, g, x, p );
if( DBG_CIPHER ) {
progress('\n');
log_mpidump("dsa p= ", p );
log_mpidump("dsa q= ", q );
log_mpidump("dsa g= ", g );
log_mpidump("dsa y= ", y );
log_mpidump("dsa x= ", x );
}
/* copy the stuff to the key structures */
sk->p = p;
sk->q = q;
sk->g = g;
sk->y = y;
sk->x = x;
/* now we can test our keys (this should never fail!) */
test_keys( sk, qbits );
}
/****************
* Test whether the secret key is valid.
* Returns: if this is a valid key.
*/
static int
check_secret_key( DSA_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;
}
/****************
* Make a DSA signature from HASH and put it into r and s.
*
* Without generating the k this function runs in
* about 26ms on a 300 Mhz Mobile Pentium
*/
static void
sign(MPI r, MPI s, MPI hash, DSA_secret_key *skey )
{
MPI k;
MPI kinv;
MPI tmp;
mpi_normalize (hash);
/* select a random k with 0 < k < q */
k = gen_k( skey->q );
/* r = (a^k mod p) mod q */
mpi_powm( r, skey->g, k, skey->p );
mpi_fdiv_r( r, r, skey->q );
/* kinv = k^(-1) mod q */
kinv = mpi_alloc( mpi_get_nlimbs(k) );
mpi_invm(kinv, k, skey->q );
/* s = (kinv * ( hash + x * r)) mod q */
tmp = mpi_alloc( mpi_get_nlimbs(skey->p) );
mpi_mul( tmp, skey->x, r );
mpi_add( tmp, tmp, hash );
mpi_mulm( s , kinv, tmp, skey->q );
mpi_free(k);
mpi_free(kinv);
mpi_free(tmp);
}
/****************
* Returns true if the signature composed from R and S is valid.
*
* Without the checks this function runs in
* about 31ms on a 300 Mhz Mobile Pentium
*/
static int
verify(MPI r, MPI s, MPI hash, DSA_public_key *pkey )
{
int rc;
MPI w, u1, u2, v;
MPI base[3];
MPI exponent[3];
if( !(mpi_cmp_ui( r, 0 ) > 0 && mpi_cmp( r, pkey->q ) < 0) )
return 0; /* assertion 0 < r < q failed */
if( !(mpi_cmp_ui( s, 0 ) > 0 && mpi_cmp( s, pkey->q ) < 0) )
return 0; /* assertion 0 < s < q failed */
w = mpi_alloc( mpi_get_nlimbs(pkey->q) );
u1 = mpi_alloc( mpi_get_nlimbs(pkey->q) );
u2 = mpi_alloc( mpi_get_nlimbs(pkey->q) );
v = mpi_alloc( mpi_get_nlimbs(pkey->p) );
/* w = s^(-1) mod q */
mpi_invm( w, s, pkey->q );
/* u1 = (hash * w) mod q */
mpi_mulm( u1, hash, w, pkey->q );
/* u2 = r * w mod q */
mpi_mulm( u2, r, w, pkey->q );
/* v = g^u1 * y^u2 mod p mod q */
base[0] = pkey->g; exponent[0] = u1;
base[1] = pkey->y; exponent[1] = u2;
base[2] = NULL; exponent[2] = NULL;
mpi_mulpowm( v, base, exponent, pkey->p );
mpi_fdiv_r( v, v, pkey->q );
rc = !mpi_cmp( v, r );
mpi_free(w);
mpi_free(u1);
mpi_free(u2);
mpi_free(v);
return rc;
}
/*********************************************
************** interface ******************
*********************************************/
/* DSA2 has a variable-sized q, which adds an extra parameter to the
pubkey generation. I'm doing this as a different function as it is
only called from one place and is thus cleaner than revamping the
pubkey_generate interface to carry an extra parameter which would
be meaningless for all algorithms other than DSA. */
int
dsa2_generate( int algo, unsigned nbits, unsigned qbits,
MPI *skey, MPI **retfactors )
{
DSA_secret_key sk;
if( algo != PUBKEY_ALGO_DSA )
return G10ERR_PUBKEY_ALGO;
generate( &sk, nbits, qbits, retfactors );
skey[0] = sk.p;
skey[1] = sk.q;
skey[2] = sk.g;
skey[3] = sk.y;
skey[4] = sk.x;
return 0;
}
int
dsa_generate( int algo, unsigned nbits, MPI *skey, MPI **retfactors )
{
return dsa2_generate(algo,nbits,160,skey,retfactors);
}
int
dsa_check_secret_key( int algo, MPI *skey )
{
DSA_secret_key sk;
if( algo != PUBKEY_ALGO_DSA )
return G10ERR_PUBKEY_ALGO;
if( !skey[0] || !skey[1] || !skey[2] || !skey[3] || !skey[4] )
return G10ERR_BAD_MPI;
sk.p = skey[0];
sk.q = skey[1];
sk.g = skey[2];
sk.y = skey[3];
sk.x = skey[4];
if( !check_secret_key( &sk ) )
return G10ERR_BAD_SECKEY;
return 0;
}
int
dsa_sign( int algo, MPI *resarr, MPI data, MPI *skey )
{
DSA_secret_key sk;
if( algo != PUBKEY_ALGO_DSA )
return G10ERR_PUBKEY_ALGO;
if( !data || !skey[0] || !skey[1] || !skey[2] || !skey[3] || !skey[4] )
return G10ERR_BAD_MPI;
sk.p = skey[0];
sk.q = skey[1];
sk.g = skey[2];
sk.y = skey[3];
sk.x = skey[4];
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
dsa_verify( int algo, MPI hash, MPI *data, MPI *pkey )
{
DSA_public_key pk;
if( algo != PUBKEY_ALGO_DSA )
return G10ERR_PUBKEY_ALGO;
if( !data[0] || !data[1] || !hash
|| !pkey[0] || !pkey[1] || !pkey[2] || !pkey[3] )
return G10ERR_BAD_MPI;
pk.p = pkey[0];
pk.q = pkey[1];
pk.g = pkey[2];
pk.y = pkey[3];
if( !verify( data[0], data[1], hash, &pk ) )
return G10ERR_BAD_SIGN;
return 0;
}
unsigned
dsa_get_nbits( int algo, MPI *pkey )
{
if( algo != PUBKEY_ALGO_DSA )
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
*/
const char *
dsa_get_info( int algo, int *npkey, int *nskey, int *nenc, int *nsig,
int *use )
{
*npkey = 4;
*nskey = 5;
*nenc = 0;
*nsig = 2;
switch( algo ) {
case PUBKEY_ALGO_DSA: *use = PUBKEY_USAGE_SIG; return "DSA";
default: *use = 0; return NULL;
}
}
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