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dsa-common.c
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/* dsa-common.c - Common code for DSA
* Copyright (C) 1998, 1999 Free Software Foundation, Inc.
* Copyright (C) 2013 g10 Code GmbH
*
* This file is part of Libgcrypt.
*
* Libgcrypt is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* Libgcrypt 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 Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser 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 "g10lib.h"
#include "mpi.h"
#include "cipher.h"
#include "pubkey-internal.h"
/*
* Generate a random secret exponent K less than Q.
* Note that ECDSA uses this code also to generate D.
*/
gcry_mpi_t
_gcry_dsa_gen_k (gcry_mpi_t q, int security_level)
{
gcry_mpi_t k = mpi_alloc_secure (mpi_get_nlimbs (q));
unsigned int nbits = mpi_get_nbits (q);
unsigned int nbytes = (nbits+7)/8;
char *rndbuf = NULL;
/* To learn why we don't use mpi_mod to get the requested bit size,
read the paper: "The Insecurity of the Digital Signature
Algorithm with Partially Known Nonces" by Nguyen and Shparlinski.
Journal of Cryptology, New York. Vol 15, nr 3 (2003) */
if (DBG_CIPHER)
log_debug ("choosing a random k of %u bits at seclevel %d\n",
nbits, security_level);
for (;;)
{
if ( !rndbuf || nbits < 32 )
{
xfree (rndbuf);
rndbuf = _gcry_random_bytes_secure (nbytes, security_level);
}
else
{ /* Change only some of the higher bits. We could improve
this by directly requesting more memory at the first call
to get_random_bytes() and use these extra bytes here.
However the required management code is more complex and
thus we better use this simple method. */
char *pp = _gcry_random_bytes_secure (4, security_level);
memcpy (rndbuf, pp, 4);
xfree (pp);
}
_gcry_mpi_set_buffer (k, rndbuf, nbytes, 0);
/* Make sure we have the requested number of bits. This code
looks a bit funny but it is easy to understand if you
consider that mpi_set_highbit clears all higher bits. We
don't have a clear_highbit, thus we first set the high bit
and then clear it again. */
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)
log_debug ("\tk too large - again\n");
continue; /* no */
}
if (!(mpi_cmp_ui (k, 0) > 0)) /* check: k > 0 */
{
if (DBG_CIPHER)
log_debug ("\tk is zero - again\n");
continue; /* no */
}
break; /* okay */
}
xfree (rndbuf);
return k;
}
/* Turn VALUE into an octet string and store it in an allocated buffer
at R_FRAME. If the resulting octet string is shorter than NBYTES
the result will be left padded with zeroes. If VALUE does not fit
into NBYTES an error code is returned. */
static gpg_err_code_t
int2octets (unsigned char **r_frame, gcry_mpi_t value, size_t nbytes)
{
gpg_err_code_t rc;
size_t nframe, noff, n;
unsigned char *frame;
rc = _gcry_mpi_print (GCRYMPI_FMT_USG, NULL, 0, &nframe, value);
if (rc)
return rc;
if (nframe > nbytes)
return GPG_ERR_TOO_LARGE; /* Value too long to fit into NBYTES. */
noff = (nframe < nbytes)? nbytes - nframe : 0;
n = nframe + noff;
frame = mpi_is_secure (value)? xtrymalloc_secure (n) : xtrymalloc (n);
if (!frame)
return gpg_err_code_from_syserror ();
if (noff)
memset (frame, 0, noff);
nframe += noff;
rc = _gcry_mpi_print (GCRYMPI_FMT_USG, frame+noff, nframe-noff, NULL, value);
if (rc)
{
xfree (frame);
return rc;
}
*r_frame = frame;
return 0;
}
/* Connert the bit string BITS of length NBITS into an octet string
with a length of (QBITS+7)/8 bytes. On success store the result at
R_FRAME. */
static gpg_err_code_t
bits2octets (unsigned char **r_frame,
const void *bits, unsigned int nbits,
gcry_mpi_t q, unsigned int qbits)
{
gpg_err_code_t rc;
gcry_mpi_t z1;
/* z1 = bits2int (b) */
rc = _gcry_mpi_scan (&z1, GCRYMPI_FMT_USG, bits, (nbits+7)/8, NULL);
if (rc)
return rc;
if (nbits > qbits)
mpi_rshift (z1, z1, nbits - qbits);
/* z2 - z1 mod q */
if (mpi_cmp (z1, q) >= 0)
mpi_sub (z1, z1, q);
/* Convert to an octet string. */
rc = int2octets (r_frame, z1, (qbits+7)/8);
mpi_free (z1);
return rc;
}
/*
* Generate a deterministic secret exponent K less than DSA_Q. H1 is
* the to be signed digest with a length of HLEN bytes. HALGO is the
* algorithm used to create the hash. On success the value for K is
* stored at R_K.
*/
gpg_err_code_t
_gcry_dsa_gen_rfc6979_k (gcry_mpi_t *r_k,
gcry_mpi_t dsa_q, gcry_mpi_t dsa_x,
const unsigned char *h1, unsigned int hlen,
int halgo, unsigned int extraloops)
{
gpg_err_code_t rc;
unsigned char *V = NULL;
unsigned char *K = NULL;
unsigned char *x_buf = NULL;
unsigned char *h1_buf = NULL;
gcry_md_hd_t hd = NULL;
unsigned char *t = NULL;
gcry_mpi_t k = NULL;
unsigned int tbits, qbits;
int i;
qbits = mpi_get_nbits (dsa_q);
if (!qbits || !h1 || !hlen)
return GPG_ERR_EINVAL;
if (_gcry_md_get_algo_dlen (halgo) != hlen)
return GPG_ERR_DIGEST_ALGO;
/* Step b: V = 0x01 0x01 0x01 ... 0x01 */
V = xtrymalloc (hlen);
if (!V)
{
rc = gpg_err_code_from_syserror ();
goto leave;
}
for (i=0; i < hlen; i++)
V[i] = 1;
/* Step c: K = 0x00 0x00 0x00 ... 0x00 */
K = xtrycalloc (1, hlen);
if (!K)
{
rc = gpg_err_code_from_syserror ();
goto leave;
}
rc = int2octets (&x_buf, dsa_x, (qbits+7)/8);
if (rc)
goto leave;
rc = bits2octets (&h1_buf, h1, hlen*8, dsa_q, qbits);
if (rc)
goto leave;
/* Create a handle to compute the HMACs. */
rc = _gcry_md_open (&hd, halgo, (GCRY_MD_FLAG_SECURE | GCRY_MD_FLAG_HMAC));
if (rc)
goto leave;
/* Step d: K = HMAC_K(V || 0x00 || int2octets(x) || bits2octets(h1) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
_gcry_md_write (hd, "", 1);
_gcry_md_write (hd, x_buf, (qbits+7)/8);
_gcry_md_write (hd, h1_buf, (qbits+7)/8);
memcpy (K, _gcry_md_read (hd, 0), hlen);
/* Step e: V = HMAC_K(V) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
memcpy (V, _gcry_md_read (hd, 0), hlen);
/* Step f: K = HMAC_K(V || 0x01 || int2octets(x) || bits2octets(h1) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
_gcry_md_write (hd, "\x01", 1);
_gcry_md_write (hd, x_buf, (qbits+7)/8);
_gcry_md_write (hd, h1_buf, (qbits+7)/8);
memcpy (K, _gcry_md_read (hd, 0), hlen);
/* Step g: V = HMAC_K(V) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
memcpy (V, _gcry_md_read (hd, 0), hlen);
/* Step h. */
t = xtrymalloc ((qbits+7)/8+hlen);
if (!t)
{
rc = gpg_err_code_from_syserror ();
goto leave;
}
again:
for (tbits = 0; tbits < qbits;)
{
/* V = HMAC_K(V) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
memcpy (V, _gcry_md_read (hd, 0), hlen);
/* T = T || V */
memcpy (t+(tbits+7)/8, V, hlen);
tbits += 8*hlen;
}
/* k = bits2int (T) */
mpi_free (k);
k = NULL;
rc = _gcry_mpi_scan (&k, GCRYMPI_FMT_USG, t, (tbits+7)/8, NULL);
if (rc)
goto leave;
if (tbits > qbits)
mpi_rshift (k, k, tbits - qbits);
/* Check: k < q and k > 1 */
if (!(mpi_cmp (k, dsa_q) < 0 && mpi_cmp_ui (k, 0) > 0))
{
/* K = HMAC_K(V || 0x00) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
_gcry_md_write (hd, "", 1);
memcpy (K, _gcry_md_read (hd, 0), hlen);
/* V = HMAC_K(V) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
memcpy (V, _gcry_md_read (hd, 0), hlen);
goto again;
}
/* The caller may have requested that we introduce some extra loops.
This is for example useful if the caller wants another value for
K because the last returned one yielded an R of 0. Because this
is very unlikely we implement it in a straightforward way. */
if (extraloops)
{
extraloops--;
/* K = HMAC_K(V || 0x00) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
_gcry_md_write (hd, "", 1);
memcpy (K, _gcry_md_read (hd, 0), hlen);
/* V = HMAC_K(V) */
rc = _gcry_md_setkey (hd, K, hlen);
if (rc)
goto leave;
_gcry_md_write (hd, V, hlen);
memcpy (V, _gcry_md_read (hd, 0), hlen);
goto again;
}
/* log_mpidump (" k", k); */
leave:
xfree (t);
_gcry_md_close (hd);
xfree (h1_buf);
xfree (x_buf);
xfree (K);
xfree (V);
if (rc)
mpi_free (k);
else
*r_k = k;
return rc;
}
/*
* Truncate opaque hash value to qbits for DSA.
* Non-opaque input is not truncated, in hope that user
* knows what is passed. It is not possible to correctly
* trucate non-opaque inputs.
*/
gpg_err_code_t
_gcry_dsa_normalize_hash (gcry_mpi_t input,
gcry_mpi_t *out,
unsigned int qbits)
{
gpg_err_code_t rc = 0;
const void *abuf;
unsigned int abits;
gcry_mpi_t hash;
if (mpi_is_opaque (input))
{
abuf = mpi_get_opaque (input, &abits);
rc = _gcry_mpi_scan (&hash, GCRYMPI_FMT_USG, abuf, (abits+7)/8, NULL);
if (rc)
return rc;
if (abits > qbits)
mpi_rshift (hash, hash, abits - qbits);
}
else
hash = input;
*out = hash;
return rc;
}

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