crypto_kdf.c

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/*
     This file is part of GNUnet.
     Copyright (C) 2010 GNUnet e.V.
 
     GNUnet is free software: you can redistribute it and/or modify it
     under the terms of the GNU Affero General Public License as published
     by the Free Software Foundation, either version 3 of the License,
     or (at your option) any later version.
 
     GNUnet 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
     Affero General Public License for more details.
 
     You should have received a copy of the GNU Affero General Public License
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
     SPDX-License-Identifier: AGPL3.0-or-later
 */
 
#include "platform.h"
#include <gcrypt.h>
 
 
#include "gnunet_util_lib.h"
 
#define LOG(kind, ...) GNUNET_log_from (kind, "util-crypto-kdf", __VA_ARGS__)
 
 
enum GNUNET_GenericReturnValue
GNUNET_CRYPTO_kdf_v (void *result,
                     size_t out_len,
                     const void *xts,
                     size_t xts_len,
                     const void *skm,
                     size_t skm_len,
                     va_list argp)
{
  /*
   * "Finally, we point out to a particularly advantageous instantiation using
   * HMAC-SHA512 as XTR and HMAC-SHA256 in PRF* (in which case the output from SHA-512 is
   * truncated to 256 bits). This makes sense in two ways: First, the extraction part is where we need a
   * stronger hash function due to the unconventional demand from the hash function in the extraction
   * setting. Second, as shown in Section 6, using HMAC with a truncated output as an extractor
   * allows to prove the security of HKDF under considerably weaker assumptions on the underlying
   * hash function."
   *
   * http://eprint.iacr.org/2010/264
   */
  return GNUNET_CRYPTO_hkdf_v (result,
                               out_len,
                               GCRY_MD_SHA512,
                               GCRY_MD_SHA256,
                               xts,
                               xts_len,
                               skm,
                               skm_len,
                               argp);
}
 
 
enum GNUNET_GenericReturnValue
GNUNET_CRYPTO_kdf (void *result,
                   size_t out_len,
                   const void *xts,
                   size_t xts_len,
                   const void *skm,
                   size_t skm_len, ...)
{
  va_list argp;
  int ret;
 
  va_start (argp, skm_len);
  ret = GNUNET_CRYPTO_kdf_v (result,
                             out_len,
                             xts,
                             xts_len,
                             skm,
                             skm_len,
                             argp);
  va_end (argp);
 
  return ret;
}
 
 
void
GNUNET_CRYPTO_kdf_mod_mpi (gcry_mpi_t *r,
                           gcry_mpi_t n,
                           const void *xts, size_t xts_len,
                           const void *skm, size_t skm_len,
                           const char *ctx)
{
  gcry_error_t rc;
  unsigned int nbits;
  size_t rsize;
  uint16_t ctr;
 
  nbits = gcry_mpi_get_nbits (n);
  /* GNUNET_assert (nbits > 512); */
  ctr = 0;
  while (1)
  {
    /* Ain't clear if n is always divisible by 8 */
    size_t bsize = (nbits - 1) / 8 + 1;
    uint8_t buf[bsize];
    uint16_t ctr_nbo = htons (ctr);
 
    rc = GNUNET_CRYPTO_kdf (buf,
                            bsize,
                            xts, xts_len,
                            skm, skm_len,
                            ctx, strlen (ctx),
                            &ctr_nbo, sizeof(ctr_nbo),
                            NULL, 0);
    GNUNET_assert (GNUNET_YES == rc);
    rc = gcry_mpi_scan (r,
                        GCRYMPI_FMT_USG,
                        (const unsigned char *) buf,
                        bsize,
                        &rsize);
    GNUNET_assert (GPG_ERR_NO_ERROR == rc);  /* Allocation error? */
    GNUNET_assert (rsize == bsize);
    gcry_mpi_clear_highbit (*r,
                            nbits);
    GNUNET_assert (0 ==
                   gcry_mpi_test_bit (*r,
                                      nbits));
    ++ctr;
    /* We reject this FDH if either *r > n and retry with another ctr */
    if (0 > gcry_mpi_cmp (*r, n))
      break;
    gcry_mpi_release (*r);
  }
}
 
 
/* end of crypto_kdf.c */