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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.
*/
/**
* @file src/util/crypto_kdf.c
* @brief Key derivation
* @author Nils Durner
* @author Jeffrey Burdges <burdges@gnunet.org>
*/
#include <gcrypt.h>
#include "platform.h"
#include "gnunet_crypto_lib.h"
#define LOG(kind,...) GNUNET_log_from (kind, "util-crypto-kdf", __VA_ARGS__)
/**
* @brief Derive key
* @param result buffer for the derived key, allocated by caller
* @param out_len desired length of the derived key
* @param xts salt
* @param xts_len length of @a xts
* @param skm source key material
* @param skm_len length of @a skm
* @param argp va_list of void * & size_t pairs for context chunks
* @return #GNUNET_YES on success
*/
int
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);
}
/**
* @brief Derive key
* @param result buffer for the derived key, allocated by caller
* @param out_len desired length of the derived key
* @param xts salt
* @param xts_len length of @a xts
* @param skm source key material
* @param skm_len length of @a skm
* @param ... void * & size_t pairs for context chunks
* @return #GNUNET_YES on success
*/
int
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;
}
/**
* Deterministically generate a pseudo-random number uniformly from the
* integers modulo a libgcrypt mpi.
*
* @param[out] r MPI value set to the FDH
* @param n MPI to work modulo
* @param xts salt
* @param xts_len length of @a xts
* @param skm source key material
* @param skm_len length of @a skm
* @param ctx context string
*/
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;
unsigned int 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 */
uint8_t buf[ (nbits-1)/8 + 1 ];
rc = GNUNET_CRYPTO_kdf (buf,
sizeof (buf),
xts, xts_len,
skm, skm_len,
ctx, strlen(ctx),
&ctr, sizeof(ctr),
NULL, 0);
GNUNET_assert (GNUNET_YES == rc);
rc = gcry_mpi_scan (r,
GCRYMPI_FMT_USG,
(const unsigned char *) buf,
sizeof (buf),
&rsize);
GNUNET_assert (0 == rc); /* Allocation erro? */
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 */
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