libbrandt/crypto.c

/* This file is part of libbrandt.
 * Copyright (C) 2016 GNUnet e.V.
 *
 * libbrandt 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.
 *
 * libbrandt 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
 * libbrandt.  If not, see <http://www.gnu.org/licenses/>.
 */

/**
 * @file crypto.c
 * @brief Implementation of the crypto primitives.
 */

#include <arpa/inet.h>

#include "crypto.h"
#include "util.h"

#define CURVE "Ed25519"

/* --- RANDOM --- */

void
brandt_rand_poll ()
{
	static unsigned char rand_amount = 255;

	if (!(rand_amount--))
		gcry_fast_random_poll ();
}

/* --- HASHING --- */

/**
 * Hash block of given size.
 *
 * @param block the data to #brandt_hash, length is given as a second argument
 * @param size the length of the data to #brandt_hash in @a block
 * @param ret pointer to where to write the hashcode
 */
void
brandt_hash (const void *block, size_t size, struct brandt_hash_code *ret)
{
	gcry_md_hash_buffer (GCRY_MD_SHA512, ret, block, size);
}

/* --- MPI --- */

/**
 * If target != size, move @a target bytes to the end of the size-sized
 * buffer and zero out the first @a target - @a size bytes.
 *
 * @param buf original buffer
 * @param size number of bytes in @a buf
 * @param target target size of the buffer
 */
static void
adjust (void *buf, size_t size, size_t target)
{
	char *p = buf;

	if (size < target)
	{
		memmove (&p[target - size], buf, size);
		memset (buf, 0, target - size);
	}
}

/**
 * Output the given MPI value to the given buffer in
 * network byte order.
 * The MPI @a val may not be negative.
 *
 * @param buf where to output to
 * @param size number of bytes in @a buf
 * @param val value to write to @a buf
 */
void
brandt_mpi_print_unsigned (void *buf, size_t size, gcry_mpi_t val)
{
	size_t       rsize;
	gcry_error_t rc;

	if (gcry_mpi_get_flag (val, GCRYMPI_FLAG_OPAQUE))
	{
		/* Store opaque MPIs left aligned into the buffer.  */
		unsigned int nbits;
		const void   *p;

		p = gcry_mpi_get_opaque (val, &nbits);
		brandt_assert (NULL != p);
		rsize = (nbits + 7) / 8;
		if (rsize > size)
			rsize = size;
		memcpy (buf, p, rsize);
		if (rsize < size)
			memset (((char *)buf) + rsize, 0, size - rsize);
	}
	else
	{
		/* Store regular MPIs as unsigned integers right aligned into the buffer. */
		rsize = size;
		rc = gcry_mpi_print (GCRYMPI_FMT_USG, buf, rsize, &rsize, val);
		brandt_assert_gpgerr (rc);
		adjust (buf, rsize, size);
	}
}

/**
 * Convert data buffer into MPI value.
 * The buffer is interpreted as network
 * byte order, unsigned integer.
 *
 * @param result where to store MPI value (allocated)
 * @param data raw data (GCRYMPI_FMT_USG)
 * @param size number of bytes in @a data
 */
void
brandt_mpi_scan_unsigned (gcry_mpi_t *result, const void *data, size_t size)
{
	gcry_error_t rc;

	rc = gcry_mpi_scan (result, GCRYMPI_FMT_USG, data, size, &size);
	brandt_assert_gpgerr (rc);
}

/* --- ECDHE --- */

/**
 * Convert the given private key from the network format to the
 * S-expression that can be used by libgcrypt.
 *
 * @param priv private key to decode
 * @return NULL on error
 */
static gcry_sexp_t
decode_private_ecdhe_key (const struct brandt_dhe_skey *priv)
{
	gcry_sexp_t  result;
	gcry_error_t rc;

	rc = gcry_sexp_build (&result, NULL,
	                      "(private-key(ecc(curve \"" CURVE "\")"
	                      "(d %b)))",
	                      (int)sizeof (priv->d), priv->d);
	brandt_assert_gpgerr (rc);
	return result;
}

/**
 * Extract values from an S-expression.
 *
 * @param array where to store the result(s)
 * @param sexp S-expression to parse
 * @param topname top-level name in the S-expression that is of interest
 * @param elems names of the elements to extract
 * @return 0 on success
 */
static int
key_from_sexp (gcry_mpi_t *array, gcry_sexp_t sexp, const char *topname,
               const char *elems)
{
	gcry_sexp_t  list;
	gcry_sexp_t  l2;
	const char   *s;
	unsigned int i;
	unsigned int idx;

	list = gcry_sexp_find_token (sexp, topname, 0);
	if (!list)
		return 1;
	l2 = gcry_sexp_cadr (list);
	gcry_sexp_release (list);
	list = l2;
	if (!list)
		return 2;
	idx = 0;
	for (s = elems; *s; s++, idx++)
	{
		l2 = gcry_sexp_find_token (list, s, 1);
		if (!l2)
		{
			for (i = 0; i < idx; i++)
			{
				gcry_free (array[i]);
				array[i] = NULL;
			}
			gcry_sexp_release (list);
			return 3;                 /* required parameter not found */
		}
		array[idx] = gcry_sexp_nth_mpi (l2, 1, GCRYMPI_FMT_USG);
		gcry_sexp_release (l2);
		if (!array[idx])
		{
			for (i = 0; i < idx; i++)
			{
				gcry_free (array[i]);
				array[i] = NULL;
			}
			gcry_sexp_release (list);
			return 4;                 /* required parameter is invalid */
		}
	}
	gcry_sexp_release (list);
	return 0;
}

/**
 * Create a new private key.
 *
 * @param priv where to write the private key
 */
void
brandt_ecdhe_key_create (struct brandt_dhe_skey *priv)
{
	gcry_sexp_t  priv_sexp;
	gcry_sexp_t  s_keyparam;
	gcry_mpi_t   d;
	gcry_error_t rc;

	rc = gcry_sexp_build (&s_keyparam, NULL, "(genkey(ecc(curve \"" CURVE "\")"
	                      "(flags)))")
	     brandt_assert_gpgerr (rc);
	rc = gcry_pk_genkey (&priv_sexp, s_keyparam)
	     brandt_assert_gpgerr (rc);
	gcry_sexp_release (s_keyparam);
	rc = key_from_sexp (&d, priv_sexp, "private-key", "d")
	     brandt_assert_gpgerr (rc);
	gcry_sexp_release (priv_sexp);
	brandt_mpi_print_unsigned (priv->d, sizeof (priv->d), d);
	gcry_mpi_release (d);
}

/**
 * Extract the public key for the given private key.
 *
 * @param priv the private key
 * @param pub where to write the public key
 */
void
brandt_ecdhe_key_get_public (const struct brandt_dhe_skey *priv,
                             struct brandt_dhe_pkey       *pub)
{
	gcry_sexp_t  sexp;
	gcry_ctx_t   ctx;
	gcry_mpi_t   q;
	gcry_error_t rc;

	sexp = decode_private_ecdhe_key (priv);
	brandt_assert (NULL != sexp);
	rc = gcry_mpi_ec_new (&ctx, sexp, NULL);
	brandt_assert_gpgerr (rc);
	gcry_sexp_release (sexp);
	q = gcry_mpi_ec_get_mpi ("q@eddsa", ctx, 0);
	brandt_assert (NULL != q);
	brandt_mpi_print_unsigned (pub->q_y, sizeof (pub->q_y), q);
	gcry_mpi_release (q);
	gcry_ctx_release (ctx);
}

/**
 * Derive key material from a public and a private ECDHE key.
 *
 * @param priv private key to use for the ECDH (x)
 * @param pub public key to use for the ECDH (yG)
 * @param key_material where to write the key material (xyG)
 * @return 0 on error, 1 on success
 */
int
brandt_ecdhe (const struct brandt_dhe_skey *priv,
              const struct brandt_dhe_pkey *pub,
              struct brandt_hash_code      *key_material)
{
	gcry_error_t     rc;
	int              rc2;
	gcry_mpi_point_t result;
	gcry_mpi_point_t q;
	gcry_mpi_t       d;
	gcry_ctx_t       ctx;
	gcry_sexp_t      pub_sexpr;
	gcry_mpi_t       result_x;
	unsigned char    xbuf[256 / 8];
	size_t           rsize;

	/* first, extract the q = dP value from the public key */
	if (0 != gcry_sexp_build (&pub_sexpr, NULL,
	                          "(public-key(ecc(curve " CURVE ")(q %b)))",
	                          (int)sizeof (pub->q_y), pub->q_y))
		return 0;
	rc = gcry_mpi_ec_new (&ctx, pub_sexpr, NULL);
	brandt_assert_gpgerr (rc);
	gcry_sexp_release (pub_sexpr);
	q = gcry_mpi_ec_get_point ("q", ctx, 0);

	/* second, extract the d value from our private key */
	brandt_mpi_scan_unsigned (&d, priv->d, sizeof (priv->d));

	/* then call the 'multiply' function, to compute the product */
	result = gcry_mpi_point_new (0);
	gcry_mpi_ec_mul (result, d, q, ctx);
	gcry_mpi_point_release (q);
	gcry_mpi_release (d);

	/* finally, convert point to string for hashing */
	result_x = gcry_mpi_new (256);
	rc = gcry_mpi_ec_get_affine (result_x, NULL, result, ctx);
	brandt_assert (0 == rc);
	gcry_mpi_point_release (result);
	gcry_ctx_release (ctx);

	rsize = sizeof (xbuf);
	rc2 = gcry_mpi_get_flag (result_x, GCRYMPI_FLAG_OPAQUE);
	brandt_assert (0 == rc2);
	/* result_x can be negative here, so we do not use 'brandt_mpi_print_unsigned'
	 * as that does not include the sign bit; x should be a 255-bit
	 * value, so with the sign it should fit snugly into the 256-bit
	 * xbuf */
	rc = gcry_mpi_print (GCRYMPI_FMT_STD, xbuf, rsize, &rsize, result_x);
	brandt_assert_gpgerr (rc);
	brandt_hash (xbuf, rsize, key_material);
	gcry_mpi_release (result_x);
	return 1;
}

/**
 * @ingroup crypto
 * Clear memory that was used to store a private key.
 *
 * @param pk location of the key
 */
void
brandt_ecdhe_key_clear (struct brandt_dhe_skey *pk)
{
	memset (pk, 0, sizeof (struct brandt_dhe_skey));
}