libbrandt/mp_priv.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 mp_priv.c
 * @brief Implementation of the m+1st price private outcome algorithm.
 * @author Markus Teich
 */

#include "platform.h"

#include <gcrypt.h>

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


void
mp_priv_prep_outcome (struct BRANDT_Auction *ad)
{
	gcry_mpi_t       factor = gcry_mpi_new (256);
	gcry_mpi_point_t subtr = gcry_mpi_point_new (0);
	gcry_mpi_point_t tmpa = gcry_mpi_point_new (0);
	gcry_mpi_point_t tmpb = gcry_mpi_point_new (0);
	gcry_mpi_point_t *tlta1;
	gcry_mpi_point_t *tltb1;
	gcry_mpi_point_t **tlta2;
	gcry_mpi_point_t **tltb2;
	gcry_mpi_point_t **tlta3;
	gcry_mpi_point_t **tltb3;

	ad->gamma3 = smc_init3 (ad->n, ad->n, ad->k);
	brandt_assert (ad->gamma3);

	ad->delta3 = smc_init3 (ad->n, ad->n, ad->k);
	brandt_assert (ad->delta3);

	/* create temporary lookup tables with partial sums */
	tlta1 = smc_init1 (ad->k);
	tltb1 = smc_init1 (ad->k);
	tlta2 = smc_init2 (ad->n, ad->k);
	tltb2 = smc_init2 (ad->n, ad->k);
	tlta3 = smc_init2 (ad->n, ad->k);
	tltb3 = smc_init2 (ad->n, ad->k);

	/* temporary lookup table for first summand (building ladder of bids) */
	for (uint16_t i = 0; i < ad->n; i++)
	{
		smc_sums_partial (tlta3[i], ad->alpha[i], ad->k, 1, 1);
		smc_sums_partial (tltb3[i], ad->beta[i], ad->k, 1, 1);
		for (uint16_t j = 0; j < ad->k; j++)
		{
			gcry_mpi_ec_sub (tlta2[i][j],
			                 tlta3[i][ad->k - 1],
			                 tlta3[i][j],
			                 ec_ctx);
			gcry_mpi_ec_sub (tltb2[i][j],
			                 tltb3[i][ad->k - 1],
			                 tltb3[i][j],
			                 ec_ctx);
		}
		brandt_assert (!ec_point_cmp (ec_zero, tlta2[i][ad->k - 1]));
		brandt_assert (!ec_point_cmp (ec_zero, tltb2[i][ad->k - 1]));
	}
	for (uint16_t j = 0; j < ad->k; j++)
	{
		/* 2L - 2I */
		smc_sum (tmpa, &tlta2[0][j], ad->n, ad->k);
		smc_sum (tmpb, &tltb2[0][j], ad->n, ad->k);
		gcry_mpi_ec_mul (tlta1[j], GCRYMPI_CONST_TWO, tmpa, ec_ctx);
		gcry_mpi_ec_mul (tltb1[j], GCRYMPI_CONST_TWO, tmpb, ec_ctx);

		/* I */
		smc_sum (tmpa, &ad->alpha[0][j], ad->n, ad->k);
		smc_sum (tmpb, &ad->beta[0][j], ad->n, ad->k);

		/* 2L - 2I + I = 2L - I */
		gcry_mpi_ec_add (tlta1[j], tlta1[j], tmpa, ec_ctx);
		gcry_mpi_ec_add (tltb1[j], tltb1[j], tmpb, ec_ctx);
	}
	/* This check only works directly after the loop when tmpa/tmpb are still
	 * the sum of the last row */
	brandt_assert (!ec_point_cmp (tmpa, tlta1[ad->k - 1]));
	brandt_assert (!ec_point_cmp (tmpb, tltb1[ad->k - 1]));

	/* temporary lookup table for second summand (hide outcome from losers) */
	gcry_mpi_set_ui (factor, ad->m);
	gcry_mpi_lshift (factor, factor, 1);
	gcry_mpi_add_ui (factor, factor, 2);
	for (uint16_t i = 0; i < ad->n; i++)
	{
		for (uint16_t j = 0; j < ad->k; j++)
		{
			gcry_mpi_ec_mul (tlta2[i][j], factor, tlta3[i][j], ec_ctx);
			gcry_mpi_ec_mul (tltb2[i][j], factor, tltb3[i][j], ec_ctx);
		}
	}

	/* temporary lookup table for subtrahend (getting M+1st highest bid) */
	gcry_mpi_sub_ui (factor, factor, 1);
	gcry_mpi_ec_mul (subtr, factor, ec_gen, ec_ctx);

	/* compute gamma and delta */
	for (uint16_t i = 0; i < ad->n; i++)
	{
		for (uint16_t j = 0; j < ad->k; j++)
		{
			/* compute inner gamma */
			gcry_mpi_ec_add (tmpa, tlta1[j], tlta2[i][j], ec_ctx);
			gcry_mpi_ec_sub (tmpa, tmpa, subtr, ec_ctx);

			/* compute inner delta */
			gcry_mpi_ec_add (tmpb, tltb1[j], tltb2[i][j], ec_ctx);

			/* copy unmasked outcome to all other bidder layers so they don't
			 * have to be recomputed to check the ZK proof_2dle's from other
			 * bidders when receiving their outcome messages */
			for (uint16_t a = 0; a < ad->n; a++)
			{
				ec_point_copy (ad->gamma3[a][i][j], tmpa);
				ec_point_copy (ad->delta3[a][i][j], tmpb);
			}
		}
	}

	gcry_mpi_release (factor);
	gcry_mpi_point_release (subtr);
	gcry_mpi_point_release (tmpa);
	gcry_mpi_point_release (tmpb);
	smc_free1 (tlta1, ad->k);
	smc_free1 (tltb1, ad->k);
	smc_free2 (tlta2, ad->n, ad->k);
	smc_free2 (tltb2, ad->n, ad->k);
	smc_free2 (tlta3, ad->n, ad->k);
	smc_free2 (tltb3, ad->n, ad->k);
}


struct BRANDT_Result *
mp_priv_determine_outcome (struct BRANDT_Auction *ad,
                           uint16_t              *len)
{
	struct BRANDT_Result *ret;
	int32_t              price = -1;
	uint16_t             winners = 0;
	uint16_t             max_winners;
	gcry_mpi_point_t     sum_gamma = gcry_mpi_point_new (0);
	gcry_mpi_point_t     sum_phi = gcry_mpi_point_new (0);

	brandt_assert (ad);

	max_winners = ad->seller_mode ? ad->m : 1;
	ret = GNUNET_new_array (max_winners, struct BRANDT_Result);
	for (uint16_t i = 0; i < ad->n; i++)
	{
		if (!ad->seller_mode && i != ad->i)
			continue;

		for (uint16_t j = 0; j < ad->k; j++)
		{
			smc_sum (sum_gamma, &ad->gamma3[0][i][j], ad->n, ad->n * ad->k);
			smc_sum (sum_phi, &ad->phi3[0][i][j], ad->n, ad->n * ad->k);
			gcry_mpi_ec_sub (sum_gamma, sum_gamma, sum_phi, ec_ctx);
			if (!ec_point_cmp (sum_gamma, ec_zero))
			{
				if (winners >= max_winners)
				{
					weprintf ("too many winners detected");
					goto fail;
				}
				if (-1 != price && j != price)
				{
					weprintf ("multiple winning prices detected");
					goto fail;
				}
				price = j;

				ret[winners].bidder = i;
				ret[winners].price = j / ad->n;
				ret[winners].status = BRANDT_bidder_won;
				winners++;
			}
		}
	}

	gcry_mpi_point_release (sum_gamma);
	gcry_mpi_point_release (sum_phi);

	if (ad->m <= ad->n && winners < max_winners && -1 != price)
		weprintf ("too few winners detected");
	if (0 == winners)
		goto fail;

	if (len)
		*len = winners;
	return ret;

fail:
	GNUNET_free (ret);
	return NULL;
}