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[u/mdw/catacomb] / share.c

/* -*-c-*-
 *
 * $Id: share.c,v 1.7 2004/04/08 01:36:15 mdw Exp $
 *
 * Shamir's secret sharing
 *
 * (c) 2000 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------* 
 *
 * This file is part of Catacomb.
 *
 * Catacomb is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 * 
 * Catacomb 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 Library General Public License for more details.
 * 
 * You should have received a copy of the GNU Library General Public
 * License along with Catacomb; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "grand.h"
#include "mp.h"
#include "mpint.h"
#include "mpbarrett.h"
#include "mprand.h"
#include "pgen.h"
#include "rabin.h"
#include "share.h"

/*----- Main code ---------------------------------------------------------*/

/* --- @share_create@ --- *
 *
 * Arguments:	@share *s@ = pointer to share context to initialize
 *		@unsigned t@ = threshold for the system
 *
 * Returns:	---
 *
 * Use:		Initializes a sharing context.
 */

void share_create(share *s, unsigned t)
{
  s->t = t;
  s->i = 0;
  s->p = 0;
  s->v = 0;
}

/* --- @share_destroy@ --- *
 *
 * Arguments:	@share *s@ = pointer to share context to destroy
 *
 * Returns:	---
 *
 * Use:		Disposes of a sharing context.  All memory is freed, all
 *		integers are dropped.
 */

void share_destroy(share *s)
{
  unsigned i;

  /* --- Dispose of the share vector --- */

  if (s->v) {
    for (i = 0; i < s->t; i++)
      mp_drop(s->v[i].y);
    xfree(s->v);
  }

  /* --- Other stuff --- */

  mp_drop(s->p);
}

/* --- @share_mkshares@ --- *
 *
 * Arguments:	@share *s@ = pointer to share context to fill in
 *		@grand *r@ = pointer to random number source
 *		@mp *n@ = the secret to share
 *
 * Returns:	---
 *
 * Use:		Initializes a sharing context to be able to create shares.
 *		The context structure is expected to be mostly filled in.  In
 *		particular, @t@ must be initialized.  If @p@ is	zero, a prime
 *		number of appropriate size is generated automatically.  If
 *		@v@ is zero, a vector of appropriate size is allocated.  You
 *		should use the macro @SHARE_INIT@ or @share_create@ to
 *		construct sharing contexts.
 */

void share_mkshares(share *s, grand *r, mp *n)
{
  unsigned i;

  /* --- If there's no prime, construct one --- */

  if (!s->p) {
    pgen_filterctx pf;
    rabin pr;
    mp *p;
    unsigned bits = (mp_octets(n) + 1) * 8;

    pf.step = 2;
    p = mprand(MP_NEW, bits, r, 1);
    s->p = pgen("p", p, p, 0, 0, 0, pgen_filter, &pf,
		rabin_iters(bits), pgen_test, &pr);
  }

  /* --- Construct the polynomial --- */

  if (!s->v)
    s->v = xmalloc(s->t * sizeof(share_pt));
  for (i = 0; i < s->t - 1; i++)
    s->v[i].y = mprand_range(MP_NEWSEC, s->p, r, 0);
  s->v[s->t - 1].y = mp_copy(n);
}

/* --- @share_get@ --- *
 *
 * Arguments:	@share *s@ = pointer to share conext
 *		@mp *d@ = destination for the share
 *		@unsigned x@ = share index to fetch
 *
 * Returns:	The share, as requested.
 *
 * Use:		Extracts a share from the system.  You may extract @MPW_MAX@
 *		shares, or @s->p@ shares from the system, whichever is
 *		smaller.  Shares are indexed from 0.
 */

mp *share_get(share *s, mp *d, unsigned x)
{
  mpbarrett mb;
  mpw uw = x + 1;
  mp u;
  unsigned i;

  /* --- Various bits of initialization --- */

  mp_build(&u, &uw, &uw + 1);
  mp_drop(d);

  /* --- Evaluate the polynomial at %$x = i + 1$% --- */

  d = MP_ZERO;
  mpbarrett_create(&mb, s->p);
  for (i = 0; i < s->t; i++) {
    d = mp_mul(d, d, &u);
    d = mp_add(d, d, s->v[i].y);
    d = mpbarrett_reduce(&mb, d, d);
  }
  mpbarrett_destroy(&mb);

  return (d);
}

/* --- @share_add@ --- *
 *
 * Arguments:	@share *s@ = pointer to sharing context
 *		@unsigned x@ = which share number this is
 *		@mp *y@ = the share value
 *
 * Returns:	Number of shares required before recovery may be performed.
 *
 * Use:		Adds a share to the context.  The context must have been
 *		initialized with the correct prime @p@ and threshold @t@.
 */

unsigned share_add(share *s, unsigned x, mp *y)
{
  /* --- If no vector has been allocated, create one --- */

  if (!s->v) {
    unsigned i;
    s->v = xmalloc(s->t * sizeof(share_pt));
    s->i = 0;
    for (i = 0; i < s->t; i++)
      s->v[i].y = 0;
  }

  assert(((void)"Share context is full", s->i < s->t));

  /* --- Store the share in the vector --- */

  s->v[s->i].x = x + 1;
  s->v[s->i].y = mp_copy(y);
  s->i++;

  /* --- Done --- */

  return (s->t - s->i);
}

/* --- @share_combine@ --- *
 *
 * Arguments:	@share *s@ = pointer to share context
 *
 * Returns:	The secret, as a multiprecision integer.
 *
 * Use:		Reconstructs a secret, given enough shares.
 */

mp *share_combine(share *s)
{
  mp *a = MP_ZERO;
  mpbarrett mb;
  unsigned i, j;
  mp ii, jj;
  mpw iiw, jjw;
  mp *m = MP_NEW;

  /* --- Sanity checking --- */

  assert(((void)"Not enough shares yet", s->i == s->t));

  /* --- Initialization --- */

  mpbarrett_create(&mb, s->p);
  mp_build(&ii, &iiw, &iiw + 1);
  mp_build(&jj, &jjw, &jjw + 1);

  /* --- Grind through the shares --- */

  for (i = 0; i < s->t; i++) {
    mp *c = MP_ONE;

    iiw = s->v[i].x;
    for (j = 0; j < s->t; j++) {
      if (i == j)
	continue;
      jjw = s->v[j].x;
      if (s->v[j].x >= s->v[i].x)
	m = mp_sub(m, &jj, &ii);
      else {
	m = mp_sub(m, &ii, &jj);
	m = mp_sub(m, s->p, m);
      }
      m = mp_modinv(m, m, s->p);
      c = mp_mul(c, c, &jj);
      c = mpbarrett_reduce(&mb, c, c);
      c = mp_mul(c, c, m);
      c = mpbarrett_reduce(&mb, c, c);
    }
    c = mp_mul(c, c, s->v[i].y);
    c = mpbarrett_reduce(&mb, c, c);
    a = mp_add(a, a, c);
    mp_drop(c);
  }

  a = mpbarrett_reduce(&mb, a, a);
  mp_drop(m);
  mpbarrett_destroy(&mb);
  return (a);
}

/*----- Test rig ----------------------------------------------------------*/

#ifdef TEST_RIG

#include "fibrand.h"

static int verify(grand *r)
{
  unsigned n = r->ops->range(r, 16) + 8;
  unsigned t = r->ops->range(r, n - 1) + 1;
  unsigned len = r->ops->range(r, 160);

  mp **v = xmalloc(t * sizeof(mp *));
  unsigned *p = xmalloc(n * sizeof(unsigned));
  mp *sec = mprand(MP_NEW, len, r, 0);
  share s;
  mp *pp;
  mp *ss;
  unsigned i;

  int ok = 1;

  for (i = 0; i < n; i++)
    p[i] = i;
  for (i = 0; i < t; i++) {
    unsigned long j = r->ops->range(r, n - i);
    unsigned x = p[i];
    p[i] = p[i + j];
    p[i + j] = x;
  }

  share_create(&s, t);
  share_mkshares(&s, r, sec);
  for (i = 0; i < t; i++)
    v[i] = share_get(&s, MP_NEW, p[i]);
  pp = mp_copy(s.p);
  share_destroy(&s);
  assert(mparena_count(MPARENA_GLOBAL) + mparena_count(MPARENA_SECURE) == t + 2);

  share_create(&s, t);
  s.p = pp;
  for (i = 0; i < t; i++)
    share_add(&s, p[i], v[i]);
  ss = share_combine(&s);
  share_destroy(&s);

  if (!MP_EQ(sec, ss)) {
    ok = 0;
    fprintf(stderr, "\nbad recombination of shares\n");
  };

  mp_drop(sec);
  mp_drop(ss);

  for (i = 0; i < t; i++)
    mp_drop(v[i]);

  xfree(v);
  xfree(p);

  assert(mparena_count(MPARENA_GLOBAL) + mparena_count(MPARENA_SECURE) == 0);
  return (ok);
}

int main(void)
{
  grand *r = fibrand_create(0);
  unsigned i;
  int ok = 1;

  fputs("share: ", stdout);
  for (i = 0; i < 40; i++) {
    if (!verify(r))
      ok = 0;
    fputc('.', stdout);
    fflush(stdout);
  }

  if (ok)
    fputs(" ok\n", stdout);
  else
    fputs(" failed\n", stdout);
  return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
}

#endif

/*----- That's all, folks -------------------------------------------------*/