Fast but nonstandard secret sharing system.

[u/mdw/catacomb] / gfshare.h

/* -*-c-*-
 *
 * $Id: gfshare.h,v 1.1 2000/06/17 10:56:30 mdw Exp $
 *
 * Secret sharing over %$\gf(2^8)$%
 *
 * (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.
 */

/*----- Revision history --------------------------------------------------* 
 *
 * $Log: gfshare.h,v $
 * Revision 1.1  2000/06/17 10:56:30  mdw
 * Fast but nonstandard secret sharing system.
 *
 */

#ifndef CATACOMB_GFSHARE_H
#define CATACOMB_GFSHARE_H

#ifdef __cplusplus
  extern "C" {
#endif

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

#include <mLib/bits.h>

#ifndef CATACOMB_GRAND_H
#  include "grand.h"
#endif

/*----- Data structures ---------------------------------------------------*/

/* --- A secret sharing context --- */

typedef struct gfshare_pt {
  octet x;				/* %$x$%-coordinate of the share */
  octet *y;				/* Pointer to share payload */
} gfshare_pt;

typedef struct gfshare {
  unsigned t;				/* Threshold */
  unsigned n;				/* The number of shares to make */
  unsigned i;				/* Next free slot in vector */
  size_t sz;				/* Size of the secret and shares */
  octet *s;				/* The secret */
  gfshare_pt *v;			/* Vector of share information */
} gfshare;

#define GFSHARE_INIT(t, n, sz) { t, n, 0, sz, 0, 0 }

/*----- Functions provided ------------------------------------------------*/

/* --- @gfshare_create@ --- *
 *
 * Arguments:	@gfshare *s@ = pointer to share context to initialize
 *		@unsigned t, n@ = threshold parameters for the system
 *		@size_t sz@ = size of the secret
 *
 * Returns:	---
 *
 * Use:		Initializes a sharing context.
 */

extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, unsigned /*n*/,
			   size_t /*sz*/);

/* --- @gfshare_destroy@ --- *
 *
 * Arguments:	@gfshare *s@ = pointer to share context to destroy
 *
 * Returns:	---
 *
 * Use:		Disposes of a sharing context.  The allocations for the
 *		individual shares and the vector @v@ are freed; the secret is
 *		left alone.
 */

extern void gfshare_destroy(gfshare */*s*/);

/* --- @gfshare_mkshares@ --- *
 *
 * Arguments:	@gfshare *s@ = pointer to share context to fill in
 *		@grand *r@ = pointer to random number source
 *
 * Returns:	---
 *
 * Use:		Generates @c->n@ secret shares, such that any @c->t@ of them
 *		may be used to recover the secret.
 *
 *		The context structure is expected to be mostly filled in.  In
 *		particular, @t@, @n@, @ssz@ and @s@ must be initialized.  If
 *		@v@ is zero, a vector of appropriate size is allocated.  You
 *		should use the macro @GFSHARE_INIT@ or @gfshare_create@ to
 *		construct sharing contexts.
 */

extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/);

/* --- @gfshare_add@ --- *
 *
 * Arguments:	@gfshare *s@ = pointer to sharing context
 *		@unsigned x@ = which share number this is
 *		@const octet *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 threshold @t@.
 */

extern unsigned gfshare_add(gfshare */*s*/,
			    unsigned /*x*/, const octet */*y*/);

/* --- @gfshare_combine@ --- *
 *
 * Arguments:	@gfshare *s@ = pointer to share context
 *		@octet *buf@ = pointer to output buffer for the secret
 *
 * Returns:	---
 *
 * Use:		Reconstructs a secret, given enough shares.
 */

extern void gfshare_combine(gfshare */*s*/, octet */*buf*/);

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

#ifdef __cplusplus
  }
#endif

#endif