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   1 /* -*-c-*-
   2  *
   3  * $Id: gfshare.h,v 1.2 2000/06/17 11:05:27 mdw Exp $
   4  *
   5  * Secret sharing over %$\gf(2^8)$%
   6  *
   7  * (c) 2000 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  14  * Catacomb is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * Catacomb is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with Catacomb; if not, write to the Free
  26  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: gfshare.h,v $
  33  * Revision 1.2  2000/06/17 11:05:27  mdw
  34  * Add a commentary on the system.
  35  *
  36  * Revision 1.1  2000/06/17 10:56:30  mdw
  37  * Fast but nonstandard secret sharing system.
  38  *
  39  */
  40
  41 /*----- Notes on the system -----------------------------------------------*
  42  *
  43  * This uses a variant of Shamir's secret sharing system.  Shamir's original
  44  * system used polynomials modulo a large prime.  This implementation instead
  45  * uses the field %$\gf(2^8)$%, represented by
  46  *
  47  *   %$\gf(2)[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
  48  *
  49  * and shares each byte of the secret independently.  It is therefore limited
  50  * to 255 players, although this probably isn't a serious limitation in
  51  * practice.
  52  *
  53  * Share creation and reconstruction is extremely efficient.  Contrast the
  54  * performance of the straightforward implementation based on multiprecision
  55  * arithmetic.
  56  */
  57
  58 #ifndef CATACOMB_GFSHARE_H
  59 #define CATACOMB_GFSHARE_H
  60
  61 #ifdef __cplusplus
  62   extern "C" {
  63 #endif
  64
  65 /*----- Header files ------------------------------------------------------*/
  66
  67 #include <mLib/bits.h>
  68
  69 #ifndef CATACOMB_GRAND_H
  70 #  include "grand.h"
  71 #endif
  72
  73 /*----- Data structures ---------------------------------------------------*/
  74
  75 /* --- A secret sharing context --- */
  76
  77 typedef struct gfshare_pt {
  78   octet x;                              /* %$x$%-coordinate of the share */
  79   octet *y;                             /* Pointer to share payload */
  80 } gfshare_pt;
  81
  82 typedef struct gfshare {
  83   unsigned t;                           /* Threshold */
  84   unsigned n;                           /* The number of shares to make */
  85   unsigned i;                           /* Next free slot in vector */
  86   size_t sz;                            /* Size of the secret and shares */
  87   octet *s;                             /* The secret */
  88   gfshare_pt *v;                        /* Vector of share information */
  89 } gfshare;
  90
  91 #define GFSHARE_INIT(t, n, sz) { t, n, 0, sz, 0, 0 }
  92
  93 /*----- Functions provided ------------------------------------------------*/
  94
  95 /* --- @gfshare_create@ --- *
  96  *
  97  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  98  *              @unsigned t, n@ = threshold parameters for the system
  99  *              @size_t sz@ = size of the secret
 100  *
 101  * Returns:     ---
 102  *
 103  * Use:         Initializes a sharing context.
 104  */
 105
 106 extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, unsigned /*n*/,
 107                            size_t /*sz*/);
 108
 109 /* --- @gfshare_destroy@ --- *
 110  *
 111  * Arguments:   @gfshare *s@ = pointer to share context to destroy
 112  *
 113  * Returns:     ---
 114  *
 115  * Use:         Disposes of a sharing context.  The allocations for the
 116  *              individual shares and the vector @v@ are freed; the secret is
 117  *              left alone.
 118  */
 119
 120 extern void gfshare_destroy(gfshare */*s*/);
 121
 122 /* --- @gfshare_mkshares@ --- *
 123  *
 124  * Arguments:   @gfshare *s@ = pointer to share context to fill in
 125  *              @grand *r@ = pointer to random number source
 126  *
 127  * Returns:     ---
 128  *
 129  * Use:         Generates @c->n@ secret shares, such that any @c->t@ of them
 130  *              may be used to recover the secret.
 131  *
 132  *              The context structure is expected to be mostly filled in.  In
 133  *              particular, @t@, @n@, @ssz@ and @s@ must be initialized.  If
 134  *              @v@ is zero, a vector of appropriate size is allocated.  You
 135  *              should use the macro @GFSHARE_INIT@ or @gfshare_create@ to
 136  *              construct sharing contexts.
 137  */
 138
 139 extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/);
 140
 141 /* --- @gfshare_add@ --- *
 142  *
 143  * Arguments:   @gfshare *s@ = pointer to sharing context
 144  *              @unsigned x@ = which share number this is
 145  *              @const octet *y@ = the share value
 146  *
 147  * Returns:     Number of shares required before recovery may be performed.
 148  *
 149  * Use:         Adds a share to the context.  The context must have been
 150  *              initialized with the correct threshold @t@.
 151  */
 152
 153 extern unsigned gfshare_add(gfshare */*s*/,
 154                             unsigned /*x*/, const octet */*y*/);
 155
 156 /* --- @gfshare_combine@ --- *
 157  *
 158  * Arguments:   @gfshare *s@ = pointer to share context
 159  *              @octet *buf@ = pointer to output buffer for the secret
 160  *
 161  * Returns:     ---
 162  *
 163  * Use:         Reconstructs a secret, given enough shares.
 164  */
 165
 166 extern void gfshare_combine(gfshare */*s*/, octet */*buf*/);
 167
 168 /*----- That's all, folks -------------------------------------------------*/
 169
 170 #ifdef __cplusplus
 171   }
 172 #endif
 173
 174 #endif