git.distorted.org.uk Git - u/mdw/catacomb/blob - misc/gfshare.h

   1 /* -*-c-*-
   2  *
   3  * Secret sharing over %$\gf{2^8}$%
   4  *
   5  * (c) 2000 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Notes on the system -----------------------------------------------*
  29  *
  30  * This uses a variant of Shamir's secret sharing system.  Shamir's original
  31  * system used polynomials modulo a large prime.  This implementation instead
  32  * uses the field %$\gf{2^8}$%, represented by
  33  *
  34  *   %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
  35  *
  36  * and shares each byte of the secret independently.  It is therefore limited
  37  * to 255 players, although this probably isn't a serious limitation in
  38  * practice.
  39  *
  40  * Share creation and reconstruction is extremely efficient.  Contrast the
  41  * performance of the straightforward implementation based on multiprecision
  42  * arithmetic.
  43  */
  44
  45 #ifndef CATACOMB_GFSHARE_H
  46 #define CATACOMB_GFSHARE_H
  47
  48 #ifdef __cplusplus
  49   extern "C" {
  50 #endif
  51
  52 /*----- Header files ------------------------------------------------------*/
  53
  54 #include <mLib/bits.h>
  55
  56 #ifndef CATACOMB_GRAND_H
  57 #  include "grand.h"
  58 #endif
  59
  60 /*----- Data structures ---------------------------------------------------*/
  61
  62 /* --- A secret sharing context --- */
  63
  64 typedef struct gfshare {
  65   unsigned t;                           /* Threshold */
  66   unsigned i;                           /* Next free slot in vector */
  67   size_t sz;                            /* Size of the secret and shares */
  68   octet *v;                             /* Vector of share information */
  69 } gfshare;
  70
  71 #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 }
  72
  73 #define GFSHARE_INDEX(s, i) ((s)->v[(i) * ((s)->sz + 1)])
  74
  75 /*----- Functions provided ------------------------------------------------*/
  76
  77 /* --- @gfshare_create@ --- *
  78  *
  79  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  80  *              @unsigned t@ = threshold for the system
  81  *              @size_t sz@ = size of the secret
  82  *
  83  * Returns:     ---
  84  *
  85  * Use:         Initializes a sharing context.
  86  */
  87
  88 extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/);
  89
  90 /* --- @gfshare_destroy@ --- *
  91  *
  92  * Arguments:   @gfshare *s@ = pointer to share context to destroy
  93  *
  94  * Returns:     ---
  95  *
  96  * Use:         Disposes of a sharing context.  The allocations for the
  97  *              individual shares and the vector @v@ are freed; the secret is
  98  *              left alone.
  99  */
 100
 101 extern void gfshare_destroy(gfshare */*s*/);
 102
 103 /* --- @gfshare_mkshares@ --- *
 104  *
 105  * Arguments:   @gfshare *s@ = pointer to share context to fill in
 106  *              @grand *r@ = pointer to random number source
 107  *              @const void *buf@ = pointer to the secret to share
 108  *
 109  * Returns:     ---
 110  *
 111  * Use:         Initializes a sharing context to be able to create shares.
 112  *              The context structure is expected to be mostly filled in.  In
 113  *              particular, @t@ must be initialized.  If @v@ is zero, a
 114  *              vector of appropriate size is allocated.  You should use the
 115  *              macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
 116  *              contexts.
 117  */
 118
 119 extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/,
 120                              const void */*buf*/);
 121
 122 /* --- @gfshare_get@ --- *
 123  *
 124  * Arguments:   @gfshare *s@ = pointer to share conext
 125  *              @unsigned x@ = share index to fetch
 126  *              @void *buf@ = pointer to output buffer
 127  *
 128  * Returns:     ---
 129  *
 130  * Use:         Extracts a share from the system.  You may extract up to 255
 131  *              shares from the system.  Shares are indexed from 0.
 132  */
 133
 134 extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/);
 135
 136 /* --- @gfshare_addedp@ --- *
 137  *
 138  * Arguments:   @gfshare *s@ = pointer to sharing context
 139  *              @unsigned x@ = which share number to check
 140  *
 141  * Returns:     Nonzero if share @x@ has been added already, zero if it
 142  *              hasn't.
 143  */
 144
 145 extern int gfshare_addedp(gfshare */*s*/, unsigned /*x*/);
 146
 147 /* --- @gfshare_add@ --- *
 148  *
 149  * Arguments:   @gfshare *s@ = pointer to sharing context
 150  *              @unsigned x@ = which share number this is
 151  *              @const void *y@ = the share value
 152  *
 153  * Returns:     Number of shares required before recovery may be performed.
 154  *
 155  * Use:         Adds a share to the context.  The context must have been
 156  *              initialized with the correct threshold @t@.
 157  */
 158
 159 extern unsigned gfshare_add(gfshare */*s*/,
 160                             unsigned /*x*/, const void */*y*/);
 161
 162 /* --- @gfshare_combine@ --- *
 163  *
 164  * Arguments:   @gfshare *s@ = pointer to share context
 165  *              @void *buf@ = pointer to output buffer for the secret
 166  *
 167  * Returns:     ---
 168  *
 169  * Use:         Reconstructs a secret, given enough shares.
 170  */
 171
 172 extern void gfshare_combine(gfshare */*s*/, void */*buf*/);
 173
 174 /*----- That's all, folks -------------------------------------------------*/
 175
 176 #ifdef __cplusplus
 177   }
 178 #endif
 179
 180 #endif