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

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Shamir's secret sharing
   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 /*----- Notes on the sharing system ---------------------------------------*
  31  *
  32  * Shamir's secret-sharing system is based on polynomial interpolation modulo
  33  * a prime number.  It is `perfect' in that fewer participants than the
  34  * threshold can derive no information about the secret by pooling their
  35  * shares, and `ideal' in that the shares are the same size as the secret.
  36  *
  37  * This implementation stays close to the definition, in order to support
  38  * other schemes for (e.g.) threshold cryptography.  It is, however, rather
  39  * slow.
  40  */
  41
  42 #ifndef CATACOMB_SHARE_H
  43 #define CATACOMB_SHARE_H
  44
  45 #ifdef __cplusplus
  46   extern "C" {
  47 #endif
  48
  49 /*----- Header files ------------------------------------------------------*/
  50
  51 #ifndef CATACOMB_GRAND_H
  52 #  include "grand.h"
  53 #endif
  54
  55 #ifndef CATACOMB_MP_H
  56 #  include "mp.h"
  57 #endif
  58
  59 /*----- Data structures ---------------------------------------------------*/
  60
  61 /* --- A secret sharing context --- */
  62
  63 typedef struct share_pt {
  64   unsigned x;                           /* Index of this share */
  65   mp *y;                                /* Payload of this share */
  66 } share_pt;
  67
  68 typedef struct share {
  69   unsigned t;                           /* Threshold */
  70   unsigned i;                           /* Next free slot in the vector */
  71   mp *p;                                /* Modulus for arithmetic */
  72   share_pt *v;                          /* Vector of share information */
  73 } share;
  74
  75 #define SHARE_INIT(t) { t, 0, 0, 0 }
  76
  77 /*----- Functions provided ------------------------------------------------*/
  78
  79 /* --- @share_create@ --- *
  80  *
  81  * Arguments:   @share *s@ = pointer to share context to initialize
  82  *              @unsigned t@ = threshold for the system
  83  *
  84  * Returns:     ---
  85  *
  86  * Use:         Initializes a sharing context.
  87  */
  88
  89 extern void share_create(share */*s*/, unsigned /*t*/);
  90
  91 /* --- @share_destroy@ --- *
  92  *
  93  * Arguments:   @share *s@ = pointer to share context to destroy
  94  *
  95  * Returns:     ---
  96  *
  97  * Use:         Disposes of a sharing context.  All memory is freed, all
  98  *              integers are dropped.
  99  */
 100
 101 extern void share_destroy(share */*s*/);
 102
 103 /* --- @share_mkshares@ --- *
 104  *
 105  * Arguments:   @share *s@ = pointer to share context to fill in
 106  *              @grand *r@ = pointer to random number source
 107  *              @mp *n@ = 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 @p@ is zero, a prime
 114  *              number of appropriate size is generated automatically.  If
 115  *              @v@ is zero, a vector of appropriate size is allocated.  You
 116  *              should use the macro @SHARE_INIT@ or @share_create@ to
 117  *              construct sharing contexts.
 118  */
 119
 120 extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/);
 121
 122 /* --- @share_get@ --- *
 123  *
 124  * Arguments:   @share *s@ = pointer to share conext
 125  *              @mp *d@ = destination for the share
 126  *              @unsigned x@ = share index to fetch
 127  *
 128  * Returns:     The share, as requested.
 129  *
 130  * Use:         Extracts a share from the system.  You may extract @MPW_MAX@
 131  *              shares, or @s->p@ shares from the system, whichever is
 132  *              smaller.  Shares are indexed from 0.
 133  */
 134
 135 extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/);
 136
 137 /* --- @share_addedp@ --- *
 138  *
 139  * Arguments:   @share *s@ = pointer to sharing context
 140  *              @unsigned x@ = which share number to check
 141  *
 142  * Returns:     Nonzero if share @x@ has been added already, zero if it
 143  *              hasn't.
 144  */
 145
 146 extern int share_addedp(share */*s*/, unsigned /*x*/);
 147
 148 /* --- @share_add@ --- *
 149  *
 150  * Arguments:   @share *s@ = pointer to sharing context
 151  *              @unsigned x@ = which share number this is
 152  *              @mp *y@ = the share value
 153  *
 154  * Returns:     Number of shares required before recovery may be performed.
 155  *
 156  * Use:         Adds a share to the context.  The context must have been
 157  *              initialized with the correct prime @p@ and threshold @t@.
 158  */
 159
 160 extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/);
 161
 162 /* --- @share_combine@ --- *
 163  *
 164  * Arguments:   @share *s@ = pointer to share context
 165  *
 166  * Returns:     The secret, as a multiprecision integer.
 167  *
 168  * Use:         Reconstructs a secret, given enough shares.
 169  */
 170
 171 extern mp *share_combine(share */*s*/);
 172
 173 /*----- That's all, folks -------------------------------------------------*/
 174
 175 #ifdef __cplusplus
 176   }
 177 #endif
 178
 179 #endif