| 1 | /* -*-c-*- |
| 2 | * |
| 3 | * Shamir's secret sharing |
| 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 sharing system ---------------------------------------* |
| 29 | * |
| 30 | * Shamir's secret-sharing system is based on polynomial interpolation modulo |
| 31 | * a prime number. It is `perfect' in that fewer participants than the |
| 32 | * threshold can derive no information about the secret by pooling their |
| 33 | * shares, and `ideal' in that the shares are the same size as the secret. |
| 34 | * |
| 35 | * This implementation stays close to the definition, in order to support |
| 36 | * other schemes for (e.g.) threshold cryptography. It is, however, rather |
| 37 | * slow. |
| 38 | */ |
| 39 | |
| 40 | #ifndef CATACOMB_SHARE_H |
| 41 | #define CATACOMB_SHARE_H |
| 42 | |
| 43 | #ifdef __cplusplus |
| 44 | extern "C" { |
| 45 | #endif |
| 46 | |
| 47 | /*----- Header files ------------------------------------------------------*/ |
| 48 | |
| 49 | #ifndef CATACOMB_GRAND_H |
| 50 | # include "grand.h" |
| 51 | #endif |
| 52 | |
| 53 | #ifndef CATACOMB_MP_H |
| 54 | # include "mp.h" |
| 55 | #endif |
| 56 | |
| 57 | /*----- Data structures ---------------------------------------------------*/ |
| 58 | |
| 59 | /* --- A secret sharing context --- */ |
| 60 | |
| 61 | typedef struct share_pt { |
| 62 | unsigned x; /* Index of this share */ |
| 63 | mp *y; /* Payload of this share */ |
| 64 | } share_pt; |
| 65 | |
| 66 | typedef struct share { |
| 67 | unsigned t; /* Threshold */ |
| 68 | unsigned i; /* Next free slot in the vector */ |
| 69 | mp *p; /* Modulus for arithmetic */ |
| 70 | share_pt *v; /* Vector of share information */ |
| 71 | } share; |
| 72 | |
| 73 | #define SHARE_INIT(t) { t, 0, 0, 0 } |
| 74 | |
| 75 | /*----- Functions provided ------------------------------------------------*/ |
| 76 | |
| 77 | /* --- @share_create@ --- * |
| 78 | * |
| 79 | * Arguments: @share *s@ = pointer to share context to initialize |
| 80 | * @unsigned t@ = threshold for the system |
| 81 | * |
| 82 | * Returns: --- |
| 83 | * |
| 84 | * Use: Initializes a sharing context. |
| 85 | */ |
| 86 | |
| 87 | extern void share_create(share */*s*/, unsigned /*t*/); |
| 88 | |
| 89 | /* --- @share_destroy@ --- * |
| 90 | * |
| 91 | * Arguments: @share *s@ = pointer to share context to destroy |
| 92 | * |
| 93 | * Returns: --- |
| 94 | * |
| 95 | * Use: Disposes of a sharing context. All memory is freed, all |
| 96 | * integers are dropped. |
| 97 | */ |
| 98 | |
| 99 | extern void share_destroy(share */*s*/); |
| 100 | |
| 101 | /* --- @share_mkshares@ --- * |
| 102 | * |
| 103 | * Arguments: @share *s@ = pointer to share context to fill in |
| 104 | * @grand *r@ = pointer to random number source |
| 105 | * @mp *n@ = the secret to share |
| 106 | * |
| 107 | * Returns: --- |
| 108 | * |
| 109 | * Use: Initializes a sharing context to be able to create shares. |
| 110 | * The context structure is expected to be mostly filled in. In |
| 111 | * particular, @t@ must be initialized. If @p@ is zero, a prime |
| 112 | * number of appropriate size is generated automatically. If |
| 113 | * @v@ is zero, a vector of appropriate size is allocated. You |
| 114 | * should use the macro @SHARE_INIT@ or @share_create@ to |
| 115 | * construct sharing contexts. |
| 116 | */ |
| 117 | |
| 118 | extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/); |
| 119 | |
| 120 | /* --- @share_get@ --- * |
| 121 | * |
| 122 | * Arguments: @share *s@ = pointer to share conext |
| 123 | * @mp *d@ = destination for the share |
| 124 | * @unsigned x@ = share index to fetch |
| 125 | * |
| 126 | * Returns: The share, as requested. |
| 127 | * |
| 128 | * Use: Extracts a share from the system. You may extract @MPW_MAX@ |
| 129 | * shares, or @s->p@ shares from the system, whichever is |
| 130 | * smaller. Shares are indexed from 0. |
| 131 | */ |
| 132 | |
| 133 | extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/); |
| 134 | |
| 135 | /* --- @share_addedp@ --- * |
| 136 | * |
| 137 | * Arguments: @share *s@ = pointer to sharing context |
| 138 | * @unsigned x@ = which share number to check |
| 139 | * |
| 140 | * Returns: Nonzero if share @x@ has been added already, zero if it |
| 141 | * hasn't. |
| 142 | */ |
| 143 | |
| 144 | extern int share_addedp(share */*s*/, unsigned /*x*/); |
| 145 | |
| 146 | /* --- @share_add@ --- * |
| 147 | * |
| 148 | * Arguments: @share *s@ = pointer to sharing context |
| 149 | * @unsigned x@ = which share number this is |
| 150 | * @mp *y@ = the share value |
| 151 | * |
| 152 | * Returns: Number of shares required before recovery may be performed. |
| 153 | * |
| 154 | * Use: Adds a share to the context. The context must have been |
| 155 | * initialized with the correct prime @p@ and threshold @t@. |
| 156 | */ |
| 157 | |
| 158 | extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/); |
| 159 | |
| 160 | /* --- @share_combine@ --- * |
| 161 | * |
| 162 | * Arguments: @share *s@ = pointer to share context |
| 163 | * |
| 164 | * Returns: The secret, as a multiprecision integer. |
| 165 | * |
| 166 | * Use: Reconstructs a secret, given enough shares. |
| 167 | */ |
| 168 | |
| 169 | extern mp *share_combine(share */*s*/); |
| 170 | |
| 171 | /*----- That's all, folks -------------------------------------------------*/ |
| 172 | |
| 173 | #ifdef __cplusplus |
| 174 | } |
| 175 | #endif |
| 176 | |
| 177 | #endif |