--- /dev/null
+/* -*-c-*-
+ *
+ * Shamir's secret sharing
+ *
+ * (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.
+ */
+
+/*----- Notes on the sharing system ---------------------------------------*
+ *
+ * Shamir's secret-sharing system is based on polynomial interpolation modulo
+ * a prime number. It is `perfect' in that fewer participants than the
+ * threshold can derive no information about the secret by pooling their
+ * shares, and `ideal' in that the shares are the same size as the secret.
+ *
+ * This implementation stays close to the definition, in order to support
+ * other schemes for (e.g.) threshold cryptography. It is, however, rather
+ * slow.
+ */
+
+#ifndef CATACOMB_SHARE_H
+#define CATACOMB_SHARE_H
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+/*----- Header files ------------------------------------------------------*/
+
+#ifndef CATACOMB_GRAND_H
+# include "grand.h"
+#endif
+
+#ifndef CATACOMB_MP_H
+# include "mp.h"
+#endif
+
+/*----- Data structures ---------------------------------------------------*/
+
+/* --- A secret sharing context --- */
+
+typedef struct share_pt {
+ unsigned x; /* Index of this share */
+ mp *y; /* Payload of this share */
+} share_pt;
+
+typedef struct share {
+ unsigned t; /* Threshold */
+ unsigned i; /* Next free slot in the vector */
+ mp *p; /* Modulus for arithmetic */
+ share_pt *v; /* Vector of share information */
+} share;
+
+#define SHARE_INIT(t) { t, 0, 0, 0 }
+
+/*----- Functions provided ------------------------------------------------*/
+
+/* --- @share_create@ --- *
+ *
+ * Arguments: @share *s@ = pointer to share context to initialize
+ * @unsigned t@ = threshold for the system
+ *
+ * Returns: ---
+ *
+ * Use: Initializes a sharing context.
+ */
+
+extern void share_create(share */*s*/, unsigned /*t*/);
+
+/* --- @share_destroy@ --- *
+ *
+ * Arguments: @share *s@ = pointer to share context to destroy
+ *
+ * Returns: ---
+ *
+ * Use: Disposes of a sharing context. All memory is freed, all
+ * integers are dropped.
+ */
+
+extern void share_destroy(share */*s*/);
+
+/* --- @share_mkshares@ --- *
+ *
+ * Arguments: @share *s@ = pointer to share context to fill in
+ * @grand *r@ = pointer to random number source
+ * @mp *n@ = the secret to share
+ *
+ * Returns: ---
+ *
+ * Use: Initializes a sharing context to be able to create shares.
+ * The context structure is expected to be mostly filled in. In
+ * particular, @t@ must be initialized. If @p@ is zero, a prime
+ * number of appropriate size is generated automatically. If
+ * @v@ is zero, a vector of appropriate size is allocated. You
+ * should use the macro @SHARE_INIT@ or @share_create@ to
+ * construct sharing contexts.
+ */
+
+extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/);
+
+/* --- @share_get@ --- *
+ *
+ * Arguments: @share *s@ = pointer to share conext
+ * @mp *d@ = destination for the share
+ * @unsigned x@ = share index to fetch
+ *
+ * Returns: The share, as requested.
+ *
+ * Use: Extracts a share from the system. You may extract @MPW_MAX@
+ * shares, or @s->p@ shares from the system, whichever is
+ * smaller. Shares are indexed from 0.
+ */
+
+extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/);
+
+/* --- @share_addedp@ --- *
+ *
+ * Arguments: @share *s@ = pointer to sharing context
+ * @unsigned x@ = which share number to check
+ *
+ * Returns: Nonzero if share @x@ has been added already, zero if it
+ * hasn't.
+ */
+
+extern int share_addedp(share */*s*/, unsigned /*x*/);
+
+/* --- @share_add@ --- *
+ *
+ * Arguments: @share *s@ = pointer to sharing context
+ * @unsigned x@ = which share number this is
+ * @mp *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 prime @p@ and threshold @t@.
+ */
+
+extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/);
+
+/* --- @share_combine@ --- *
+ *
+ * Arguments: @share *s@ = pointer to share context
+ *
+ * Returns: The secret, as a multiprecision integer.
+ *
+ * Use: Reconstructs a secret, given enough shares.
+ */
+
+extern mp *share_combine(share */*s*/);
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#ifdef __cplusplus
+ }
+#endif
+
+#endif
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