--- /dev/null
+/* -*-c-*-
+ *
+ * Secret sharing over %$\gf{2^8}$%
+ *
+ * (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 system -----------------------------------------------*
+ *
+ * This uses a variant of Shamir's secret sharing system. Shamir's original
+ * system used polynomials modulo a large prime. This implementation instead
+ * uses the field %$\gf{2^8}$%, represented by
+ *
+ * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
+ *
+ * and shares each byte of the secret independently. It is therefore limited
+ * to 255 players, although this probably isn't a serious limitation in
+ * practice.
+ *
+ * Share creation and reconstruction is extremely efficient. Contrast the
+ * performance of the straightforward implementation based on multiprecision
+ * arithmetic.
+ */
+
+#ifndef CATACOMB_GFSHARE_H
+#define CATACOMB_GFSHARE_H
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <mLib/bits.h>
+
+#ifndef CATACOMB_GRAND_H
+# include "grand.h"
+#endif
+
+/*----- Data structures ---------------------------------------------------*/
+
+/* --- A secret sharing context --- */
+
+typedef struct gfshare {
+ unsigned t; /* Threshold */
+ unsigned i; /* Next free slot in vector */
+ size_t sz; /* Size of the secret and shares */
+ octet *v; /* Vector of share information */
+} gfshare;
+
+#define GFSHARE_INIT(t, sz) { t, 0, sz, 0 }
+
+#define GFSHARE_INDEX(s, i) ((s)->v[(i) * ((s)->sz + 1)])
+
+/*----- Functions provided ------------------------------------------------*/
+
+/* --- @gfshare_create@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to initialize
+ * @unsigned t@ = threshold for the system
+ * @size_t sz@ = size of the secret
+ *
+ * Returns: ---
+ *
+ * Use: Initializes a sharing context.
+ */
+
+extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/);
+
+/* --- @gfshare_destroy@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to destroy
+ *
+ * Returns: ---
+ *
+ * Use: Disposes of a sharing context. The allocations for the
+ * individual shares and the vector @v@ are freed; the secret is
+ * left alone.
+ */
+
+extern void gfshare_destroy(gfshare */*s*/);
+
+/* --- @gfshare_mkshares@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to fill in
+ * @grand *r@ = pointer to random number source
+ * @const void *buf@ = pointer to 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 @v@ is zero, a
+ * vector of appropriate size is allocated. You should use the
+ * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
+ * contexts.
+ */
+
+extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/,
+ const void */*buf*/);
+
+/* --- @gfshare_get@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share conext
+ * @unsigned x@ = share index to fetch
+ * @void *buf@ = pointer to output buffer
+ *
+ * Returns: ---
+ *
+ * Use: Extracts a share from the system. You may extract up to 255
+ * shares from the system. Shares are indexed from 0.
+ */
+
+extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/);
+
+/* --- @gfshare_addedp@ --- *
+ *
+ * Arguments: @gfshare *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 gfshare_addedp(gfshare */*s*/, unsigned /*x*/);
+
+/* --- @gfshare_add@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to sharing context
+ * @unsigned x@ = which share number this is
+ * @const void *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 threshold @t@.
+ */
+
+extern unsigned gfshare_add(gfshare */*s*/,
+ unsigned /*x*/, const void */*y*/);
+
+/* --- @gfshare_combine@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context
+ * @void *buf@ = pointer to output buffer for the secret
+ *
+ * Returns: ---
+ *
+ * Use: Reconstructs a secret, given enough shares.
+ */
+
+extern void gfshare_combine(gfshare */*s*/, void */*buf*/);
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#ifdef __cplusplus
+ }
+#endif
+
+#endif
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