X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/misc/gfshare.h

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+/* -*-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