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+/* -*-c-*-
+ *
+ * The X25519 key-agreement algorithm
+ *
+ * (c) 2017 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------*
+ *
+ * This file is part of secnet.
+ * See README for full list of copyright holders.
+ *
+ * secnet is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version d of the License, or
+ * (at your option) any later version.
+ *
+ * secnet 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
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * version 3 along with secnet; if not, see
+ * https://www.gnu.org/licenses/gpl.html.
+ *
+ * This file was originally part of Catacomb, but has been automatically
+ * modified for incorporation into secnet: see `import-catacomb-crypto'
+ * for details.
+ *
+ * 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.
+ */
+
+#ifndef CATACOMB_X25519_H
+#define CATACOMB_X25519_H
+
+#ifdef __cplusplus
+  extern "C" {
+#endif
+
+/*----- Notes on the X25519 key-agreement algorithm -----------------------*
+ *
+ * This is X25519, as described in Daniel J. Bernstein, `Curve25519: new
+ * Diffie--Hellman speed records', PKC 2006,
+ * https://cr.yp.to/ecdh/curve25519-20060209.pdf
+ *
+ * Since then, the name `Curve25519' has shifted somewhat, to refer to the
+ * specific elliptic curve used, and the x-coordinate Diffie--Hellman
+ * operation is now named `X25519'.
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include "fake-mLib-bits.h"
+
+/*----- Important constants -----------------------------------------------*/
+
+#define X25519_KEYSZ 32u
+#define X25519_PUBSZ 32u
+#define X25519_OUTSZ 32u
+
+extern const octet x25519_base[32];
+
+/*----- Functions provided ------------------------------------------------*/
+
+/* --- @x25519@ --- *
+ *
+ * Arguments:	@octet zz[X25519_OUTSZ]@ = where to put the result
+ *		@const octet k[X25519_KEYSZ]@ = pointer to private key
+ *		@const octet qx[X25519_PUBSZ]@ = pointer to public value
+ *
+ * Returns:	---
+ *
+ * Use:		Calculates X25519 of @k@ and @qx@.
+ *
+ *		Note that there is disagreement over whether the most
+ *		significant bit of @qx@ (i.e., the value @qx[31]&0x80@)
+ *		should be ignored or counted towards the represented value.
+ *		Historically implementations respected the bit; later
+ *		convention seems to be to ignore it.  This implementation
+ *		honours the bit: a caller who wants to ignore the bit can
+ *		easily clear it, while caller who wants to respect it has a
+ *		difficult job if this function ignores it.
+ */
+
+extern void x25519(octet /*zz*/[X25519_OUTSZ],
+		   const octet /*k*/[X25519_KEYSZ],
+		   const octet /*qx*/[X25519_PUBSZ]);
+
+/*----- That's all, folks -------------------------------------------------*/
+
+#ifdef __cplusplus
+  }
+#endif
+
+#endif