pub/, progs/: Implement Bernstein's X25519 key-exchange algorithm.

[catacomb] / math / f25519.h

/* -*-c-*-
 *
 * Arithmetic modulo 2^255 - 19
 *
 * (c) 2017 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.
 */

#ifndef CATACOMB_F25519_H
#define CATACOMB_F25519_H

#ifdef __cplusplus
  extern "C" {
#endif

/*----- Header files ------------------------------------------------------*/

#include <mLib/bits.h>

#ifndef CATACOMB_QFARITH_H
#  include "qfarith.h"
#endif

/*----- Data structures ---------------------------------------------------*/

typedef union {
  int32 p26[10];
  int16 p10[26];
} f25519;

#if !defined(F25519_IMPL) && defined(HAVE_INT64)
#  define F25519_IMPL 26
#endif

#ifndef F25519_IMPL
#  define F25519_IMPL 10
#endif

/*----- Functions provided ------------------------------------------------*/

/* --- @f25519_set@ --- *
 *
 * Arguments:	@f25519 *z@ = where to write the result
 *		@int a@ = a small-ish constant
 *
 * Returns:	---
 *
 * Use:		Sets @z@ to equal @a@.
 */

extern void f25519_set(f25519 */*x*/, int /*a*/);

/* --- @f25519_load@ --- *
 *
 * Arguments:	@f25519 *z@ = where to store the result
 *		@const octet xv[32]@ = source to read
 *
 * Returns:	---
 *
 * Use:		Reads an element of %$\gf{2^{255} - 19}$% in external
 *		representation from @xv@ and stores it in @z@.
 *
 *		External representation is little-endian base-256.  Elements
 *		have multiple encodings, which are not produced by correct
 *		software; use of noncanonical encodings is not an error, and
 *		toleration of them is considered a performance feature.
 *
 *		Some specifications, e.g., RFC7748, require the topmost bit
 *		(i.e., bit 7 of @wv[31]@) to be ignored.  Callers
 *		implementing such specifications should clear the bit
 *		explicitly.  (It's much easier for a caller who wants the bit
 *		to be ignored to clear it than for a caller who wants the bit
 *		to be significant to apply the necessary change by hand.)
 */

extern void f25519_load(f25519 */*z*/, const octet /*xv*/[32]);

/* --- @f25519_store@ --- *
 *
 * Arguments:	@octet zv[32]@ = where to write the result
 *		@const f25519 *x@ = the field element to write
 *
 * Returns:	---
 *
 * Use:		Stores a field element in the given octet vector in external
 *		representation.  A canonical encoding is always stored, so,
 *		in particular, the top bit of @xv[31]@ is always left clear.
 */

extern void f25519_store(octet /*zv*/[32], const f25519 */*x*/);

/* --- @f25519_condswap@ --- *
 *
 * Arguments:	@f25519 *x, *y@ = two operands
 *		@uint32 m@ = a mask
 *
 * Returns:	---
 *
 * Use:		If @m@ is zero, do nothing; if @m@ is all-bits-set, then
 *		exchange @x@ and @y@.  If @m@ has some other value, then
 *		scramble @x@ and @y@ in an unhelpful way.
 */

extern void f25519_condswap(f25519 */*x*/, f25519 */*y*/, uint32 /*m*/);

/* --- @f25519_add@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@ or @y@)
 *		@const f25519 *x, *y@ = two operands
 *
 * Returns:	---
 *
 * Use:		Set @z@ to the sum %$x + y$%.
 */

extern void f25519_add(f25519 */*z*/,
		       const f25519 */*x*/, const f25519 */*y*/);

/* --- @f25519_sub@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@ or @y@)
 *		@const f25519 *x, *y@ = two operands
 *
 * Returns:	---
 *
 * Use:		Set @z@ to the difference %$x - y$%.
 */

extern void f25519_sub(f25519 */*z*/,
		       const f25519 */*x*/, const f25519 */*y*/);

/* --- @f25519_mulconst@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@)
 *		@const f25519 *x@ = an operand
 *		@long a@ = a small-ish constant; %$|a| < 2^{20}$%.
 *
 * Returns:	---
 *
 * Use:		Set @z@ to the product %$a x$%.
 */

extern void f25519_mulconst(f25519 */*z*/, const f25519 */*x*/, long /*a*/);

/* --- @f25519_mul@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@ or @y@)
 *		@const f25519 *x, *y@ = two operands
 *
 * Returns:	---
 *
 * Use:		Set @z@ to the product %$x y$%.
 */

extern void f25519_mul(f25519 */*z*/,
		       const f25519 */*x*/, const f25519 */*y*/);

/* --- @f25519_sqr@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@ or @y@)
 *		@const f25519 *x@ = an operand
 *
 * Returns:	---
 *
 * Use:		Set @z@ to the square %$x^2$%.
 */

extern void f25519_sqr(f25519 */*z*/, const f25519 */*x*/);

/* --- @f25519_inv@ --- *
 *
 * Arguments:	@f25519 *z@ = where to put the result (may alias @x@)
 *		@const f25519 *x@ = an operand
 *
 * Returns:	---
 *
 * Use:		Stores in @z@ the multiplicative inverse %$x^{-1}$%.  If
 *		%$x = 0$% then @z@ is set to zero.  This is considered a
 *		feature.
 */

extern void f25519_inv(f25519 */*z*/, const f25519 */*x*/);

/*----- That's all, folks -------------------------------------------------*/

#ifdef __cplusplus
  }
#endif

#endif