mdw@git.distorted.org.uk Git - catacomb/blob - pub/x25519.h

   1 /* -*-c-*-
   2  *
   3  * The X25519 key-agreement algorithm
   4  *
   5  * (c) 2017 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 #ifndef CATACOMB_X25519_H
  29 #define CATACOMB_X25519_H
  30
  31 #ifdef __cplusplus
  32   extern "C" {
  33 #endif
  34
  35 /*----- Notes on the X25519 key-agreement algorithm -----------------------*
  36  *
  37  * This is X25519, as described in Daniel J. Bernstein, `Curve25519: new
  38  * Diffie--Hellman speed records', PKC 2006,
  39  * https://cr.yp.to/ecdh/curve25519-20060209.pdf
  40  *
  41  * Since then, the name `Curve25519' has shifted somewhat, to refer to the
  42  * specific elliptic curve used, and the x-coordinate Diffie--Hellman
  43  * operation is now named `X25519'.
  44  */
  45
  46 /*----- Header files ------------------------------------------------------*/
  47
  48 #include <mLib/bits.h>
  49
  50 #ifndef CATACOMB_KEY_H
  51 #  include "key.h"
  52 #endif
  53
  54 /*----- Important constants -----------------------------------------------*/
  55
  56 #define X25519_KEYSZ 32u
  57 #define X25519_PUBSZ 32u
  58 #define X25519_OUTSZ 32u
  59
  60 extern const octet x25519_base[32];
  61
  62 /*----- Key fetching ------------------------------------------------------*/
  63
  64 typedef struct x25519_priv { key_bin priv, pub; } x25519_priv;
  65 typedef struct x25519_pub { key_bin pub; } x25519_pub;
  66
  67 extern const key_fetchdef x25519_pubfetch[], x25519_privfetch[];
  68 #define X25519_PUBFETCHSZ 3
  69 #define X25519_PRIVFETCHSZ 6
  70
  71 /*----- Functions provided ------------------------------------------------*/
  72
  73 /* --- @x25519@ --- *
  74  *
  75  * Arguments:   @octet zz[X25519_OUTSZ]@ = where to put the result
  76  *              @const octet k[X25519_KEYSZ]@ = pointer to private key
  77  *              @const octet qx[X25519_PUBSZ]@ = pointer to public value
  78  *
  79  * Returns:     ---
  80  *
  81  * Use:         Calculates X25519 of @k@ and @qx@.
  82  *
  83  *              Note that there is disagreement over whether the most
  84  *              significant bit of @qx@ (i.e., the value @qx[31]&0x80@)
  85  *              should be ignored or counted towards the represented value.
  86  *              Historically implementations respected the bit; later
  87  *              convention seems to be to ignore it.  This implementation
  88  *              honours the bit: a caller who wants to ignore the bit can
  89  *              easily clear it, while caller who wants to respect it has a
  90  *              difficult job if this function ignores it.
  91  */
  92
  93 extern void x25519(octet /*zz*/[X25519_OUTSZ],
  94                    const octet /*k*/[X25519_KEYSZ],
  95                    const octet /*qx*/[X25519_PUBSZ]);
  96
  97 /*----- That's all, folks -------------------------------------------------*/
  98
  99 #ifdef __cplusplus
 100   }
 101 #endif
 102
 103 #endif