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fc2d44af MW |
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'. | |
6dbc50ef MW |
44 | * |
45 | * The @x25519@ function essentially performs incompatible cofactor | |
46 | * multiplication on the elliptic curve %$E(k)$% containing points %$(x, y)$% | |
47 | * in %$\proj^2(k)$% satisfying the Montgomery-form equation | |
48 | * | |
49 | * %$y^3 = x^3 + 486662 x^2 + x$% , | |
50 | * | |
51 | * where $k = \gf{p}$, with $p = 2^{255} - 19$%. The curve has | |
52 | * %$n = (p + 1) + 221938542218978828286815502327069187962$% points; this is | |
53 | * eight times a prime %$\ell$%. The points with %$x$%-coordinate 9 have | |
54 | * order %$\ell$%. | |
fc2d44af MW |
55 | */ |
56 | ||
57 | /*----- Header files ------------------------------------------------------*/ | |
58 | ||
59 | #include <mLib/bits.h> | |
60 | ||
61 | #ifndef CATACOMB_KEY_H | |
62 | # include "key.h" | |
63 | #endif | |
64 | ||
65 | /*----- Important constants -----------------------------------------------*/ | |
66 | ||
67 | #define X25519_KEYSZ 32u | |
68 | #define X25519_PUBSZ 32u | |
69 | #define X25519_OUTSZ 32u | |
70 | ||
71 | extern const octet x25519_base[32]; | |
72 | ||
73 | /*----- Key fetching ------------------------------------------------------*/ | |
74 | ||
75 | typedef struct x25519_priv { key_bin priv, pub; } x25519_priv; | |
76 | typedef struct x25519_pub { key_bin pub; } x25519_pub; | |
77 | ||
78 | extern const key_fetchdef x25519_pubfetch[], x25519_privfetch[]; | |
79 | #define X25519_PUBFETCHSZ 3 | |
80 | #define X25519_PRIVFETCHSZ 6 | |
81 | ||
82 | /*----- Functions provided ------------------------------------------------*/ | |
83 | ||
84 | /* --- @x25519@ --- * | |
85 | * | |
86 | * Arguments: @octet zz[X25519_OUTSZ]@ = where to put the result | |
87 | * @const octet k[X25519_KEYSZ]@ = pointer to private key | |
88 | * @const octet qx[X25519_PUBSZ]@ = pointer to public value | |
89 | * | |
90 | * Returns: --- | |
91 | * | |
92 | * Use: Calculates X25519 of @k@ and @qx@. | |
93 | * | |
94 | * Note that there is disagreement over whether the most | |
95 | * significant bit of @qx@ (i.e., the value @qx[31]&0x80@) | |
96 | * should be ignored or counted towards the represented value. | |
97 | * Historically implementations respected the bit; later | |
98 | * convention seems to be to ignore it. This implementation | |
99 | * honours the bit: a caller who wants to ignore the bit can | |
100 | * easily clear it, while caller who wants to respect it has a | |
101 | * difficult job if this function ignores it. | |
102 | */ | |
103 | ||
104 | extern void x25519(octet /*zz*/[X25519_OUTSZ], | |
105 | const octet /*k*/[X25519_KEYSZ], | |
106 | const octet /*qx*/[X25519_PUBSZ]); | |
107 | ||
108 | /*----- That's all, folks -------------------------------------------------*/ | |
109 | ||
110 | #ifdef __cplusplus | |
111 | } | |
112 | #endif | |
113 | ||
114 | #endif |