| 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 secnet. |
| 11 | * See README for full list of copyright holders. |
| 12 | * |
| 13 | * secnet is free software; you can redistribute it and/or modify it |
| 14 | * under the terms of the GNU General Public License as published by |
| 15 | * the Free Software Foundation; either version d of the License, or |
| 16 | * (at your option) any later version. |
| 17 | * |
| 18 | * secnet is distributed in the hope that it will be useful, but |
| 19 | * WITHOUT ANY WARRANTY; without even the implied warranty of |
| 20 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 21 | * General Public License for more details. |
| 22 | * |
| 23 | * You should have received a copy of the GNU General Public License |
| 24 | * version 3 along with secnet; if not, see |
| 25 | * https://www.gnu.org/licenses/gpl.html. |
| 26 | * |
| 27 | * This file was originally part of Catacomb, but has been automatically |
| 28 | * modified for incorporation into secnet: see `import-catacomb-crypto' |
| 29 | * for details. |
| 30 | * |
| 31 | * Catacomb is free software; you can redistribute it and/or modify |
| 32 | * it under the terms of the GNU Library General Public License as |
| 33 | * published by the Free Software Foundation; either version 2 of the |
| 34 | * License, or (at your option) any later version. |
| 35 | * |
| 36 | * Catacomb is distributed in the hope that it will be useful, |
| 37 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 38 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 39 | * GNU Library General Public License for more details. |
| 40 | * |
| 41 | * You should have received a copy of the GNU Library General Public |
| 42 | * License along with Catacomb; if not, write to the Free |
| 43 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
| 44 | * MA 02111-1307, USA. |
| 45 | */ |
| 46 | |
| 47 | /*----- Header files ------------------------------------------------------*/ |
| 48 | |
| 49 | #include "fake-mLib-bits.h" |
| 50 | |
| 51 | #include "montladder.h" |
| 52 | #include "f25519.h" |
| 53 | #include "x25519.h" |
| 54 | |
| 55 | /*----- Important constants -----------------------------------------------*/ |
| 56 | |
| 57 | const octet x25519_base[32] = { 9, 0, /* ... */ }; |
| 58 | |
| 59 | #define A0 121665 |
| 60 | |
| 61 | /*----- Main code ---------------------------------------------------------*/ |
| 62 | |
| 63 | /* --- @x25519@ --- * |
| 64 | * |
| 65 | * Arguments: @octet zz[X25519_OUTSZ]@ = where to put the result |
| 66 | * @const octet k[X25519_KEYSZ]@ = pointer to private key |
| 67 | * @const octet qx[X25519_PUBSZ]@ = pointer to public value |
| 68 | * |
| 69 | * Returns: --- |
| 70 | * |
| 71 | * Use: Calculates X25519 of @k@ and @qx@. |
| 72 | * |
| 73 | * Note that there is disagreement over whether the most |
| 74 | * significant bit of @qx@ (i.e., the value @qx[31]&0x80@) |
| 75 | * should be ignored or counted towards the represented value. |
| 76 | * Historically implementations respected the bit; later |
| 77 | * convention seems to be to ignore it. This implementation |
| 78 | * honours the bit: a caller who wants to ignore the bit can |
| 79 | * easily clear it, while caller who wants to respect it has a |
| 80 | * difficult job if this function ignores it. |
| 81 | */ |
| 82 | |
| 83 | void x25519(octet zz[X25519_OUTSZ], |
| 84 | const octet k[X25519_KEYSZ], |
| 85 | const octet qx[X25519_PUBSZ]) |
| 86 | { |
| 87 | uint32 kw[8]; |
| 88 | f25519 x1; |
| 89 | |
| 90 | /* Load and clamp the key. The low bits are cleared to kill the small |
| 91 | * subgroups on the curve and its twist, and a high bit is set to guard |
| 92 | * against careless implementations, though this isn't one of those. |
| 93 | */ |
| 94 | kw[0] = LOAD32_L(k + 0); kw[1] = LOAD32_L(k + 4); |
| 95 | kw[2] = LOAD32_L(k + 8); kw[3] = LOAD32_L(k + 12); |
| 96 | kw[4] = LOAD32_L(k + 16); kw[5] = LOAD32_L(k + 20); |
| 97 | kw[6] = LOAD32_L(k + 24); kw[7] = LOAD32_L(k + 28); |
| 98 | kw[0] &= 0xfffffff8; kw[7] = (kw[7]&0x3fffffff) | 0x40000000; |
| 99 | |
| 100 | /* And run the ladder. */ |
| 101 | f25519_load(&x1, qx); |
| 102 | #define MULA0(z, x) do { f25519_mulconst((z), (x), A0); } while (0) |
| 103 | MONT_LADDER(f25519, MULA0, kw, 8, 32, &x1, &x1); |
| 104 | #undef MULA0 |
| 105 | f25519_store(zz, &x1); |
| 106 | } |
| 107 | |
| 108 | /*----- That's all, folks -------------------------------------------------*/ |