_ Git - BearSSL/blob - src/ec/ecdsa_i15_sign_raw.c

   1 /*
   2  * Copyright (c) 2017 Thomas Pornin <pornin@bolet.org>
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining
   5  * a copy of this software and associated documentation files (the
   6  * "Software"), to deal in the Software without restriction, including
   7  * without limitation the rights to use, copy, modify, merge, publish,
   8  * distribute, sublicense, and/or sell copies of the Software, and to
   9  * permit persons to whom the Software is furnished to do so, subject to
  10  * the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be
  13  * included in all copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  16  * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  17  * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  18  * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  19  * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  20  * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  21  * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22  * SOFTWARE.
  23  */
  24
  25 #include "inner.h"
  26
  27 #define I15_LEN     ((BR_MAX_EC_SIZE + 29) / 15)
  28 #define POINT_LEN   (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1))
  29 #define ORDER_LEN   ((BR_MAX_EC_SIZE + 7) >> 3)
  30
  31 /* see bearssl_ec.h */
  32 size_t
  33 br_ecdsa_i15_sign_raw(const br_ec_impl *impl,
  34         const br_hash_class *hf, const void *hash_value,
  35         const br_ec_private_key *sk, void *sig)
  36 {
  37         /*
  38          * IMPORTANT: this code is fit only for curves with a prime
  39          * order. This is needed so that modular reduction of the X
  40          * coordinate of a point can be done with a simple subtraction.
  41          * We also rely on the last byte of the curve order to be distinct
  42          * from 0 and 1.
  43          */
  44         const br_ec_curve_def *cd;
  45         uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], x[I15_LEN];
  46         uint16_t m[I15_LEN], k[I15_LEN], t1[I15_LEN], t2[I15_LEN];
  47         unsigned char tt[ORDER_LEN << 1];
  48         unsigned char eU[POINT_LEN];
  49         size_t hash_len, nlen, ulen;
  50         uint16_t n0i;
  51         uint32_t ctl;
  52         br_hmac_drbg_context drbg;
  53
  54         /*
  55          * If the curve is not supported, then exit with an error.
  56          */
  57         if (((impl->supported_curves >> sk->curve) & 1) == 0) {
  58                 return 0;
  59         }
  60
  61         /*
  62          * Get the curve parameters (generator and order).
  63          */
  64         switch (sk->curve) {
  65         case BR_EC_secp256r1:
  66                 cd = &br_secp256r1;
  67                 break;
  68         case BR_EC_secp384r1:
  69                 cd = &br_secp384r1;
  70                 break;
  71         case BR_EC_secp521r1:
  72                 cd = &br_secp521r1;
  73                 break;
  74         default:
  75                 return 0;
  76         }
  77
  78         /*
  79          * Get modulus.
  80          */
  81         nlen = cd->order_len;
  82         br_i15_decode(n, cd->order, nlen);
  83         n0i = br_i15_ninv15(n[1]);
  84
  85         /*
  86          * Get private key as an i15 integer. This also checks that the
  87          * private key is well-defined (not zero, and less than the
  88          * curve order).
  89          */
  90         if (!br_i15_decode_mod(x, sk->x, sk->xlen, n)) {
  91                 return 0;
  92         }
  93         if (br_i15_iszero(x)) {
  94                 return 0;
  95         }
  96
  97         /*
  98          * Get hash length.
  99          */
 100         hash_len = (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;
 101
 102         /*
 103          * Truncate and reduce the hash value modulo the curve order.
 104          */
 105         br_ecdsa_i15_bits2int(m, hash_value, hash_len, n[0]);
 106         br_i15_sub(m, n, br_i15_sub(m, n, 0) ^ 1);
 107
 108         /*
 109          * RFC 6979 generation of the "k" value.
 110          *
 111          * The process uses HMAC_DRBG (with the hash function used to
 112          * process the message that is to be signed). The seed is the
 113          * concatenation of the encodings of the private key and
 114          * the hash value (after truncation and modular reduction).
 115          */
 116         br_i15_encode(tt, nlen, x);
 117         br_i15_encode(tt + nlen, nlen, m);
 118         br_hmac_drbg_init(&drbg, hf, tt, nlen << 1);
 119         for (;;) {
 120                 br_hmac_drbg_generate(&drbg, tt, nlen);
 121                 br_ecdsa_i15_bits2int(k, tt, nlen, n[0]);
 122                 if (br_i15_iszero(k)) {
 123                         continue;
 124                 }
 125                 if (br_i15_sub(k, n, 0)) {
 126                         break;
 127                 }
 128         }
 129
 130         /*
 131          * Compute k*G and extract the X coordinate, then reduce it
 132          * modulo the curve order. Since we support only curves with
 133          * prime order, that reduction is only a matter of computing
 134          * a subtraction.
 135          */
 136         br_i15_encode(tt, nlen, k);
 137         ulen = impl->mulgen(eU, tt, nlen, sk->curve);
 138         br_i15_zero(r, n[0]);
 139         br_i15_decode(r, &eU[1], ulen >> 1);
 140         r[0] = n[0];
 141         br_i15_sub(r, n, br_i15_sub(r, n, 0) ^ 1);
 142
 143         /*
 144          * Compute 1/k in double-Montgomery representation. We do so by
 145          * first converting _from_ Montgomery representation (twice),
 146          * then using a modular exponentiation.
 147          */
 148         br_i15_from_monty(k, n, n0i);
 149         br_i15_from_monty(k, n, n0i);
 150         memcpy(tt, cd->order, nlen);
 151         tt[nlen - 1] -= 2;
 152         br_i15_modpow(k, tt, nlen, n, n0i, t1, t2);
 153
 154         /*
 155          * Compute s = (m+xr)/k (mod n).
 156          * The k[] array contains R^2/k (double-Montgomery representation);
 157          * we thus can use direct Montgomery multiplications and conversions
 158          * from Montgomery, avoiding any call to br_i15_to_monty() (which
 159          * is slower).
 160          */
 161         br_i15_from_monty(m, n, n0i);
 162         br_i15_montymul(t1, x, r, n, n0i);
 163         ctl = br_i15_add(t1, m, 1);
 164         ctl |= br_i15_sub(t1, n, 0) ^ 1;
 165         br_i15_sub(t1, n, ctl);
 166         br_i15_montymul(s, t1, k, n, n0i);
 167
 168         /*
 169          * Encode r and s in the signature.
 170          */
 171         br_i15_encode(sig, nlen, r);
 172         br_i15_encode((unsigned char *)sig + nlen, nlen, s);
 173         return nlen << 1;
 174 }