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 30 /* see bearssl_ec.h */ 31 uint32_t 32 br_ecdsa_i15_vrfy_raw(const br_ec_impl *impl, 33 const void *hash, size_t hash_len, 34 const br_ec_public_key *pk, 35 const void *sig, size_t sig_len) 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 */ 42 const br_ec_curve_def *cd; 43 uint16_t n[I15_LEN], r[I15_LEN], s[I15_LEN], t1[I15_LEN], t2[I15_LEN]; 44 unsigned char tx[(BR_MAX_EC_SIZE + 7) >> 3]; 45 unsigned char ty[(BR_MAX_EC_SIZE + 7) >> 3]; 46 unsigned char eU[POINT_LEN]; 47 size_t nlen, rlen, ulen; 48 uint16_t n0i; 49 uint32_t res; 50 51 /* 52 * If the curve is not supported, then report an error. 53 */ 54 if (((impl->supported_curves >> pk->curve) & 1) == 0) { 55 return 0; 56 } 57 58 /* 59 * Get the curve parameters (generator and order). 60 */ 61 switch (pk->curve) { 62 case BR_EC_secp256r1: 63 cd = &br_secp256r1; 64 break; 65 case BR_EC_secp384r1: 66 cd = &br_secp384r1; 67 break; 68 case BR_EC_secp521r1: 69 cd = &br_secp521r1; 70 break; 71 default: 72 return 0; 73 } 74 75 /* 76 * Signature length must be even. 77 */ 78 if (sig_len & 1) { 79 return 0; 80 } 81 rlen = sig_len >> 1; 82 83 /* 84 * Public key point must have the proper size for this curve. 85 */ 86 if (pk->qlen != cd->generator_len) { 87 return 0; 88 } 89 90 /* 91 * Get modulus; then decode the r and s values. They must be 92 * lower than the modulus, and s must not be null. 93 */ 94 nlen = cd->order_len; 95 br_i15_decode(n, cd->order, nlen); 96 n0i = br_i15_ninv15(n[1]); 97 if (!br_i15_decode_mod(r, sig, rlen, n)) { 98 return 0; 99 } 100 if (!br_i15_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) { 101 return 0; 102 } 103 if (br_i15_iszero(s)) { 104 return 0; 105 } 106 107 /* 108 * Invert s. We do that with a modular exponentiation; we use 109 * the fact that for all the curves we support, the least 110 * significant byte is not 0 or 1, so we can subtract 2 without 111 * any carry to process. 112 * We also want 1/s in Montgomery representation, which can be 113 * done by converting _from_ Montgomery representation before 114 * the inversion (because (1/s)*R = 1/(s/R)). 115 */ 116 br_i15_from_monty(s, n, n0i); 117 memcpy(tx, cd->order, nlen); 118 tx[nlen - 1] -= 2; 119 br_i15_modpow(s, tx, nlen, n, n0i, t1, t2); 120 121 /* 122 * Truncate the hash to the modulus length (in bits) and reduce 123 * it modulo the curve order. The modular reduction can be done 124 * with a subtraction since the truncation already reduced the 125 * value to the modulus bit length. 126 */ 127 br_ecdsa_i15_bits2int(t1, hash, hash_len, n[0]); 128 br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1); 129 130 /* 131 * Multiply the (truncated, reduced) hash value with 1/s, result in 132 * t2, encoded in ty. 133 */ 134 br_i15_montymul(t2, t1, s, n, n0i); 135 br_i15_encode(ty, nlen, t2); 136 137 /* 138 * Multiply r with 1/s, result in t1, encoded in tx. 139 */ 140 br_i15_montymul(t1, r, s, n, n0i); 141 br_i15_encode(tx, nlen, t1); 142 143 /* 144 * Compute the point x*Q + y*G. 145 */ 146 ulen = cd->generator_len; 147 memcpy(eU, pk->q, ulen); 148 res = impl->muladd(eU, NULL, ulen, 149 tx, nlen, ty, nlen, cd->curve); 150 151 /* 152 * Get the X coordinate, reduce modulo the curve order, and 153 * compare with the 'r' value. 154 * 155 * The modular reduction can be done with subtractions because 156 * we work with curves of prime order, so the curve order is 157 * close to the field order (Hasse's theorem). 158 */ 159 br_i15_zero(t1, n[0]); 160 br_i15_decode(t1, &eU[1], ulen >> 1); 161 t1[0] = n[0]; 162 br_i15_sub(t1, n, br_i15_sub(t1, n, 0) ^ 1); 163 res &= ~br_i15_sub(t1, r, 1); 164 res &= br_i15_iszero(t1); 165 return res; 166 } 167