1 /* 2 * Copyright (c) 2016 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 I31_LEN ((BR_MAX_EC_SIZE + 61) / 31) 28 #define POINT_LEN (1 + (((BR_MAX_EC_SIZE + 7) >> 3) << 1)) 29 30 /* see bearssl_ec.h */ 31 uint32_t 32 br_ecdsa_i31_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 uint32_t n[I31_LEN], r[I31_LEN], s[I31_LEN], t1[I31_LEN], t2[I31_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 uint32_t n0i, res; 49 50 /* 51 * If the curve is not supported, then report an error. 52 */ 53 if (((impl->supported_curves >> pk->curve) & 1) == 0) { 54 return 0; 55 } 56 57 /* 58 * Get the curve parameters (generator and order). 59 */ 60 switch (pk->curve) { 61 case BR_EC_secp256r1: 62 cd = &br_secp256r1; 63 break; 64 case BR_EC_secp384r1: 65 cd = &br_secp384r1; 66 break; 67 case BR_EC_secp521r1: 68 cd = &br_secp521r1; 69 break; 70 default: 71 return 0; 72 } 73 74 /* 75 * Signature length must be even. 76 */ 77 if (sig_len & 1) { 78 return 0; 79 } 80 rlen = sig_len >> 1; 81 82 /* 83 * Public key point must have the proper size for this curve. 84 */ 85 if (pk->qlen != cd->generator_len) { 86 return 0; 87 } 88 89 /* 90 * Get modulus; then decode the r and s values. They must be 91 * lower than the modulus, and s must not be null. 92 */ 93 nlen = cd->order_len; 94 br_i31_decode(n, cd->order, nlen); 95 n0i = br_i31_ninv31(n[1]); 96 if (!br_i31_decode_mod(r, sig, rlen, n)) { 97 return 0; 98 } 99 if (!br_i31_decode_mod(s, (const unsigned char *)sig + rlen, rlen, n)) { 100 return 0; 101 } 102 if (br_i31_iszero(s)) { 103 return 0; 104 } 105 106 /* 107 * Invert s. We do that with a modular exponentiation; we use 108 * the fact that for all the curves we support, the least 109 * significant byte is not 0 or 1, so we can subtract 2 without 110 * any carry to process. 111 * We also want 1/s in Montgomery representation, which can be 112 * done by converting _from_ Montgomery representation before 113 * the inversion (because (1/s)*R = 1/(s/R)). 114 */ 115 br_i31_from_monty(s, n, n0i); 116 memcpy(tx, cd->order, nlen); 117 tx[nlen - 1] -= 2; 118 br_i31_modpow(s, tx, nlen, n, n0i, t1, t2); 119 120 /* 121 * Truncate the hash to the modulus length (in bits) and reduce 122 * it modulo the curve order. The modular reduction can be done 123 * with a subtraction since the truncation already reduced the 124 * value to the modulus bit length. 125 */ 126 br_ecdsa_i31_bits2int(t1, hash, hash_len, n[0]); 127 br_i31_sub(t1, n, br_i31_sub(t1, n, 0) ^ 1); 128 129 /* 130 * Multiply the (truncated, reduced) hash value with 1/s, result in 131 * t2, encoded in ty. 132 */ 133 br_i31_montymul(t2, t1, s, n, n0i); 134 br_i31_encode(ty, nlen, t2); 135 136 /* 137 * Multiply r with 1/s, result in t1, encoded in tx. 138 */ 139 br_i31_montymul(t1, r, s, n, n0i); 140 br_i31_encode(tx, nlen, t1); 141 142 /* 143 * Compute the point x*Q + y*G. 144 */ 145 ulen = cd->generator_len; 146 memcpy(eU, pk->q, ulen); 147 res = impl->muladd(eU, NULL, ulen, 148 tx, nlen, ty, nlen, cd->curve); 149 150 /* 151 * Get the X coordinate, reduce modulo the curve order, and 152 * compare with the 'r' value. 153 * 154 * The modular reduction can be done with subtractions because 155 * we work with curves of prime order, so the curve order is 156 * close to the field order (Hasse's theorem). 157 */ 158 br_i31_zero(t1, n[0]); 159 br_i31_decode(t1, &eU[1], ulen >> 1); 160 t1[0] = n[0]; 161 br_i31_sub(t1, n, br_i31_sub(t1, n, 0) ^ 1); 162 res &= ~br_i31_sub(t1, r, 1); 163 res &= br_i31_iszero(t1); 164 return res; 165 } 166