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 /* 28 * As a strict minimum, we need four buffers that can hold a 29 * modular integer. 30 */ 31 #define TLEN (4 * (2 + ((BR_MAX_RSA_SIZE + 30) / 31))) 32 33 /* see bearssl_rsa.h */ 34 uint32_t 35 br_rsa_i31_public(unsigned char *x, size_t xlen, 36 const br_rsa_public_key *pk) 37 { 38 const unsigned char *n; 39 size_t nlen; 40 uint32_t tmp[1 + TLEN]; 41 uint32_t *m, *a, *t; 42 size_t fwlen; 43 long z; 44 uint32_t m0i, r; 45 46 /* 47 * Get the actual length of the modulus, and see if it fits within 48 * our stack buffer. We also check that the length of x[] is valid. 49 */ 50 n = pk->n; 51 nlen = pk->nlen; 52 while (nlen > 0 && *n == 0) { 53 n ++; 54 nlen --; 55 } 56 if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) { 57 return 0; 58 } 59 z = (long)nlen << 3; 60 fwlen = 1; 61 while (z > 0) { 62 z -= 31; 63 fwlen ++; 64 } 65 /* 66 * Round up length to an even number. 67 */ 68 fwlen += (fwlen & 1); 69 70 /* 71 * The modulus gets decoded into m[]. 72 * The value to exponentiate goes into a[]. 73 * The temporaries for modular exponentiation are in t[]. 74 */ 75 m = tmp; 76 a = m + fwlen; 77 t = m + 2 * fwlen; 78 79 /* 80 * Decode the modulus. 81 */ 82 br_i31_decode(m, n, nlen); 83 m0i = br_i31_ninv31(m[1]); 84 85 /* 86 * Note: if m[] is even, then m0i == 0. Otherwise, m0i must be 87 * an odd integer. 88 */ 89 r = m0i & 1; 90 91 /* 92 * Decode x[] into a[]; we also check that its value is proper. 93 */ 94 r &= br_i31_decode_mod(a, x, xlen, m); 95 96 /* 97 * Compute the modular exponentiation. 98 */ 99 br_i31_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen); 100 101 /* 102 * Encode the result. 103 */ 104 br_i31_encode(x, xlen, a); 105 return r; 106 } 107