_ Git - BearSSL/blob - src/rsa/rsa_i15_pubexp.c

   1 /*
   2  * Copyright (c) 2018 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  * Recompute public exponent, based on factor p and reduced private
  29  * exponent dp.
  30  */
  31 static uint32_t
  32 get_pubexp(const unsigned char *pbuf, size_t plen,
  33         const unsigned char *dpbuf, size_t dplen)
  34 {
  35         /*
  36          * dp is the inverse of e modulo p-1. If p = 3 mod 4, then
  37          * p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
  38          * thus, dp must be odd.
  39          *
  40          * We compute the inverse of dp modulo (p-1)/2. This requires
  41          * first reducing dp modulo (p-1)/2 (this can be done with a
  42          * conditional subtract, no need to use the generic modular
  43          * reduction function); then, we use moddiv.
  44          */
  45
  46         uint16_t tmp[6 * ((BR_MAX_RSA_FACTOR + 29) / 15)];
  47         uint16_t *p, *dp, *x;
  48         size_t len;
  49         uint32_t e;
  50
  51         /*
  52          * Compute actual factor length (in bytes) and check that it fits
  53          * under our size constraints.
  54          */
  55         while (plen > 0 && *pbuf == 0) {
  56                 pbuf ++;
  57                 plen --;
  58         }
  59         if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
  60                 return 0;
  61         }
  62
  63         /*
  64          * Compute actual reduced exponent length (in bytes) and check that
  65          * it is not longer than p.
  66          */
  67         while (dplen > 0 && *dpbuf == 0) {
  68                 dpbuf ++;
  69                 dplen --;
  70         }
  71         if (dplen > plen || dplen == 0
  72                 || (dplen == plen && dpbuf[0] > pbuf[0]))
  73         {
  74                 return 0;
  75         }
  76
  77         /*
  78          * Verify that p = 3 mod 4 and that dp is odd.
  79          */
  80         if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
  81                 return 0;
  82         }
  83
  84         /*
  85          * Decode p and compute (p-1)/2.
  86          */
  87         p = tmp;
  88         br_i15_decode(p, pbuf, plen);
  89         len = (p[0] + 31) >> 4;
  90         br_i15_rshift(p, 1);
  91
  92         /*
  93          * Decode dp and make sure its announced bit length matches that of
  94          * p (we already know that the size of dp, in bits, does not exceed
  95          * the size of p, so we just have to copy the header word).
  96          */
  97         dp = p + len;
  98         memset(dp, 0, len * sizeof *dp);
  99         br_i15_decode(dp, dpbuf, dplen);
 100         dp[0] = p[0];
 101
 102         /*
 103          * Subtract (p-1)/2 from dp if necessary.
 104          */
 105         br_i15_sub(dp, p, NOT(br_i15_sub(dp, p, 0)));
 106
 107         /*
 108          * If another subtraction is needed, then this means that the
 109          * value was invalid. We don't care to leak information about
 110          * invalid keys.
 111          */
 112         if (br_i15_sub(dp, p, 0) == 0) {
 113                 return 0;
 114         }
 115
 116         /*
 117          * Invert dp modulo (p-1)/2. If the inversion fails, then the
 118          * key value was invalid.
 119          */
 120         x = dp + len;
 121         br_i15_zero(x, p[0]);
 122         x[1] = 1;
 123         if (br_i15_moddiv(x, dp, p, br_i15_ninv15(p[1]), x + len) == 0) {
 124                 return 0;
 125         }
 126
 127         /*
 128          * We now have an inverse. We must set it to zero (error) if its
 129          * length is greater than 32 bits and/or if it is an even integer.
 130          * Take care that the bit_length function returns an encoded
 131          * bit length.
 132          */
 133         e = (uint32_t)x[1] | ((uint32_t)x[2] << 15) | ((uint32_t)x[3] << 30);
 134         e &= -LT(br_i15_bit_length(x + 1, len - 1), 35);
 135         e &= -(e & 1);
 136         return e;
 137 }
 138
 139 /* see bearssl_rsa.h */
 140 uint32_t
 141 br_rsa_i15_compute_pubexp(const br_rsa_private_key *sk)
 142 {
 143         /*
 144          * Get the public exponent from both p and q. This is the right
 145          * exponent if we get twice the same value.
 146          */
 147         uint32_t ep, eq;
 148
 149         ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
 150         eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
 151         return ep & -EQ(ep, eq);
 152 }