_ Git - BearSSL/blob - src/rsa/rsa_i15_priv.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 U      (2 + ((BR_MAX_RSA_FACTOR + 14) / 15))
  28 #define TLEN   (8 * U)
  29
  30 /* obsolete
  31 static void
  32 print_int(const char *name, const uint16_t *x)
  33 {
  34         extern int printf(const char *fmt, ...);
  35         unsigned char tmp[1000];
  36         size_t u, len;
  37
  38         len = (x[0] - (x[0] >> 4) + 7) >> 3;
  39         br_i15_encode(tmp, len, x);
  40         printf("%s = ", name);
  41         for (u = 0; u < len; u ++) {
  42                 printf("%02X", tmp[u]);
  43         }
  44         printf("\n");
  45 }
  46 */
  47
  48 /* see bearssl_rsa.h */
  49 uint32_t
  50 br_rsa_i15_private(unsigned char *x, const br_rsa_private_key *sk)
  51 {
  52         const unsigned char *p, *q;
  53         size_t plen, qlen;
  54         size_t fwlen;
  55         uint16_t p0i, q0i;
  56         size_t xlen;
  57         uint16_t tmp[1 + TLEN];
  58         long z;
  59         uint16_t *mp, *mq, *s1, *s2, *t1, *t2, *t3;
  60         uint32_t r;
  61
  62         /*
  63          * Compute the actual lengths of p and q, in bytes.
  64          * These lengths are not considered secret (we cannot really hide
  65          * them anyway in constant-time code).
  66          */
  67         p = sk->p;
  68         plen = sk->plen;
  69         while (plen > 0 && *p == 0) {
  70                 p ++;
  71                 plen --;
  72         }
  73         q = sk->q;
  74         qlen = sk->qlen;
  75         while (qlen > 0 && *q == 0) {
  76                 q ++;
  77                 qlen --;
  78         }
  79
  80         /*
  81          * Compute the maximum factor length, in words.
  82          */
  83         z = (long)(plen > qlen ? plen : qlen) << 3;
  84         fwlen = 1;
  85         while (z > 0) {
  86                 z -= 15;
  87                 fwlen ++;
  88         }
  89         /*
  90          * Round up the word length to an even number.
  91          */
  92         fwlen += (fwlen & 1);
  93
  94         /*
  95          * We need to fit at least 6 values in the stack buffer.
  96          */
  97         if (6 * fwlen > TLEN) {
  98                 return 0;
  99         }
 100
 101         /*
 102          * Compute signature length (in bytes).
 103          */
 104         xlen = (sk->n_bitlen + 7) >> 3;
 105
 106         /*
 107          * Ensure 32-bit alignment for value words.
 108          */
 109         mq = tmp;
 110         if (((uintptr_t)mq & 2) == 0) {
 111                 mq ++;
 112         }
 113
 114         /*
 115          * Decode q.
 116          */
 117         br_i15_decode(mq, q, qlen);
 118
 119         /*
 120          * Compute s2 = x^dq mod q.
 121          */
 122         q0i = br_i15_ninv15(mq[1]);
 123         s2 = mq + fwlen;
 124         br_i15_decode_reduce(s2, x, xlen, mq);
 125         r = br_i15_modpow_opt(s2, sk->dq, sk->dqlen, mq, q0i,
 126                 mq + 2 * fwlen, TLEN - 2 * fwlen);
 127
 128         /*
 129          * Decode p.
 130          */
 131         mp = mq + 2 * fwlen;
 132         br_i15_decode(mp, p, plen);
 133
 134         /*
 135          * Compute s1 = x^dq mod q.
 136          */
 137         p0i = br_i15_ninv15(mp[1]);
 138         s1 = mq + 3 * fwlen;
 139         br_i15_decode_reduce(s1, x, xlen, mp);
 140         r &= br_i15_modpow_opt(s1, sk->dp, sk->dplen, mp, p0i,
 141                 mq + 4 * fwlen, TLEN - 4 * fwlen);
 142
 143         /*
 144          * Compute:
 145          *   h = (s1 - s2)*(1/q) mod p
 146          * s1 is an integer modulo p, but s2 is modulo q. PKCS#1 is
 147          * unclear about whether p may be lower than q (some existing,
 148          * widely deployed implementations of RSA don't tolerate p < q),
 149          * but we want to support that occurrence, so we need to use the
 150          * reduction function.
 151          *
 152          * Since we use br_i15_decode_reduce() for iq (purportedly, the
 153          * inverse of q modulo p), we also tolerate improperly large
 154          * values for this parameter.
 155          */
 156         t1 = mq + 4 * fwlen;
 157         t2 = mq + 5 * fwlen;
 158         br_i15_reduce(t2, s2, mp);
 159         br_i15_add(s1, mp, br_i15_sub(s1, t2, 1));
 160         br_i15_to_monty(s1, mp);
 161         br_i15_decode_reduce(t1, sk->iq, sk->iqlen, mp);
 162         br_i15_montymul(t2, s1, t1, mp, p0i);
 163
 164         /*
 165          * h is now in t2. We compute the final result:
 166          *   s = s2 + q*h
 167          * All these operations are non-modular.
 168          *
 169          * We need mq, s2 and t2. We use the t3 buffer as destination.
 170          * The buffers mp, s1 and t1 are no longer needed, so we can
 171          * reuse them for t3. Moreover, the first step of the computation
 172          * is to copy s2 into t3, after which s2 is not needed. Right
 173          * now, mq is in slot 0, s2 is in slot 1, and t2 in slot 5.
 174          * Therefore, we have ample room for t3 by simply using s2.
 175          */
 176         t3 = s2;
 177         br_i15_mulacc(t3, mq, t2);
 178
 179         /*
 180          * Encode the result. Since we already checked the value of xlen,
 181          * we can just use it right away.
 182          */
 183         br_i15_encode(x, xlen, t3);
 184
 185         /*
 186          * The only error conditions remaining at that point are invalid
 187          * values for p and q (even integers).
 188          */
 189         return p0i & q0i & r;
 190 }