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/*
* Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/*
* Recompute public exponent, based on factor p and reduced private
* exponent dp.
*/
static uint32_t
get_pubexp(const unsigned char *pbuf, size_t plen,
const unsigned char *dpbuf, size_t dplen)
{
/*
* dp is the inverse of e modulo p-1. If p = 3 mod 4, then
* p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
* thus, dp must be odd.
*
* We compute the inverse of dp modulo (p-1)/2. This requires
* first reducing dp modulo (p-1)/2 (this can be done with a
* conditional subtract, no need to use the generic modular
* reduction function); then, we use moddiv.
*/
uint16_t tmp[6 * ((BR_MAX_RSA_FACTOR + 29) / 15)];
uint16_t *p, *dp, *x;
size_t len;
uint32_t e;
/*
* Compute actual factor length (in bytes) and check that it fits
* under our size constraints.
*/
while (plen > 0 && *pbuf == 0) {
pbuf ++;
plen --;
}
if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
return 0;
}
/*
* Compute actual reduced exponent length (in bytes) and check that
* it is not longer than p.
*/
while (dplen > 0 && *dpbuf == 0) {
dpbuf ++;
dplen --;
}
if (dplen > plen || dplen == 0
|| (dplen == plen && dpbuf[0] > pbuf[0]))
{
return 0;
}
/*
* Verify that p = 3 mod 4 and that dp is odd.
*/
if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
return 0;
}
/*
* Decode p and compute (p-1)/2.
*/
p = tmp;
br_i15_decode(p, pbuf, plen);
len = (p[0] + 31) >> 4;
br_i15_rshift(p, 1);
/*
* Decode dp and make sure its announced bit length matches that of
* p (we already know that the size of dp, in bits, does not exceed
* the size of p, so we just have to copy the header word).
*/
dp = p + len;
memset(dp, 0, len * sizeof *dp);
br_i15_decode(dp, dpbuf, dplen);
dp[0] = p[0];
/*
* Subtract (p-1)/2 from dp if necessary.
*/
br_i15_sub(dp, p, NOT(br_i15_sub(dp, p, 0)));
/*
* If another subtraction is needed, then this means that the
* value was invalid. We don't care to leak information about
* invalid keys.
*/
if (br_i15_sub(dp, p, 0) == 0) {
return 0;
}
/*
* Invert dp modulo (p-1)/2. If the inversion fails, then the
* key value was invalid.
*/
x = dp + len;
br_i15_zero(x, p[0]);
x[1] = 1;
if (br_i15_moddiv(x, dp, p, br_i15_ninv15(p[1]), x + len) == 0) {
return 0;
}
/*
* We now have an inverse. We must set it to zero (error) if its
* length is greater than 32 bits and/or if it is an even integer.
* Take care that the bit_length function returns an encoded
* bit length.
*/
e = (uint32_t)x[1] | ((uint32_t)x[2] << 15) | ((uint32_t)x[3] << 30);
e &= -LT(br_i15_bit_length(x + 1, len - 1), 35);
e &= -(e & 1);
return e;
}
/* see bearssl_rsa.h */
uint32_t
br_rsa_i15_compute_pubexp(const br_rsa_private_key *sk)
{
/*
* Get the public exponent from both p and q. This is the right
* exponent if we get twice the same value.
*/
uint32_t ep, eq;
ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
return ep & -EQ(ep, eq);
}
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