153 lines
4.1 KiB
C
153 lines
4.1 KiB
C
/*
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* Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
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*
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* Permission is hereby granted, free of charge, to any person obtaining
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* a copy of this software and associated documentation files (the
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* "Software"), to deal in the Software without restriction, including
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* without limitation the rights to use, copy, modify, merge, publish,
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* distribute, sublicense, and/or sell copies of the Software, and to
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* permit persons to whom the Software is furnished to do so, subject to
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* the following conditions:
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*
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* The above copyright notice and this permission notice shall be
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* included in all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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* SOFTWARE.
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*/
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#include "inner.h"
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/*
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* Recompute public exponent, based on factor p and reduced private
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* exponent dp.
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*/
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static uint32_t
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get_pubexp(const unsigned char *pbuf, size_t plen,
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const unsigned char *dpbuf, size_t dplen)
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{
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/*
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* dp is the inverse of e modulo p-1. If p = 3 mod 4, then
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* p-1 = 2*((p-1)/2). Taken modulo 2, e is odd and has inverse 1;
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* thus, dp must be odd.
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*
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* We compute the inverse of dp modulo (p-1)/2. This requires
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* first reducing dp modulo (p-1)/2 (this can be done with a
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* conditional subtract, no need to use the generic modular
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* reduction function); then, we use moddiv.
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*/
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uint32_t tmp[6 * ((BR_MAX_RSA_FACTOR + 61) / 31)];
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uint32_t *p, *dp, *x;
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size_t len;
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uint32_t e;
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/*
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* Compute actual factor length (in bytes) and check that it fits
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* under our size constraints.
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*/
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while (plen > 0 && *pbuf == 0) {
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pbuf ++;
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plen --;
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}
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if (plen == 0 || plen < 5 || plen > (BR_MAX_RSA_FACTOR / 8)) {
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return 0;
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}
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/*
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* Compute actual reduced exponent length (in bytes) and check that
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* it is not longer than p.
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*/
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while (dplen > 0 && *dpbuf == 0) {
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dpbuf ++;
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dplen --;
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}
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if (dplen > plen || dplen == 0
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|| (dplen == plen && dpbuf[0] > pbuf[0]))
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{
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return 0;
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}
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/*
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* Verify that p = 3 mod 4 and that dp is odd.
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*/
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if ((pbuf[plen - 1] & 3) != 3 || (dpbuf[dplen - 1] & 1) != 1) {
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return 0;
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}
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/*
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* Decode p and compute (p-1)/2.
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*/
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p = tmp;
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br_i31_decode(p, pbuf, plen);
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len = (p[0] + 63) >> 5;
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br_i31_rshift(p, 1);
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/*
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* Decode dp and make sure its announced bit length matches that of
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* p (we already know that the size of dp, in bits, does not exceed
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* the size of p, so we just have to copy the header word).
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*/
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dp = p + len;
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memset(dp, 0, len * sizeof *dp);
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br_i31_decode(dp, dpbuf, dplen);
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dp[0] = p[0];
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/*
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* Subtract (p-1)/2 from dp if necessary.
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*/
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br_i31_sub(dp, p, NOT(br_i31_sub(dp, p, 0)));
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/*
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* If another subtraction is needed, then this means that the
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* value was invalid. We don't care to leak information about
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* invalid keys.
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*/
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if (br_i31_sub(dp, p, 0) == 0) {
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return 0;
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}
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/*
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* Invert dp modulo (p-1)/2. If the inversion fails, then the
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* key value was invalid.
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*/
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x = dp + len;
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br_i31_zero(x, p[0]);
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x[1] = 1;
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if (br_i31_moddiv(x, dp, p, br_i31_ninv31(p[1]), x + len) == 0) {
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return 0;
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}
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/*
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* We now have an inverse. We must set it to zero (error) if its
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* length is greater than 32 bits and/or if it is an even integer.
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* Take care that the bit_length function returns an encoded
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* bit length.
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*/
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e = (uint32_t)x[1] | ((uint32_t)x[2] << 31);
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e &= -LT(br_i31_bit_length(x + 1, len - 1), 34);
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e &= -(e & 1);
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return e;
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}
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/* see bearssl_rsa.h */
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uint32_t
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br_rsa_i31_compute_pubexp(const br_rsa_private_key *sk)
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{
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/*
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* Get the public exponent from both p and q. This is the right
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* exponent if we get twice the same value.
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*/
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uint32_t ep, eq;
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ep = get_pubexp(sk->p, sk->plen, sk->dp, sk->dplen);
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eq = get_pubexp(sk->q, sk->qlen, sk->dq, sk->dqlen);
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return ep & -EQ(ep, eq);
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}
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