Differential D16334 Diff 55390 head/contrib/bearssl/src/int/i15_modpow2.c

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head/contrib/bearssl/src/int/i15_modpow2.c

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				/*
				* Copyright (c) 2017 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"

				/* see inner.h */
				uint32_t
				br_i15_modpow_opt(uint16_t *x,
				const unsigned char *e, size_t elen,
				const uint16_t m, uint16_t m0i, uint16_t tmp, size_t twlen)
				{
				size_t mlen, mwlen;
				uint16_t t1, t2, *base;
				size_t u, v;
				uint32_t acc;
				int acc_len, win_len;

				/*
				* Get modulus size.
				*/
				mwlen = (m[0] + 31) >> 4;
				mlen = mwlen * sizeof m[0];
				mwlen += (mwlen & 1);
				t1 = tmp;
				t2 = tmp + mwlen;

				/*
				* Compute possible window size, with a maximum of 5 bits.
				* When the window has size 1 bit, we use a specific code
				* that requires only two temporaries. Otherwise, for a
				* window of k bits, we need 2^k+1 temporaries.
				*/
				if (twlen < (mwlen << 1)) {
				return 0;
				}
				for (win_len = 5; win_len > 1; win_len --) {
				if ((((uint32_t)1 << win_len) + 1) * mwlen <= twlen) {
				break;
				}
				}

				/*
				* Everything is done in Montgomery representation.
				*/
				br_i15_to_monty(x, m);

				/*
				* Compute window contents. If the window has size one bit only,
				* then t2 is set to x; otherwise, t2[0] is left untouched, and
				* t2[k] is set to x^k (for k >= 1).
				*/
				if (win_len == 1) {
				memcpy(t2, x, mlen);
				} else {
				memcpy(t2 + mwlen, x, mlen);
				base = t2 + mwlen;
				for (u = 2; u < ((unsigned)1 << win_len); u ++) {
				br_i15_montymul(base + mwlen, base, x, m, m0i);
				base += mwlen;
				}
				}

				/*
				* We need to set x to 1, in Montgomery representation. This can
				* be done efficiently by setting the high word to 1, then doing
				* one word-sized shift.
				*/
				br_i15_zero(x, m[0]);
				x[(m[0] + 15) >> 4] = 1;
				br_i15_muladd_small(x, 0, m);

				/*
				* We process bits from most to least significant. At each
				* loop iteration, we have acc_len bits in acc.
				*/
				acc = 0;
				acc_len = 0;
				while (acc_len > 0 \|\| elen > 0) {
				int i, k;
				uint32_t bits;

				/*
				* Get the next bits.
				*/
				k = win_len;
				if (acc_len < win_len) {
				if (elen > 0) {
				acc = (acc << 8) \| *e ++;
				elen --;
				acc_len += 8;
				} else {
				k = acc_len;
				}
				}
				bits = (acc >> (acc_len - k)) & (((uint32_t)1 << k) - 1);
				acc_len -= k;

				/*
				* We could get exactly k bits. Compute k squarings.
				*/
				for (i = 0; i < k; i ++) {
				br_i15_montymul(t1, x, x, m, m0i);
				memcpy(x, t1, mlen);
				}

				/*
				* Window lookup: we want to set t2 to the window
				* lookup value, assuming the bits are non-zero. If
				* the window length is 1 bit only, then t2 is
				* already set; otherwise, we do a constant-time lookup.
				*/
				if (win_len > 1) {
				br_i15_zero(t2, m[0]);
				base = t2 + mwlen;
				for (u = 1; u < ((uint32_t)1 << k); u ++) {
				uint32_t mask;

				mask = -EQ(u, bits);
				for (v = 1; v < mwlen; v ++) {
				t2[v] \|= mask & base[v];
				}
				base += mwlen;
				}
				}

				/*
				* Multiply with the looked-up value. We keep the
				* product only if the exponent bits are not all-zero.
				*/
				br_i15_montymul(t1, x, t2, m, m0i);
				CCOPY(NEQ(bits, 0), x, t1, mlen);
				}

				/*
				* Convert back from Montgomery representation, and exit.
				*/
				br_i15_from_monty(x, m, m0i);
				return 1;
				}