diff options
Diffstat (limited to 'third_party/bearssl/src/i31_modpow2.c')
| -rw-r--r-- | third_party/bearssl/src/i31_modpow2.c | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/third_party/bearssl/src/i31_modpow2.c b/third_party/bearssl/src/i31_modpow2.c new file mode 100644 index 0000000..0b8f8cf --- /dev/null +++ b/third_party/bearssl/src/i31_modpow2.c @@ -0,0 +1,160 @@ +/* + * Copyright (c) 2017 Thomas Pornin <[email protected]> + * + * 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_i31_modpow_opt(uint32_t *x, + const unsigned char *e, size_t elen, + const uint32_t *m, uint32_t m0i, uint32_t *tmp, size_t twlen) +{ + size_t mlen, mwlen; + uint32_t *t1, *t2, *base; + size_t u, v; + uint32_t acc; + int acc_len, win_len; + + /* + * Get modulus size. + */ + mwlen = (m[0] + 63) >> 5; + 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_i31_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_i31_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_i31_zero(x, m[0]); + x[(m[0] + 31) >> 5] = 1; + br_i31_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_i31_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_i31_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_i31_montymul(t1, x, t2, m, m0i); + CCOPY(NEQ(bits, 0), x, t1, mlen); + } + + /* + * Convert back from Montgomery representation, and exit. + */ + br_i31_from_monty(x, m, m0i); + return 1; +} |