diff options
Diffstat (limited to 'third_party/bearssl/src/i31_montmul.c')
| -rw-r--r-- | third_party/bearssl/src/i31_montmul.c | 127 |
1 files changed, 127 insertions, 0 deletions
diff --git a/third_party/bearssl/src/i31_montmul.c b/third_party/bearssl/src/i31_montmul.c new file mode 100644 index 0000000..758f8f4 --- /dev/null +++ b/third_party/bearssl/src/i31_montmul.c @@ -0,0 +1,127 @@ +/* + * Copyright (c) 2016 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 */ +void +br_i31_montymul(uint32_t *d, const uint32_t *x, const uint32_t *y, + const uint32_t *m, uint32_t m0i) +{ + /* + * Each outer loop iteration computes: + * d <- (d + xu*y + f*m) / 2^31 + * We have xu <= 2^31-1 and f <= 2^31-1. + * Thus, if d <= 2*m-1 on input, then: + * 2*m-1 + 2*(2^31-1)*m <= (2^32)*m-1 + * and the new d value is less than 2*m. + * + * We represent d over 31-bit words, with an extra word 'dh' + * which can thus be only 0 or 1. + */ + size_t len, len4, u, v; + uint32_t dh; + + len = (m[0] + 31) >> 5; + len4 = len & ~(size_t)3; + br_i31_zero(d, m[0]); + dh = 0; + for (u = 0; u < len; u ++) { + /* + * The carry for each operation fits on 32 bits: + * d[v+1] <= 2^31-1 + * xu*y[v+1] <= (2^31-1)*(2^31-1) + * f*m[v+1] <= (2^31-1)*(2^31-1) + * r <= 2^32-1 + * (2^31-1) + 2*(2^31-1)*(2^31-1) + (2^32-1) = 2^63 - 2^31 + * After division by 2^31, the new r is then at most 2^32-1 + * + * Using a 32-bit carry has performance benefits on 32-bit + * systems; however, on 64-bit architectures, we prefer to + * keep the carry (r) in a 64-bit register, thus avoiding some + * "clear high bits" operations. + */ + uint32_t f, xu; +#if BR_64 + uint64_t r; +#else + uint32_t r; +#endif + + xu = x[u + 1]; + f = MUL31_lo((d[1] + MUL31_lo(x[u + 1], y[1])), m0i); + + r = 0; + for (v = 0; v < len4; v += 4) { + uint64_t z; + + z = (uint64_t)d[v + 1] + MUL31(xu, y[v + 1]) + + MUL31(f, m[v + 1]) + r; + r = z >> 31; + d[v + 0] = (uint32_t)z & 0x7FFFFFFF; + z = (uint64_t)d[v + 2] + MUL31(xu, y[v + 2]) + + MUL31(f, m[v + 2]) + r; + r = z >> 31; + d[v + 1] = (uint32_t)z & 0x7FFFFFFF; + z = (uint64_t)d[v + 3] + MUL31(xu, y[v + 3]) + + MUL31(f, m[v + 3]) + r; + r = z >> 31; + d[v + 2] = (uint32_t)z & 0x7FFFFFFF; + z = (uint64_t)d[v + 4] + MUL31(xu, y[v + 4]) + + MUL31(f, m[v + 4]) + r; + r = z >> 31; + d[v + 3] = (uint32_t)z & 0x7FFFFFFF; + } + for (; v < len; v ++) { + uint64_t z; + + z = (uint64_t)d[v + 1] + MUL31(xu, y[v + 1]) + + MUL31(f, m[v + 1]) + r; + r = z >> 31; + d[v] = (uint32_t)z & 0x7FFFFFFF; + } + + /* + * Since the new dh can only be 0 or 1, the addition of + * the old dh with the carry MUST fit on 32 bits, and + * thus can be done into dh itself. + */ + dh += r; + d[len] = dh & 0x7FFFFFFF; + dh >>= 31; + } + + /* + * We must write back the bit length because it was overwritten in + * the loop (not overwriting it would require a test in the loop, + * which would yield bigger and slower code). + */ + d[0] = m[0]; + + /* + * d[] may still be greater than m[] at that point; notably, the + * 'dh' word may be non-zero. + */ + br_i31_sub(d, m, NEQ(dh, 0) | NOT(br_i31_sub(d, m, 0))); +} |