diff options
Diffstat (limited to 'third_party/bearssl/src/i31_decmod.c')
| -rw-r--r-- | third_party/bearssl/src/i31_decmod.c | 124 |
1 files changed, 124 insertions, 0 deletions
diff --git a/third_party/bearssl/src/i31_decmod.c b/third_party/bearssl/src/i31_decmod.c new file mode 100644 index 0000000..3cd7bfe --- /dev/null +++ b/third_party/bearssl/src/i31_decmod.c @@ -0,0 +1,124 @@ +/* + * 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 */ +uint32_t +br_i31_decode_mod(uint32_t *x, const void *src, size_t len, const uint32_t *m) +{ + /* + * Two-pass algorithm: in the first pass, we determine whether the + * value fits; in the second pass, we do the actual write. + * + * During the first pass, 'r' contains the comparison result so + * far: + * 0x00000000 value is equal to the modulus + * 0x00000001 value is greater than the modulus + * 0xFFFFFFFF value is lower than the modulus + * + * Since we iterate starting with the least significant bytes (at + * the end of src[]), each new comparison overrides the previous + * except when the comparison yields 0 (equal). + * + * During the second pass, 'r' is either 0xFFFFFFFF (value fits) + * or 0x00000000 (value does not fit). + * + * We must iterate over all bytes of the source, _and_ possibly + * some extra virtual bytes (with value 0) so as to cover the + * complete modulus as well. We also add 4 such extra bytes beyond + * the modulus length because it then guarantees that no accumulated + * partial word remains to be processed. + */ + const unsigned char *buf; + size_t mlen, tlen; + int pass; + uint32_t r; + + buf = src; + mlen = (m[0] + 31) >> 5; + tlen = (mlen << 2); + if (tlen < len) { + tlen = len; + } + tlen += 4; + r = 0; + for (pass = 0; pass < 2; pass ++) { + size_t u, v; + uint32_t acc; + int acc_len; + + v = 1; + acc = 0; + acc_len = 0; + for (u = 0; u < tlen; u ++) { + uint32_t b; + + if (u < len) { + b = buf[len - 1 - u]; + } else { + b = 0; + } + acc |= (b << acc_len); + acc_len += 8; + if (acc_len >= 31) { + uint32_t xw; + + xw = acc & (uint32_t)0x7FFFFFFF; + acc_len -= 31; + acc = b >> (8 - acc_len); + if (v <= mlen) { + if (pass) { + x[v] = r & xw; + } else { + uint32_t cc; + + cc = (uint32_t)CMP(xw, m[v]); + r = MUX(EQ(cc, 0), r, cc); + } + } else { + if (!pass) { + r = MUX(EQ(xw, 0), r, 1); + } + } + v ++; + } + } + + /* + * When we reach this point at the end of the first pass: + * r is either 0, 1 or -1; we want to set r to 0 if it + * is equal to 0 or 1, and leave it to -1 otherwise. + * + * When we reach this point at the end of the second pass: + * r is either 0 or -1; we want to leave that value + * untouched. This is a subcase of the previous. + */ + r >>= 1; + r |= (r << 1); + } + + x[0] = m[0]; + return r & (uint32_t)1; +} |