53 lines
2.1 KiB
C
53 lines
2.1 KiB
C
/* Copyright (c) 2014, Intel Corporation.
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*
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* Permission to use, copy, modify, and/or distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
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* SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
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* OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
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* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
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#include "ecp_nistz.h"
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#if defined(__GNUC__)
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#pragma GCC diagnostic ignored "-Wconversion"
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#endif
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/* Fills |str| with the bytewise little-endian encoding of |scalar|, where
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* |scalar| has |num_limbs| limbs. |str| is padded with zeros at the end up
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* to |str_len| bytes. Actually, |str_len| must be exactly one byte more than
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* needed to encode |num_limbs| losslessly, so that there is an extra byte at
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* the end. The extra byte is useful because the caller will be breaking |str|
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* up into windows of a number of bits (5 or 7) that isn't divisible by 8, and
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* so it is useful for it to be able to read an extra zero byte. */
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void gfp_little_endian_bytes_from_scalar(uint8_t str[], size_t str_len,
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const Limb scalar[],
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size_t num_limbs) {
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debug_assert_nonsecret(str_len == (num_limbs * sizeof(Limb)) + 1);
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size_t i;
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for (i = 0; i < num_limbs * sizeof(Limb); i += sizeof(Limb)) {
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Limb d = scalar[i / sizeof(Limb)];
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str[i + 0] = d & 0xff;
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str[i + 1] = (d >> 8) & 0xff;
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str[i + 2] = (d >> 16) & 0xff;
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str[i + 3] = (d >>= 24) & 0xff;
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if (sizeof(Limb) == 8) {
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d >>= 8;
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str[i + 4] = d & 0xff;
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str[i + 5] = (d >> 8) & 0xff;
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str[i + 6] = (d >> 16) & 0xff;
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str[i + 7] = (d >> 24) & 0xff;
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}
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}
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for (; i < str_len; i++) {
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str[i] = 0;
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}
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}
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