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hash_to_field.c
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/*
* Copyright Supranational LLC
* Licensed under the Apache License, Version 2.0, see LICENSE for details.
* SPDX-License-Identifier: Apache-2.0
*/
#include "consts.h"
#include "sha256.h"
static const vec384 BLS12_381_RRRR = { /* RR^2 */
TO_LIMB_T(0xed48ac6bd94ca1e0), TO_LIMB_T(0x315f831e03a7adf8),
TO_LIMB_T(0x9a53352a615e29dd), TO_LIMB_T(0x34c04e5e921e1761),
TO_LIMB_T(0x2512d43565724728), TO_LIMB_T(0x0aa6346091755d4d)
};
#ifdef expand_message_xmd
void expand_message_xmd(unsigned char *bytes, size_t len_in_bytes,
const unsigned char *aug, size_t aug_len,
const unsigned char *msg, size_t msg_len,
const unsigned char *DST, size_t DST_len);
#else
static void sha256_init_Zpad(SHA256_CTX *ctx)
{
ctx->h[0] = 0xda5698beU;
ctx->h[1] = 0x17b9b469U;
ctx->h[2] = 0x62335799U;
ctx->h[3] = 0x779fbecaU;
ctx->h[4] = 0x8ce5d491U;
ctx->h[5] = 0xc0d26243U;
ctx->h[6] = 0xbafef9eaU;
ctx->h[7] = 0x1837a9d8U;
ctx->N = 64;
vec_zero(ctx->buf, sizeof(ctx->buf));
ctx->off = 0;
}
static void vec_xor(void *restrict ret, const void *restrict a,
const void *restrict b, size_t num)
{
limb_t *rp = (limb_t *)ret;
const limb_t *ap = (const limb_t *)a;
const limb_t *bp = (const limb_t *)b;
size_t i;
num /= sizeof(limb_t);
for (i = 0; i < num; i++)
rp[i] = ap[i] ^ bp[i];
}
static void expand_message_xmd(unsigned char *bytes, size_t len_in_bytes,
const unsigned char *aug, size_t aug_len,
const unsigned char *msg, size_t msg_len,
const unsigned char *DST, size_t DST_len)
{
union { limb_t align; unsigned char c[32]; } b_0;
union { limb_t align; unsigned char c[33+256+31]; } b_i;
unsigned char *p;
size_t i, b_i_bits, b_i_blocks;
SHA256_CTX ctx;
/*
* compose template for 'strxor(b_0, b_(i-1)) || I2OSP(i, 1) || DST_prime'
*/
if (DST_len > 255) {
sha256_init(&ctx);
sha256_update(&ctx, "H2C-OVERSIZE-DST-", 17);
sha256_update(&ctx, DST, DST_len);
sha256_final(b_0.c, &ctx);
DST = b_0.c, DST_len = 32;
}
b_i_blocks = ((33 + DST_len + 1 + 9) + 63) & -64;
vec_zero(b_i.c + b_i_blocks - 64, 64);
p = b_i.c + 33;
for (i = 0; i < DST_len; i++)
p[i] = DST[i];
p[i++] = (unsigned char)DST_len;
p[i++] = 0x80;
p[i+6] = p[i+5] = p[i+4] = p[i+3] = p[i+2] = p[i+1] = p[i+0] = 0;
b_i_bits = (33 + DST_len + 1) * 8;
p = b_i.c + b_i_blocks;
p[-2] = (unsigned char)(b_i_bits >> 8);
p[-1] = (unsigned char)(b_i_bits);
sha256_init_Zpad(&ctx); /* Z_pad | */
sha256_update(&ctx, aug, aug_len); /* | aug | */
sha256_update(&ctx, msg, msg_len); /* | msg | */
/* | I2OSP(len_in_bytes, 2) || I2OSP(0, 1) || DST_prime */
b_i.c[30] = (unsigned char)(len_in_bytes >> 8);
b_i.c[31] = (unsigned char)(len_in_bytes);
b_i.c[32] = 0;
sha256_update(&ctx, b_i.c + 30, 3 + DST_len + 1);
sha256_final(b_0.c, &ctx);
sha256_init_h(ctx.h);
vec_copy(b_i.c, b_0.c, 32);
++b_i.c[32];
sha256_block_data_order(ctx.h, b_i.c, b_i_blocks / 64);
sha256_emit(bytes, ctx.h);
len_in_bytes += 31; /* ell = ceil(len_in_bytes / b_in_bytes), with */
len_in_bytes /= 32; /* caller being responsible for accordingly large
* buffer. hash_to_field passes one with length
* divisible by 64, remember? which works... */
while (--len_in_bytes) {
sha256_init_h(ctx.h);
vec_xor(b_i.c, b_0.c, bytes, 32);
bytes += 32;
++b_i.c[32];
sha256_block_data_order(ctx.h, b_i.c, b_i_blocks / 64);
sha256_emit(bytes, ctx.h);
}
}
#endif
/*
* |nelems| is 'count * m' from spec
*/
static void hash_to_field(vec384 elems[], size_t nelems,
const unsigned char *aug, size_t aug_len,
const unsigned char *msg, size_t msg_len,
const unsigned char *DST, size_t DST_len)
{
size_t L = sizeof(vec384) + 128/8; /* ceil((ceil(log2(p)) + k) / 8) */
size_t len_in_bytes = L * nelems; /* divisible by 64, hurray! */
#if !defined(__STDC_VERSION__) || __STDC_VERSION__<199901 \
|| defined(__STDC_NO_VLA__)
limb_t *pseudo_random = alloca(len_in_bytes);
#else
limb_t pseudo_random[len_in_bytes/sizeof(limb_t)];
#endif
unsigned char *bytes;
vec768 elem;
aug_len = aug!=NULL ? aug_len : 0;
DST_len = DST!=NULL ? DST_len : 0;
expand_message_xmd((unsigned char *)pseudo_random, len_in_bytes,
aug, aug_len, msg, msg_len, DST, DST_len);
vec_zero(elem, sizeof(elem));
bytes = (unsigned char *)pseudo_random;
while (nelems--) {
limbs_from_be_bytes(elem, bytes, L);
bytes += L;
/*
* L-bytes block % P, output is in Montgomery domain...
*/
redc_mont_384(elems[0], elem, BLS12_381_P, p0);
mul_mont_384(elems[0], elems[0], BLS12_381_RRRR, BLS12_381_P, p0);
elems++;
}
}
void blst_expand_message_xmd(unsigned char *bytes, size_t len_in_bytes,
const unsigned char *msg, size_t msg_len,
const unsigned char *DST, size_t DST_len)
{
size_t buf_len = (len_in_bytes+31) & ((size_t)0-32);
unsigned char *buf_ptr = bytes;
if (buf_len > 255*32)
return;
if (buf_len != len_in_bytes)
buf_ptr = alloca(buf_len);
expand_message_xmd(buf_ptr, len_in_bytes, NULL, 0, msg, msg_len,
DST, DST_len);
if (buf_ptr != bytes) {
unsigned char *ptr = buf_ptr;
while (len_in_bytes--)
*bytes++ = *ptr++;
vec_zero(buf_ptr, buf_len);
}
}