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Optimization: use Nx32 representation for recoded bits

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The existing code needs to deal with the edge case that bit_pos >= 256,
which would lead to an out-of-bounds read from secp256k1_scalar.

Instead, recode the scalar into an array of uint32_t with enough zero
padding at the end to alleviate the issue. This also simplifies the
code, and is necessary for a security improvement in a follow-up
commit.

Original code by Peter Dettman, with modifications by Pieter Wuille.
This commit is contained in:
Peter Dettman
2021-12-28 18:13:43 -05:00
committed by Pieter Wuille
parent e03dcc44b5
commit a0d32b597d
1 changed files with 13 additions and 4 deletions
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17
src/ecmult_gen_impl.h
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@@ -56,7 +56,11 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
secp256k1_fe neg;
secp256k1_ge_storage adds;
secp256k1_scalar d;
int first = 1;
/* Array of uint32_t values large enough to store COMB_BITS bits. Only the bottom
* 8 are ever nonzero, but having the zero padding at the end if COMB_BITS>256
* avoids the need to deal with out-of-bounds reads from a scalar. */
uint32_t recoded[(COMB_BITS + 31) >> 5] = {0};
int first = 1, i;
memset(&adds, 0, sizeof(adds));
@@ -103,6 +107,11 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
/* Compute the scalar d = (gn + ctx->scalar_offset). */
secp256k1_scalar_add(&d, &ctx->scalar_offset, gn);
/* Convert to recoded array. */
for (i = 0; i < 8; ++i) {
recoded[i] = secp256k1_scalar_get_bits_limb32(&d, 32 * i, 32);
}
secp256k1_scalar_clear(&d);
/* In secp256k1_ecmult_gen_prec_table we have precomputed sums of the
* (2*d[i]-1) * 2^(i-1) * G points, for various combinations of i positions.
@@ -188,8 +197,8 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
/* Gather the mask(block)-selected bits of d into bits. They're packed:
* bits[tooth] = d[(block*COMB_TEETH + tooth)*COMB_SPACING + comb_off]. */
uint32_t bits = 0, sign, abs, index, tooth;
for (tooth = 0; tooth < COMB_TEETH && bit_pos < 256; ++tooth) {
uint32_t bit = secp256k1_scalar_get_bits_limb32(&d, bit_pos, 1);
for (tooth = 0; tooth < COMB_TEETH; ++tooth) {
uint32_t bit = (recoded[bit_pos >> 5] >> (bit_pos & 0x1f)) & 1;
bits |= bit << tooth;
bit_pos += COMB_SPACING;
}
@@ -243,7 +252,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
secp256k1_fe_clear(&neg);
secp256k1_ge_clear(&add);
memset(&adds, 0, sizeof(adds));
secp256k1_scalar_clear(&d);
memset(&recoded, 0, sizeof(recoded));
}
/* Setup blinding values for secp256k1_ecmult_gen. */