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fix: Soundness issue in bigfield's evaluate_multiply_add method #558

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merged 2 commits into from
Jun 27, 2023
Merged

fix: Soundness issue in bigfield's evaluate_multiply_add method #558

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67b1f2b
Fixing bigfield yet again
Rumata888 Jun 26, 2023
7194dd1
Remove the namespace comment
Rumata888 Jun 26, 2023
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56 changes: 29 additions & 27 deletions cpp/src/barretenberg/stdlib/primitives/bigfield/bigfield_impl.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2148,35 +2148,37 @@ void bigfield<C, T>::unsafe_evaluate_multiply_add(const bigfield& input_left,
linear_terms =
linear_terms.add_two(-remainders[2 * i].prime_basis_limb, -remainders[2 * i + 1].prime_basis_limb);
}
}
if ((remainders.size() & 1UL) == 1UL) {
linear_terms += -remainders[remainders.size() - 1].prime_basis_limb;
}
// This is where we show our identity is zero mod r (to use CRT we show it's zero mod r and mod 2^t)
field_t<C>::evaluate_polynomial_identity(
left.prime_basis_limb, to_mul.prime_basis_limb, quotient.prime_basis_limb * neg_prime, linear_terms);

// This is where we show our identity is zero mod r (to use CRT we show it's zero mod r and mod 2^t)
field_t<C>::evaluate_polynomial_identity(
left.prime_basis_limb, to_mul.prime_basis_limb, quotient.prime_basis_limb * neg_prime, linear_terms);

const uint64_t carry_lo_msb = max_lo_bits - (2 * NUM_LIMB_BITS);
const uint64_t carry_hi_msb = max_hi_bits - (2 * NUM_LIMB_BITS);
const uint64_t carry_lo_msb = max_lo_bits - (2 * NUM_LIMB_BITS);
const uint64_t carry_hi_msb = max_hi_bits - (2 * NUM_LIMB_BITS);

const barretenberg::fr carry_lo_shift(uint256_t(uint256_t(1) << carry_lo_msb));
if ((carry_hi_msb + carry_lo_msb) < field_t<C>::modulus.get_msb()) {
field_t carry_combined = carry_lo + (carry_hi * carry_lo_shift);
carry_combined = carry_combined.normalize();
const auto accumulators = ctx->decompose_into_base4_accumulators(
carry_combined.witness_index,
static_cast<size_t>(carry_lo_msb + carry_hi_msb),
"bigfield: carry_combined too large in unsafe_evaluate_multiply_add.");
field_t<C> accumulator_midpoint =
field_t<C>::from_witness_index(ctx, accumulators[static_cast<size_t>((carry_hi_msb / 2) - 1)]);
carry_hi.assert_equal(accumulator_midpoint, "bigfield multiply range check failed");
} else {
carry_lo = carry_lo.normalize();
carry_hi = carry_hi.normalize();
ctx->decompose_into_base4_accumulators(carry_lo.witness_index,
static_cast<size_t>(carry_lo_msb),
"bigfield: carry_lo too large in unsafe_evaluate_multiply_add.");
ctx->decompose_into_base4_accumulators(carry_hi.witness_index,
static_cast<size_t>(carry_hi_msb),
"bigfield: carry_hi too large in unsafe_evaluate_multiply_add.");
}
const barretenberg::fr carry_lo_shift(uint256_t(uint256_t(1) << carry_lo_msb));
if ((carry_hi_msb + carry_lo_msb) < field_t<C>::modulus.get_msb()) {
field_t carry_combined = carry_lo + (carry_hi * carry_lo_shift);
carry_combined = carry_combined.normalize();
const auto accumulators = ctx->decompose_into_base4_accumulators(
carry_combined.witness_index,
static_cast<size_t>(carry_lo_msb + carry_hi_msb),
"bigfield: carry_combined too large in unsafe_evaluate_multiply_add.");
field_t<C> accumulator_midpoint =
field_t<C>::from_witness_index(ctx, accumulators[static_cast<size_t>((carry_hi_msb / 2) - 1)]);
carry_hi.assert_equal(accumulator_midpoint, "bigfield multiply range check failed");
} else {
carry_lo = carry_lo.normalize();
carry_hi = carry_hi.normalize();
ctx->decompose_into_base4_accumulators(carry_lo.witness_index,
static_cast<size_t>(carry_lo_msb),
"bigfield: carry_lo too large in unsafe_evaluate_multiply_add.");
ctx->decompose_into_base4_accumulators(carry_hi.witness_index,
static_cast<size_t>(carry_hi_msb),
"bigfield: carry_hi too large in unsafe_evaluate_multiply_add.");
}
}
}
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