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fix: add canonical checks for fr/fq in U256Codec::deserialize_from_fields #21811

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Merged
iakovenkos merged 1 commit into from
Mar 20, 2026
Merged

fix: add canonical checks for fr/fq in U256Codec::deserialize_from_fields #21811

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9 changes: 8 additions & 1 deletion barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -250,7 +250,14 @@ class U256Codec {
BB_ASSERT_EQ(vec.size(), calc_num_fields<T>());
if constexpr (IsAnyOf<T, bool>) {
return static_cast<bool>(vec[0]);
} else if constexpr (IsAnyOf<T, uint32_t, uint64_t, uint256_t, bb::fr, fq>) {
} else if constexpr (IsAnyOf<T, bb::fr>) {
BB_ASSERT_LT(
vec[0], uint256_t(bb::fr::modulus), "Non-canonical scalar field element: value >= fr::modulus");
return static_cast<T>(vec[0]);
} else if constexpr (IsAnyOf<T, fq>) {
BB_ASSERT_LT(vec[0], uint256_t(fq::modulus), "Non-canonical base field element: value >= fq::modulus");
return static_cast<T>(vec[0]);
} else if constexpr (IsAnyOf<T, uint32_t, uint64_t, uint256_t>) {
return static_cast<T>(vec[0]);
} else if constexpr (IsAnyOf<T, bn254_commitment, grumpkin_commitment>) {
using BaseField = typename T::Fq;
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84 changes: 84 additions & 0 deletions barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -271,4 +271,88 @@ TEST_F(FieldConversionTest, RejectPointNotOnCurve)
}
#endif

// ============================================================================
// U256Codec canonical checks
// ============================================================================

/**
* @brief Test that U256Codec accepts canonical field elements.
*/
TEST_F(FieldConversionTest, U256CodecAcceptsCanonicalFr)
{
// Zero
{
std::vector<uint256_t> vec = { uint256_t(0) };
auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
EXPECT_EQ(result, bb::fr(0));
}
// One
{
std::vector<uint256_t> vec = { uint256_t(1) };
auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
EXPECT_EQ(result, bb::fr(1));
}
// modulus - 1 (largest canonical value)
{
std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) - 1 };
auto result = U256Codec::deserialize_from_fields<bb::fr>(vec);
EXPECT_EQ(result, bb::fr(bb::fr::modulus - 1));
}
}

/**
* @brief Test that U256Codec accepts canonical fq elements.
*/
TEST_F(FieldConversionTest, U256CodecAcceptsCanonicalFq)
{
using fq = grumpkin::fr;
std::vector<uint256_t> vec = { uint256_t(0) };
auto result = U256Codec::deserialize_from_fields<fq>(vec);
EXPECT_EQ(result, fq(0));

vec = { uint256_t(fq::modulus) - 1 };
result = U256Codec::deserialize_from_fields<fq>(vec);
EXPECT_EQ(result, fq(fq::modulus - 1));
}

/**
* @brief Test that U256Codec rejects non-canonical field elements (>= modulus).
* @details Security-critical: prevents Fiat-Shamir challenge grinding in Keccak transcript.
*/
#ifndef __wasm__
TEST_F(FieldConversionTest, U256CodecRejectsNonCanonicalFr)
{
// fr::modulus itself (v + 0*p where v=0 but encoded as p)
{
std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) };
EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
}
// fr::modulus + 1
{
std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) + 1 };
EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
}
// 2 * fr::modulus (another alias for 0)
{
std::vector<uint256_t> vec = { uint256_t(bb::fr::modulus) * 2 };
EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<bb::fr>(vec), "Non-canonical");
}
}

TEST_F(FieldConversionTest, U256CodecRejectsNonCanonicalFq)
{
using fq = grumpkin::fr;
// fq::modulus itself
{
std::vector<uint256_t> vec = { uint256_t(fq::modulus) };
EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<fq>(vec), "Non-canonical");
}
// fq::modulus + 1
{
std::vector<uint256_t> vec = { uint256_t(fq::modulus) + 1 };
EXPECT_THROW_OR_ABORT(U256Codec::deserialize_from_fields<fq>(vec), "Non-canonical");
}
}
#endif

} // namespace bb::field_conversion_tests
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