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feat: add poseidon2 hashing to native transcript (AztecProtocol#3718)
Change the transcript proof to be a vector of field elements instead of a vector of bytes. We want this more crucially for the stdlib transcript, but the native transcript should be implemented similarly. The key contribution is the conversion functions in field_conversion.hpp/cpp, which allow for types, in particular grumpkin::fr, to be converted to and from bb::fr elements. Some notes about why this was chosen: https://hackmd.io/2XU21v9WTx6XUSnMgZIUig. In short, it is the most efficient in terms of the number of gates. Updates the transcript deserialization/serialization and corresponding tests and makes other style updates. Resolves AztecProtocol/barretenberg#777 Also some notes from trying to resolve bb.js ACIR tests: https://hackmd.io/OrQUD_nTQLeIZVvy4zpzHA
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barretenberg/cpp/src/barretenberg/ecc/fields/field_conversion.cpp
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| #include "barretenberg/ecc/fields/field_conversion.hpp" | ||
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| namespace bb::field_conversion { | ||
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| static constexpr uint64_t NUM_CONVERSION_LIMB_BITS = 68; // set to be 68 because bigfield has 68 bit limbs | ||
| static constexpr uint64_t TOTAL_BITS = 254; | ||
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| bb::fr convert_from_bn254_frs(std::span<const bb::fr> fr_vec, bb::fr* /*unused*/) | ||
| { | ||
| ASSERT(fr_vec.size() == 1); | ||
| return fr_vec[0]; | ||
| } | ||
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| bool convert_from_bn254_frs(std::span<const bb::fr> fr_vec, bool* /*unused*/) | ||
| { | ||
| ASSERT(fr_vec.size() == 1); | ||
| return fr_vec[0] != 0; | ||
| } | ||
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| /** | ||
| * @brief Converts 2 bb::fr elements to grumpkin::fr | ||
| * @details First, this function must take in 2 bb::fr elements because the grumpkin::fr field has a larger modulus than | ||
| * the bb::fr field, so we choose to send 1 grumpkin::fr element to 2 bb::fr elements to maintain injectivity. | ||
| * For the implementation, we want to minimize the number of constraints created by the circuit form, which happens to | ||
| * use 68 bit limbs to represent a grumpkin::fr (as a bigfield). Therefore, our mapping will split a grumpkin::fr into a | ||
| * 136 bit chunk for the lower two bigfield limbs and the upper chunk for the upper two limbs. The upper chunk ends up | ||
| * being 254 - 2*68 = 118 bits as a result. This is why we check that the bb::frs must be at most 136 and 118 bits | ||
| * respectively (to ensure no overflow). Then, we converts the two chunks to a grumpkin::fr using uint256_t conversions. | ||
| * @param low_bits_in | ||
| * @param high_bits_in | ||
| * @return grumpkin::fr | ||
| */ | ||
| grumpkin::fr convert_from_bn254_frs(std::span<const bb::fr> fr_vec, grumpkin::fr* /*unused*/) | ||
| { | ||
| // Combines the two elements into one uint256_t, and then convert that to a grumpkin::fr | ||
| ASSERT(uint256_t(fr_vec[0]) < (uint256_t(1) << (NUM_CONVERSION_LIMB_BITS * 2))); // lower 136 bits | ||
| ASSERT(uint256_t(fr_vec[1]) < | ||
| (uint256_t(1) << (TOTAL_BITS - NUM_CONVERSION_LIMB_BITS * 2))); // upper 254-136=118 bits | ||
| uint256_t value = uint256_t(fr_vec[0]) + (uint256_t(fr_vec[1]) << (NUM_CONVERSION_LIMB_BITS * 2)); | ||
| grumpkin::fr result(value); | ||
| return result; | ||
| } | ||
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| curve::BN254::AffineElement convert_from_bn254_frs(std::span<const bb::fr> fr_vec, | ||
| curve::BN254::AffineElement* /*unused*/) | ||
| { | ||
| curve::BN254::AffineElement val; | ||
| val.x = convert_from_bn254_frs<grumpkin::fr>(fr_vec.subspan(0, 2)); | ||
| val.y = convert_from_bn254_frs<grumpkin::fr>(fr_vec.subspan(2, 2)); | ||
| return val; | ||
| } | ||
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| curve::Grumpkin::AffineElement convert_from_bn254_frs(std::span<const bb::fr> fr_vec, | ||
| curve::Grumpkin::AffineElement* /*unused*/) | ||
| { | ||
| ASSERT(fr_vec.size() == 2); | ||
| curve::Grumpkin::AffineElement val; | ||
| val.x = fr_vec[0]; | ||
| val.y = fr_vec[1]; | ||
| return val; | ||
| } | ||
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| /** | ||
| * @brief Converts grumpkin::fr to 2 bb::fr elements | ||
| * @details First, this function must return 2 bb::fr elements because the grumpkin::fr field has a larger modulus than | ||
| * the bb::fr field, so we choose to send 1 grumpkin::fr element to 2 bb::fr elements to maintain injectivity. | ||
| * This function the reverse of convert_from_bn254_frs(std::span<const bb::fr> fr_vec, grumpkin::fr*) by merging the two | ||
| * pairs of limbs back into the 2 bb::fr elements. For the implementation, we want to minimize the number of constraints | ||
| * created by the circuit form, which happens to use 68 bit limbs to represent a grumpkin::fr (as a bigfield). | ||
| * Therefore, our mapping will split a grumpkin::fr into a 136 bit chunk for the lower two bigfield limbs and the upper | ||
| * chunk for the upper two limbs. The upper chunk ends up being 254 - 2*68 = 118 bits as a result. We manipulate the | ||
| * value using bitwise masks and shifts to obtain our two chunks. | ||
| * @param input | ||
| * @return std::array<bb::fr, 2> | ||
| */ | ||
| std::vector<bb::fr> convert_to_bn254_frs(const grumpkin::fr& val) | ||
| { | ||
| // Goal is to slice up the 64 bit limbs of grumpkin::fr/uint256_t to mirror the 68 bit limbs of bigfield | ||
| // We accomplish this by dividing the grumpkin::fr's value into two 68*2=136 bit pieces. | ||
| constexpr uint64_t LOWER_BITS = 2 * NUM_CONVERSION_LIMB_BITS; | ||
| constexpr uint256_t LOWER_MASK = (uint256_t(1) << LOWER_BITS) - 1; | ||
| auto value = uint256_t(val); | ||
| ASSERT(value < (uint256_t(1) << TOTAL_BITS)); | ||
| std::vector<bb::fr> result(2); | ||
| result[0] = static_cast<uint256_t>(value & LOWER_MASK); | ||
| result[1] = static_cast<uint256_t>(value >> LOWER_BITS); | ||
| ASSERT(static_cast<uint256_t>(result[1]) < (uint256_t(1) << (TOTAL_BITS - LOWER_BITS))); | ||
| return result; | ||
| } | ||
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| std::vector<bb::fr> convert_to_bn254_frs(const bb::fr& val) | ||
| { | ||
| std::vector<bb::fr> fr_vec{ val }; | ||
| return fr_vec; | ||
| } | ||
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| std::vector<bb::fr> convert_to_bn254_frs(const curve::BN254::AffineElement& val) | ||
| { | ||
| auto fr_vec_x = convert_to_bn254_frs(val.x); | ||
| auto fr_vec_y = convert_to_bn254_frs(val.y); | ||
| std::vector<bb::fr> fr_vec(fr_vec_x.begin(), fr_vec_x.end()); | ||
| fr_vec.insert(fr_vec.end(), fr_vec_y.begin(), fr_vec_y.end()); | ||
| return fr_vec; | ||
| } | ||
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| std::vector<bb::fr> convert_to_bn254_frs(const curve::Grumpkin::AffineElement& val) | ||
| { | ||
| auto fr_vec_x = convert_to_bn254_frs(val.x); | ||
| auto fr_vec_y = convert_to_bn254_frs(val.y); | ||
| std::vector<bb::fr> fr_vec(fr_vec_x.begin(), fr_vec_x.end()); | ||
| fr_vec.insert(fr_vec.end(), fr_vec_y.begin(), fr_vec_y.end()); | ||
| return fr_vec; | ||
| } | ||
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| } // namespace bb::field_conversion |
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