111 lines
3.6 KiB
C++
111 lines
3.6 KiB
C++
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/// @file dpf/constrained_cmp.hpp
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/// @brief Constrained integer comparison Π_CCMP (NDSS 2025 Alg. 1).
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/// @details Given two positive integers that differ by exactly one,
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/// `1{x0 < x1}` is computed with a single AND on two derived bits.
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/// Local joint simulation opens the AND clearly; an MPC backend would
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/// replace that open with the existing Beaver AND tape.
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/// @copyright Copyright (c) 2019-2026 Ryan Henry and [others](@ref authors)
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/// @license Released under a GNU General Public v2.0 (GPLv2) license;
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/// see [LICENSE.md](@ref license) for details.
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#ifndef LIBDPF_INCLUDE_DPF_CONSTRAINED_CMP_HPP__
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#define LIBDPF_INCLUDE_DPF_CONSTRAINED_CMP_HPP__
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#include <cstdint>
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#include <type_traits>
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#include "hedley/hedley.h"
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namespace dpf
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{
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namespace detail
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{
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/// @brief Last two bits of `x`: high = bit 1, low = bit 0.
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/// @param x the `x`
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/// @return Last two bits of `x`: high = bit 1, low = bit 0
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HEDLEY_NO_THROW
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HEDLEY_ALWAYS_INLINE
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HEDLEY_CONST
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constexpr uint8_t ccmp_lo_bit(std::uint64_t x) noexcept
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{
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return static_cast<uint8_t>(x & 1u);
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}
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/// @brief Bit 1 of `x`.
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/// @param x the integer
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/// @return `(x >> 1) & 1`
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HEDLEY_NO_THROW
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HEDLEY_ALWAYS_INLINE
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HEDLEY_CONST
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constexpr uint8_t ccmp_hi_bit(std::uint64_t x) noexcept
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{
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return static_cast<uint8_t>((x >> 1) & 1u);
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}
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/// @brief Party `b`'s local share inputs for the AND: `z0 = h`, `z1 = h ⊕ l ⊕ b`.
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/// @param x the integer whose low two bits are split
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/// @param party party index, `0` or `1`
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/// @param z0 first AND input, `h`
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/// @param z1 second AND input, `h ⊕ l ⊕ party`
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/// @param l bit 0 of `x`
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HEDLEY_NO_THROW
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HEDLEY_ALWAYS_INLINE
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constexpr void ccmp_party_terms(std::uint64_t x, uint8_t party,
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uint8_t & z0, uint8_t & z1, uint8_t & l) noexcept
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{
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const uint8_t h = ccmp_hi_bit(x);
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l = ccmp_lo_bit(x);
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z0 = h;
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z1 = static_cast<uint8_t>(h ^ l ^ (party & 1u));
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}
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/// @brief Opened result of Π_CCMP when both inputs are known (local joint sim).
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/// @details Precondition: `|x0 - x1| = 1`.
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/// @param x0 the `x0`
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/// @param x1 the `x1`
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/// @return Opened result of Π_CCMP when both inputs are known (local joint sim)
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HEDLEY_NO_THROW
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HEDLEY_ALWAYS_INLINE
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HEDLEY_CONST
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constexpr uint8_t local_ccmp(std::uint64_t x0, std::uint64_t x1) noexcept
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{
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uint8_t z00 = 0, z01 = 0, l0 = 0;
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uint8_t z10 = 0, z11 = 0, l1 = 0;
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ccmp_party_terms(x0, 0, z00, z01, l0);
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ccmp_party_terms(x1, 1, z10, z11, l1);
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const uint8_t z0 = static_cast<uint8_t>(z00 ^ z10);
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const uint8_t z1 = static_cast<uint8_t>(z01 ^ z11);
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const uint8_t t = static_cast<uint8_t>(z0 & z1);
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// Party shares: y0 = t0, y1 = t1 ⊕ (l1 ∧ 1). Opened y = t ⊕ l1.
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return static_cast<uint8_t>(t ^ l1);
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}
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/// @brief Same as `local_ccmp` for any unsigned or enum-convertible integer.
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/// @tparam T0 integral type of the first operand
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/// @tparam T1 integral type of the second operand
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/// @param x0 the first integer
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/// @param x1 the second integer
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/// @return Same as `local_ccmp` for any unsigned or enum-convertible integer
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template <typename T0, typename T1>
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HEDLEY_NO_THROW
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HEDLEY_ALWAYS_INLINE
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HEDLEY_CONST
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constexpr uint8_t local_ccmp_int(T0 x0, T1 x1) noexcept
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{
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static_assert(std::is_integral_v<T0> && std::is_integral_v<T1>,
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"local_ccmp_int: integral operands");
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return local_ccmp(static_cast<std::uint64_t>(x0),
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static_cast<std::uint64_t>(x1));
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}
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} // namespace detail
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/// @brief Constrained comparison: `1{x0 < x1}` when `|x0 − x1| = 1`.
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using detail::local_ccmp;
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using detail::local_ccmp_int;
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} // namespace dpf
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#endif // LIBDPF_INCLUDE_DPF_CONSTRAINED_CMP_HPP__
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