serenity/Userland/Libraries/LibCrypto/Curves/SECP256r1.cpp

/*
 * Copyright (c) 2022, Michiel Visser <opensource@webmichiel.nl>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/ByteReader.h>
#include <AK/Endian.h>
#include <AK/Random.h>
#include <AK/String.h>
#include <AK/StringBuilder.h>
#include <AK/UFixedBigInt.h>
#include <LibCrypto/Curves/SECP256r1.h>

namespace Crypto::Curves {

static u256 import_big_endian(ReadonlyBytes data)
{
    VERIFY(data.size() == 32);

    u64 d = AK::convert_between_host_and_big_endian(ByteReader::load64(data.offset_pointer(0 * sizeof(u64))));
    u64 c = AK::convert_between_host_and_big_endian(ByteReader::load64(data.offset_pointer(1 * sizeof(u64))));
    u64 b = AK::convert_between_host_and_big_endian(ByteReader::load64(data.offset_pointer(2 * sizeof(u64))));
    u64 a = AK::convert_between_host_and_big_endian(ByteReader::load64(data.offset_pointer(3 * sizeof(u64))));

    return u256 { u128 { a, b }, u128 { c, d } };
}

static void export_big_endian(u256 const& value, Bytes data)
{
    u64 a = AK::convert_between_host_and_big_endian(value.low().low());
    u64 b = AK::convert_between_host_and_big_endian(value.low().high());
    u64 c = AK::convert_between_host_and_big_endian(value.high().low());
    u64 d = AK::convert_between_host_and_big_endian(value.high().high());

    ByteReader::store(data.offset_pointer(0 * sizeof(u64)), d);
    ByteReader::store(data.offset_pointer(1 * sizeof(u64)), c);
    ByteReader::store(data.offset_pointer(2 * sizeof(u64)), b);
    ByteReader::store(data.offset_pointer(3 * sizeof(u64)), a);
}

static u256 select(u256 const& left, u256 const& right, bool condition)
{
    // If condition = 0 return left else right
    u256 mask = (u256)condition - 1;

    return (left & mask) | (right & ~mask);
}

static u512 multiply(u256 const& left, u256 const& right)
{
    auto result = left.wide_multiply(right);
    return { result.low, result.high };
}

u256 SECP256r1::modular_reduce(u256 const& value)
{
    // Add -prime % 2^256 = 2^224-2^192-2^96+1
    bool carry = false;
    u256 other = value.addc(REDUCE_PRIME, carry);

    // Check for overflow
    return select(value, other, carry);
}

u256 SECP256r1::modular_reduce_order(u256 const& value)
{
    // Add -order % 2^256
    bool carry = false;
    u256 other = value.addc(REDUCE_ORDER, carry);

    // Check for overflow
    return select(value, other, carry);
}

u256 SECP256r1::modular_add(u256 const& left, u256 const& right, bool carry_in)
{
    bool carry = carry_in;
    u256 output = left.addc(right, carry);

    // If there is left carry, subtract p by adding 2^256 - p
    u64 t = carry;
    carry = false;
    u256 addend { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
    output = output.addc(addend, carry);

    // If there is still left carry, subtract p by adding 2^256 - p
    t = carry;
    addend = { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
    return output + addend;
}

u256 SECP256r1::modular_sub(u256 const& left, u256 const& right)
{
    bool borrow = false;
    u256 output = left.subc(right, borrow);

    // If there is left borrow, add p by subtracting 2^256 - p
    u64 t = borrow;
    borrow = false;
    u256 sub { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
    output = output.subc(sub, borrow);

    // If there is still left borrow, add p by subtracting 2^256 - p
    t = borrow;
    sub = { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
    return output - sub;
}

u256 SECP256r1::modular_multiply(u256 const& left, u256 const& right)
{
    // Modular multiplication using the Montgomery method: https://en.wikipedia.org/wiki/Montgomery_modular_multiplication
    // This requires that the inputs to this function are in Montgomery form.

    // T = left * right
    u512 mult = multiply(left, right);

    // m = ((T mod R) * curve_p')
    u512 m = multiply(mult.low(), PRIME_INVERSE_MOD_R);

    // mp = (m mod R) * curve_p
    u512 mp = multiply(m.low(), PRIME);

    // t = (T + mp)
    bool carry = false;
    mult.low().addc(mp.low(), carry);

    // output = t / R
    return modular_add(mult.high(), mp.high(), carry);
}

u256 SECP256r1::modular_square(u256 const& value)
{
    return modular_multiply(value, value);
}

u256 SECP256r1::to_montgomery(u256 const& value)
{
    return modular_multiply(value, R2_MOD_PRIME);
}

u256 SECP256r1::from_montgomery(u256 const& value)
{
    return modular_multiply(value, ONE);
}

u256 SECP256r1::modular_inverse(u256 const& value)
{
    // Modular inverse modulo the curve prime can be computed using Fermat's little theorem: a^(p-2) mod p = a^-1 mod p.
    // Calculating a^(p-2) mod p can be done using the square-and-multiply exponentiation method, as p-2 is constant.
    //
    // p-2 = 2^256 - 2^224 + 2^192 + 2^96 - 3, or written as binary:
    // 1111111111111111111111111111111100000000000000000000000000000001
    // 0000000000000000000000000000000000000000000000000000000000000000
    // 0000000000000000000000000000000011111111111111111111111111111111
    // 1111111111111111111111111111111111111111111111111111111111111101

    u256 base = value;

    // 1
    u256 result = value;
    base = modular_square(base);

    // 0
    base = modular_square(base);

    // 94*1
    for (auto i = 0; i < 94; i++) {
        result = modular_multiply(result, base);
        base = modular_square(base);
    }

    // 96*0
    for (auto i = 0; i < 96; i++) {
        base = modular_square(base);
    }

    // 1
    result = modular_multiply(result, base);
    base = modular_square(base);

    // 31*0
    for (auto i = 0; i < 31; i++) {
        base = modular_square(base);
    }

    // 32*1
    for (auto i = 0; i < 32; i++) {
        result = modular_multiply(result, base);
        base = modular_square(base);
    }

    return result;
}

void SECP256r1::point_double(JacobianPoint& output_point, JacobianPoint const& point)
{
    // Based on "Point Doubling" from http://point-at-infinity.org/ecc/Prime_Curve_Jacobian_Coordinates.html

    // if (Y == 0)
    //   return POINT_AT_INFINITY
    if (point.y.is_zero_constant_time()) {
        VERIFY_NOT_REACHED();
    }

    u256 temp;

    // Y2 = Y^2
    u256 y2 = modular_square(point.y);

    // S = 4*X*Y2
    u256 s = modular_multiply(point.x, y2);
    s = modular_add(s, s);
    s = modular_add(s, s);

    // M = 3*X^2 + a*Z^4 = 3*(X + Z^2)*(X - Z^2)
    // This specific equation from https://github.com/earlephilhower/bearssl-esp8266/blob/6105635531027f5b298aa656d44be2289b2d434f/src/ec/ec_p256_m64.c#L811-L816
    // This simplification only works because a = -3 mod p
    temp = modular_square(point.z);
    u256 m = modular_add(point.x, temp);
    temp = modular_sub(point.x, temp);
    m = modular_multiply(m, temp);
    temp = modular_add(m, m);
    m = modular_add(m, temp);

    // X' = M^2 - 2*S
    u256 xp = modular_square(m);
    xp = modular_sub(xp, s);
    xp = modular_sub(xp, s);

    // Y' = M*(S - X') - 8*Y2^2
    u256 yp = modular_sub(s, xp);
    yp = modular_multiply(yp, m);
    temp = modular_square(y2);
    temp = modular_add(temp, temp);
    temp = modular_add(temp, temp);
    temp = modular_add(temp, temp);
    yp = modular_sub(yp, temp);

    // Z' = 2*Y*Z
    u256 zp = modular_multiply(point.y, point.z);
    zp = modular_add(zp, zp);

    // return (X', Y', Z')
    output_point.x = xp;
    output_point.y = yp;
    output_point.z = zp;
}

void SECP256r1::point_add(JacobianPoint& output_point, JacobianPoint const& point_a, JacobianPoint const& point_b)
{
    // Based on "Point Addition" from  http://point-at-infinity.org/ecc/Prime_Curve_Jacobian_Coordinates.html
    if (point_a.x.is_zero_constant_time() && point_a.y.is_zero_constant_time() && point_a.z.is_zero_constant_time()) {
        output_point.x = point_b.x;
        output_point.y = point_b.y;
        output_point.z = point_b.z;
        return;
    }

    u256 temp;

    temp = modular_square(point_b.z);
    // U1 = X1*Z2^2
    u256 u1 = modular_multiply(point_a.x, temp);
    // S1 = Y1*Z2^3
    u256 s1 = modular_multiply(point_a.y, temp);
    s1 = modular_multiply(s1, point_b.z);

    temp = modular_square(point_a.z);
    // U2 = X2*Z1^2
    u256 u2 = modular_multiply(point_b.x, temp);
    // S2 = Y2*Z1^3
    u256 s2 = modular_multiply(point_b.y, temp);
    s2 = modular_multiply(s2, point_a.z);

    // if (U1 == U2)
    //   if (S1 != S2)
    //     return POINT_AT_INFINITY
    //   else
    //     return POINT_DOUBLE(X1, Y1, Z1)
    if (u1.is_equal_to_constant_time(u2)) {
        if (s1.is_equal_to_constant_time(s2)) {
            point_double(output_point, point_a);
            return;
        } else {
            VERIFY_NOT_REACHED();
        }
    }

    // H = U2 - U1
    u256 h = modular_sub(u2, u1);
    u256 h2 = modular_square(h);
    u256 h3 = modular_multiply(h2, h);
    // R = S2 - S1
    u256 r = modular_sub(s2, s1);
    // X3 = R^2 - H^3 - 2*U1*H^2
    u256 x3 = modular_square(r);
    x3 = modular_sub(x3, h3);
    temp = modular_multiply(u1, h2);
    temp = modular_add(temp, temp);
    x3 = modular_sub(x3, temp);
    // Y3 = R*(U1*H^2 - X3) - S1*H^3
    u256 y3 = modular_multiply(u1, h2);
    y3 = modular_sub(y3, x3);
    y3 = modular_multiply(y3, r);
    temp = modular_multiply(s1, h3);
    y3 = modular_sub(y3, temp);
    // Z3 = H*Z1*Z2
    u256 z3 = modular_multiply(h, point_a.z);
    z3 = modular_multiply(z3, point_b.z);
    // return (X3, Y3, Z3)
    output_point.x = x3;
    output_point.y = y3;
    output_point.z = z3;
}

void SECP256r1::convert_jacobian_to_affine(JacobianPoint& point)
{
    u256 temp;
    // X' = X/Z^2
    temp = modular_square(point.z);
    temp = modular_inverse(temp);
    point.x = modular_multiply(point.x, temp);
    // Y' = Y/Z^3
    temp = modular_square(point.z);
    temp = modular_multiply(temp, point.z);
    temp = modular_inverse(temp);
    point.y = modular_multiply(point.y, temp);
}

bool SECP256r1::is_point_on_curve(JacobianPoint const& point)
{
    // This check requires the point to be in Montgomery form, with Z=1
    u256 temp, temp2;

    // Calulcate Y^2 - X^3 - a*X - b = Y^2 - X^3 + 3*X - b
    temp = modular_square(point.y);
    temp2 = modular_square(point.x);
    temp2 = modular_multiply(temp2, point.x);
    temp = modular_sub(temp, temp2);
    temp = modular_add(temp, point.x);
    temp = modular_add(temp, point.x);
    temp = modular_add(temp, point.x);
    temp = modular_sub(temp, B_MONTGOMERY);
    temp = modular_reduce(temp);

    return temp.is_zero_constant_time();
}

ErrorOr<ByteBuffer> SECP256r1::generate_private_key()
{
    auto buffer = TRY(ByteBuffer::create_uninitialized(32));
    fill_with_random(buffer.data(), buffer.size());
    return buffer;
}

ErrorOr<ByteBuffer> SECP256r1::generate_public_key(ReadonlyBytes a)
{
    // clang-format off
    u8 generator_bytes[65] {
        0x04,
        0x6B, 0x17, 0xD1, 0xF2, 0xE1, 0x2C, 0x42, 0x47, 0xF8, 0xBC, 0xE6, 0xE5, 0x63, 0xA4, 0x40, 0xF2,
        0x77, 0x03, 0x7D, 0x81, 0x2D, 0xEB, 0x33, 0xA0, 0xF4, 0xA1, 0x39, 0x45, 0xD8, 0x98, 0xC2, 0x96,
        0x4F, 0xE3, 0x42, 0xE2, 0xFE, 0x1A, 0x7F, 0x9B, 0x8E, 0xE7, 0xEB, 0x4A, 0x7C, 0x0F, 0x9E, 0x16,
        0x2B, 0xCE, 0x33, 0x57, 0x6B, 0x31, 0x5E, 0xCE, 0xCB, 0xB6, 0x40, 0x68, 0x37, 0xBF, 0x51, 0xF5,
    };
    // clang-format on
    return compute_coordinate(a, { generator_bytes, 65 });
}

ErrorOr<ByteBuffer> SECP256r1::compute_coordinate(ReadonlyBytes scalar_bytes, ReadonlyBytes point_bytes)
{
    VERIFY(scalar_bytes.size() == 32);

    u256 scalar = import_big_endian(scalar_bytes);
    // FIXME: This will slightly bias the distribution of client secrets
    scalar = modular_reduce_order(scalar);
    if (scalar.is_zero_constant_time())
        return Error::from_string_literal("SECP256r1: scalar is zero");

    // Make sure the point is uncompressed
    if (point_bytes.size() != 65 || point_bytes[0] != 0x04)
        return Error::from_string_literal("SECP256r1: point is not uncompressed format");

    JacobianPoint point {
        import_big_endian(point_bytes.slice(1, 32)),
        import_big_endian(point_bytes.slice(33, 32)),
        1u,
    };

    // Convert the input point into Montgomery form
    point.x = to_montgomery(point.x);
    point.y = to_montgomery(point.y);
    point.z = to_montgomery(point.z);

    // Check that the point is on the curve
    if (!is_point_on_curve(point))
        return Error::from_string_literal("SECP256r1: point is not on the curve");

    JacobianPoint result;
    JacobianPoint temp_result;

    // Calculate the scalar times point multiplication in constant time
    for (auto i = 0; i < 256; i++) {
        point_add(temp_result, result, point);

        auto condition = (scalar & 1u) == 1u;
        result.x = select(result.x, temp_result.x, condition);
        result.y = select(result.y, temp_result.y, condition);
        result.z = select(result.z, temp_result.z, condition);

        point_double(point, point);
        scalar >>= 1u;
    }

    // Convert from Jacobian coordinates back to Affine coordinates
    convert_jacobian_to_affine(result);

    // Make sure the resulting point is on the curve
    VERIFY(is_point_on_curve(result));

    // Convert the result back from Montgomery form
    result.x = from_montgomery(result.x);
    result.y = from_montgomery(result.y);
    // Final modular reduction on the coordinates
    result.x = modular_reduce(result.x);
    result.y = modular_reduce(result.y);

    // Export the values into an output buffer
    auto buf = TRY(ByteBuffer::create_uninitialized(65));
    buf[0] = 0x04;
    export_big_endian(result.x, buf.bytes().slice(1, 32));
    export_big_endian(result.y, buf.bytes().slice(33, 32));
    return buf;
}

ErrorOr<ByteBuffer> SECP256r1::derive_premaster_key(ReadonlyBytes shared_point)
{
    VERIFY(shared_point.size() == 65);
    VERIFY(shared_point[0] == 0x04);

    ByteBuffer premaster_key = TRY(ByteBuffer::create_uninitialized(32));
    premaster_key.overwrite(0, shared_point.data() + 1, 32);
    return premaster_key;
}

}