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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.
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package org.bouncycastle.math.ec.custom.sec;

import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.math.raw.Mod;
import org.bouncycastle.math.raw.Nat;
import org.bouncycastle.math.raw.Nat256;
import org.bouncycastle.util.Pack;

public class SecP256K1Field
{
    // 2^256 - 2^32 - 2^9 - 2^8 - 2^7 - 2^6 - 2^4 - 1
    static final int[] P = new int[]{ 0xFFFFFC2F, 0xFFFFFFFE, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
        0xFFFFFFFF, 0xFFFFFFFF };
    private static final int[] PExt = new int[]{ 0x000E90A1, 0x000007A2, 0x00000001, 0x00000000, 0x00000000, 0x00000000,
        0x00000000, 0x00000000, 0xFFFFF85E, 0xFFFFFFFD, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
        0xFFFFFFFF };
    private static final int[] PExtInv = new int[]{ 0xFFF16F5F, 0xFFFFF85D, 0xFFFFFFFE, 0xFFFFFFFF, 0xFFFFFFFF,
        0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x000007A1, 0x00000002 };
    private static final int P7 = 0xFFFFFFFF;
    private static final int PExt15 = 0xFFFFFFFF;
    private static final int PInv33 = 0x3D1;

    public static void add(int[] x, int[] y, int[] z)
    {
        int c = Nat256.add(x, y, z);
        if (c != 0 || (z[7] == P7 && Nat256.gte(z, P)))
        {
            Nat.add33To(8, PInv33, z);
        }
    }

    public static void addExt(int[] xx, int[] yy, int[] zz)
    {
        int c = Nat.add(16, xx, yy, zz);
        if (c != 0 || (zz[15] == PExt15 && Nat.gte(16, zz, PExt)))
        {
            if (Nat.addTo(PExtInv.length, PExtInv, zz) != 0)
            {
                Nat.incAt(16, zz, PExtInv.length);
            }
        }
    }

    public static void addOne(int[] x, int[] z)
    {
        int c = Nat.inc(8, x, z);
        if (c != 0 || (z[7] == P7 && Nat256.gte(z, P)))
        {
            Nat.add33To(8, PInv33, z);
        }
    }

    public static int[] fromBigInteger(BigInteger x)
    {
        int[] z = Nat256.fromBigInteger(x);
        if (z[7] == P7 && Nat256.gte(z, P))
        {
            Nat256.subFrom(P, z);
        }
        return z;
    }

    public static void half(int[] x, int[] z)
    {
        if ((x[0] & 1) == 0)
        {
            Nat.shiftDownBit(8, x, 0, z);
        }
        else
        {
            int c = Nat256.add(x, P, z);
            Nat.shiftDownBit(8, z, c);
        }
    }

    public static void inv(int[] x, int[] z)
    {
        Mod.checkedModOddInverse(P, x, z);
    }

    public static int isZero(int[] x)
    {
        int d = 0;
        for (int i = 0; i < 8; ++i)
        {
            d |= x[i];
        }
        d = (d >>> 1) | (d & 1);
        return (d - 1) >> 31;
    }

    public static void multiply(int[] x, int[] y, int[] z)
    {
        int[] tt = Nat256.createExt();
        Nat256.mul(x, y, tt);
        reduce(tt, z);
    }

    public static void multiply(int[] x, int[] y, int[] z, int[] tt)
    {
        Nat256.mul(x, y, tt);
        reduce(tt, z);
    }

    public static void multiplyAddToExt(int[] x, int[] y, int[] zz)
    {
        int c = Nat256.mulAddTo(x, y, zz);
        if (c != 0 || (zz[15] == PExt15 && Nat.gte(16, zz, PExt)))
        {
            if (Nat.addTo(PExtInv.length, PExtInv, zz) != 0)
            {
                Nat.incAt(16, zz, PExtInv.length);
            }
        }
    }

    public static void negate(int[] x, int[] z)
    {
        if (0 != isZero(x))
        {
            Nat256.sub(P, P, z);
        }
        else
        {
            Nat256.sub(P, x, z);
        }
    }

    public static void random(SecureRandom r, int[] z)
    {
        byte[] bb = new byte[8 * 4];
        do
        {
            r.nextBytes(bb);
            Pack.littleEndianToInt(bb, 0, z, 0, 8);
        }
        while (0 == Nat.lessThan(8, z, P));
    }

    public static void randomMult(SecureRandom r, int[] z)
    {
        do
        {
            random(r, z);
        }
        while (0 != isZero(z));
    }

    public static void reduce(int[] xx, int[] z)
    {
        long cc = Nat256.mul33Add(PInv33, xx, 8, xx, 0, z, 0);
        int c = Nat256.mul33DWordAdd(PInv33, cc, z, 0);

        // assert c == 0L || c == 1L;

        if (c != 0 || (z[7] == P7 && Nat256.gte(z, P)))
        {
            Nat.add33To(8, PInv33, z);
        }
    }

    public static void reduce32(int x, int[] z)
    {
        if ((x != 0 && Nat256.mul33WordAdd(PInv33, x, z, 0) != 0)
            || (z[7] == P7 && Nat256.gte(z, P)))
        {
            Nat.add33To(8, PInv33, z);
        }
    }

    public static void square(int[] x, int[] z)
    {
        int[] tt = Nat256.createExt();
        Nat256.square(x, tt);
        reduce(tt, z);
    }

    public static void square(int[] x, int[] z, int[] tt)
    {
        Nat256.square(x, tt);
        reduce(tt, z);
    }

    public static void squareN(int[] x, int n, int[] z)
    {
//        assert n > 0;

        int[] tt = Nat256.createExt();
        Nat256.square(x, tt);
        reduce(tt, z);

        while (--n > 0)
        {
            Nat256.square(z, tt);
            reduce(tt, z);
        }
    }

    public static void squareN(int[] x, int n, int[] z, int[] tt)
    {
//        assert n > 0;

        Nat256.square(x, tt);
        reduce(tt, z);

        while (--n > 0)
        {
            Nat256.square(z, tt);
            reduce(tt, z);
        }
    }

    public static void subtract(int[] x, int[] y, int[] z)
    {
        int c = Nat256.sub(x, y, z);
        if (c != 0)
        {
            Nat.sub33From(8, PInv33, z);
        }
    }

    public static void subtractExt(int[] xx, int[] yy, int[] zz)
    {
        int c = Nat.sub(16, xx, yy, zz);
        if (c != 0)
        {
            if (Nat.subFrom(PExtInv.length, PExtInv, zz) != 0)
            {
                Nat.decAt(16, zz, PExtInv.length);
            }
        }
    }

    public static void twice(int[] x, int[] z)
    {
        int c = Nat.shiftUpBit(8, x, 0, z);
        if (c != 0 || (z[7] == P7 && Nat256.gte(z, P)))
        {
            Nat.add33To(8, PInv33, z);
        }
    }
}