SecP128R1FieldElement.java

package org.bouncycastle.math.ec.custom.sec;

import java.math.BigInteger;

import org.bouncycastle.math.ec.ECFieldElement;
import org.bouncycastle.math.raw.Mod;
import org.bouncycastle.math.raw.Nat128;
import org.bouncycastle.util.Arrays;

public class SecP128R1FieldElement extends ECFieldElement
{
    public static final BigInteger Q = SecP128R1Curve.q;

    protected int[] x;

    public SecP128R1FieldElement(BigInteger x)
    {
        if (x == null || x.signum() < 0 || x.compareTo(Q) >= 0)
        {
            throw new IllegalArgumentException("x value invalid for SecP128R1FieldElement");
        }

        this.x = SecP128R1Field.fromBigInteger(x);
    }

    public SecP128R1FieldElement()
    {
        this.x = Nat128.create();
    }

    protected SecP128R1FieldElement(int[] x)
    {
        this.x = x;
    }

    public boolean isZero()
    {
        return Nat128.isZero(x);
    }

    public boolean isOne()
    {
        return Nat128.isOne(x);
    }

    public boolean testBitZero()
    {
        return Nat128.getBit(x, 0) == 1;
    }

    public BigInteger toBigInteger()
    {
        return Nat128.toBigInteger(x);
    }

    public String getFieldName()
    {
        return "SecP128R1Field";
    }

    public int getFieldSize()
    {
        return Q.bitLength();
    }

    public ECFieldElement add(ECFieldElement b)
    {
        int[] z = Nat128.create();
        SecP128R1Field.add(x, ((SecP128R1FieldElement)b).x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement addOne()
    {
        int[] z = Nat128.create();
        SecP128R1Field.addOne(x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement subtract(ECFieldElement b)
    {
        int[] z = Nat128.create();
        SecP128R1Field.subtract(x, ((SecP128R1FieldElement)b).x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement multiply(ECFieldElement b)
    {
        int[] z = Nat128.create();
        SecP128R1Field.multiply(x, ((SecP128R1FieldElement)b).x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement divide(ECFieldElement b)
    {
//        return multiply(b.invert());
        int[] z = Nat128.create();
        Mod.invert(SecP128R1Field.P, ((SecP128R1FieldElement)b).x, z);
        SecP128R1Field.multiply(z, x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement negate()
    {
        int[] z = Nat128.create();
        SecP128R1Field.negate(x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement square()
    {
        int[] z = Nat128.create();
        SecP128R1Field.square(x, z);
        return new SecP128R1FieldElement(z);
    }

    public ECFieldElement invert()
    {
//        return new SecP128R1FieldElement(toBigInteger().modInverse(Q));
        int[] z = Nat128.create();
        Mod.invert(SecP128R1Field.P, x, z);
        return new SecP128R1FieldElement(z);
    }

    // D.1.4 91
    /**
     * return a sqrt root - the routine verifies that the calculation returns the right value - if
     * none exists it returns null.
     */
    public ECFieldElement sqrt()
    {
        /*
         * Raise this element to the exponent 2^126 - 2^95
         *
         * Breaking up the exponent's binary representation into "repunits", we get:
         *     { 31 1s } { 95 0s }
         *
         * Therefore we need an addition chain containing 31 (the length of the repunit) We use:
         *     1, 2, 4, 8, 10, 20, 30, [31]
         */

        int[] x1 = this.x;
        if (Nat128.isZero(x1) || Nat128.isOne(x1))
        {
            return this;
        }

        int[] x2 = Nat128.create();
        SecP128R1Field.square(x1, x2);
        SecP128R1Field.multiply(x2, x1, x2);
        int[] x4 = Nat128.create();
        SecP128R1Field.squareN(x2, 2, x4);
        SecP128R1Field.multiply(x4, x2, x4);
        int[] x8 = Nat128.create();
        SecP128R1Field.squareN(x4, 4, x8);
        SecP128R1Field.multiply(x8, x4, x8);
        int[] x10 = x4;
        SecP128R1Field.squareN(x8, 2, x10);
        SecP128R1Field.multiply(x10, x2, x10);
        int[] x20 = x2;
        SecP128R1Field.squareN(x10, 10, x20);
        SecP128R1Field.multiply(x20, x10, x20);
        int[] x30 = x8;
        SecP128R1Field.squareN(x20, 10, x30);
        SecP128R1Field.multiply(x30, x10, x30);
        int[] x31 = x10;
        SecP128R1Field.square(x30, x31);
        SecP128R1Field.multiply(x31, x1, x31);

        int[] t1 = x31;
        SecP128R1Field.squareN(t1, 95, t1);

        int[] t2 = x30;
        SecP128R1Field.square(t1, t2);

        return Nat128.eq(x1, t2) ? new SecP128R1FieldElement(t1) : null;
    }

    public boolean equals(Object other)
    {
        if (other == this)
        {
            return true;
        }

        if (!(other instanceof SecP128R1FieldElement))
        {
            return false;
        }

        SecP128R1FieldElement o = (SecP128R1FieldElement)other;
        return Nat128.eq(x, o.x);
    }

    public int hashCode()
    {
        return Q.hashCode() ^ Arrays.hashCode(x, 0, 4);
    }
}