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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 and up.

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package org.bouncycastle.crypto.params;

import java.math.BigInteger;

public class DHPublicKeyParameters
    extends DHKeyParameters
{
    private static final BigInteger ONE = BigInteger.valueOf(1);
    private static final BigInteger TWO = BigInteger.valueOf(2);

    private BigInteger      y;

    public DHPublicKeyParameters(
        BigInteger      y,
        DHParameters    params)
    {
        super(false, params);

        this.y = validate(y, params);
    }   

    private BigInteger validate(BigInteger y, DHParameters dhParams)
    {
        if (y == null)
        {
            throw new NullPointerException("y value cannot be null");
        }

        // TLS check
        if (y.compareTo(TWO) < 0 || y.compareTo(dhParams.getP().subtract(TWO)) > 0)
        {
            throw new IllegalArgumentException("invalid DH public key");
        }

        if (dhParams.getQ() != null)
        {
            if (ONE.equals(y.modPow(dhParams.getQ(), dhParams.getP())))
            {
                return y;
            }

            throw new IllegalArgumentException("Y value does not appear to be in correct group");
        }
        else
        {
            return y;         // we can't validate without Q.
        }
    }

    public BigInteger getY()
    {
        return y;
    }

    public int hashCode()
    {
        return y.hashCode() ^ super.hashCode();
    }

    public boolean equals(
        Object  obj)
    {
        if (!(obj instanceof DHPublicKeyParameters))
        {
            return false;
        }

        DHPublicKeyParameters   other = (DHPublicKeyParameters)obj;

        return other.getY().equals(y) && super.equals(obj);
    }
}