http://www.bouncycastle.org/java.html: 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. 
package org.bouncycastle.crypto;

import java.math.BigInteger;


The basic interface that basic Diffie-Hellman implementations
conforms to.

/**
 * The basic interface that basic Diffie-Hellman implementations
 * conforms to.
 */
public interface BasicAgreement
{
    
initialise the agreement engine.

/**
     * initialise the agreement engine.
     */
    public void init(CipherParameters param);

    
given a public key from a given party calculate the next
message in the agreement sequence. 

/**
     * given a public key from a given party calculate the next
     * message in the agreement sequence. 
     */
    public BigInteger calculateAgreement(CipherParameters pubKey);
}