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 to JDK 1.8.
  • The Legion of the Bouncy Castle Inc. 
package org.bouncycastle.asn1.eac;

import java.util.Hashtable;

public class BidirectionalMap
    extends Hashtable
{
    private static final long serialVersionUID = -7457289971962812909L;

    Hashtable reverseMap = new Hashtable();

    public Object getReverse(Object o)
    {
        return reverseMap.get(o);
    }

    public Object put(Object key, Object o)
    {
        reverseMap.put(o, key);
        return super.put(key, o);
    }

}