    001/*
    002 * $HeadURL: http://juliusdavies.ca/svn/not-yet-commons-ssl/trunk/src/java/org/apache/commons/ssl/Certificates.java $
    003 * $Revision: 121 $
    004 * $Date: 2007-11-13 21:26:57 -0800 (Tue, 13 Nov 2007) $
    005 *
    006 * ====================================================================
    007 * Licensed to the Apache Software Foundation (ASF) under one
    008 * or more contributor license agreements.  See the NOTICE file
    009 * distributed with this work for additional information
    010 * regarding copyright ownership.  The ASF licenses this file
    011 * to you under the Apache License, Version 2.0 (the
    012 * "License"); you may not use this file except in compliance
    013 * with the License.  You may obtain a copy of the License at
    014 *
    015 *   http://www.apache.org/licenses/LICENSE-2.0
    016 *
    017 * Unless required by applicable law or agreed to in writing,
    018 * software distributed under the License is distributed on an
    019 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    020 * KIND, either express or implied.  See the License for the
    021 * specific language governing permissions and limitations
    022 * under the License.
    023 * ====================================================================
    024 *
    025 * This software consists of voluntary contributions made by many
    026 * individuals on behalf of the Apache Software Foundation.  For more
    027 * information on the Apache Software Foundation, please see
    028 * <http://www.apache.org/>.
    029 *
    030 */
    031package org.apache.commons.ssl.util;
    032
    033import java.math.BigInteger;
    034import java.security.GeneralSecurityException;
    035import java.security.KeyException;
    036import java.security.KeyFactory;
    037import java.security.PrivateKey;
    038import java.security.PublicKey;
    039import java.security.interfaces.DSAParams;
    040import java.security.interfaces.DSAPrivateKey;
    041import java.security.interfaces.RSAPrivateCrtKey;
    042import java.security.spec.DSAPublicKeySpec;
    043import java.security.spec.RSAPublicKeySpec;
    044
    045/**
    046 * Utility class for deriving a public key from a given private key.
    047 *
    048 * @author Chad La Joie <lajoie OF georgetown.edu>
    049 * @since November 14th, 2007
    050 */
    051public class PublicKeyDeriver {
    052
    053    /**
    054     * Utility method for deriving a public key from a given private key.
    055     *
    056     * @param key private key for which we need a public key (DSA or RSA).
    057     * @return the corresponding public key
    058     * @throws GeneralSecurityException if it didn't work
    059     */
    060    public static PublicKey derivePublicKey(PrivateKey key) throws GeneralSecurityException {
    061        if (key instanceof DSAPrivateKey) {
    062            DSAPrivateKey dsaKey = (DSAPrivateKey) key;
    063            DSAParams keyParams = dsaKey.getParams();
    064            BigInteger g = keyParams.getG();
    065            BigInteger p = keyParams.getP();
    066            BigInteger q = keyParams.getQ();
    067            BigInteger x = dsaKey.getX();
    068            BigInteger y = q.modPow(x, p);
    069            DSAPublicKeySpec keySpec = new DSAPublicKeySpec(y, p, q, g);
    070            return KeyFactory.getInstance("DSA").generatePublic(keySpec);
    071        } else if (key instanceof RSAPrivateCrtKey) {
    072            RSAPrivateCrtKey rsaKey = (RSAPrivateCrtKey) key;
    073            BigInteger modulus = rsaKey.getModulus();
    074            BigInteger exponent = rsaKey.getPublicExponent();
    075            RSAPublicKeySpec keySpec = new RSAPublicKeySpec(modulus, exponent);
    076            return KeyFactory.getInstance("RSA").generatePublic(keySpec);
    077        } else {
    078            throw new KeyException("Private key was not a DSA or RSA key");
    079        }
    080    }
    081}
    082