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