server/src/main/java/org/apache/asyncweb/server/session/SecureRandomKeyFactory.java

/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you under the Apache License, Version 2.0 (the
 *  "License"); you may not use this file except in compliance
 *  with the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing,
 *  software distributed under the License is distributed on an
 *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
 *  under the License.
 *
 */
package org.apache.asyncweb.server.session;

import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SecureRandomKeyFactory implements HttpSessionKeyFactory {

    /**
     * The default key length provided by this factory
     */
    private static final int DEFAULT_KEY_LENGTH = 16;

    /**
     * The minimum key length supported by this factory
     */
    private static final int MINIMUM_KEY_LENGTH = 8;

    /**
     * The default algorithm employed
     */
    private static final String DEFAULT_ALGORITHM = "SHA1PRNG";

    private static final Logger LOG = LoggerFactory
            .getLogger(SecureRandomKeyFactory.class);

    private SecureRandom secureRandom;

    private int keyLength = DEFAULT_KEY_LENGTH;

    private volatile boolean isStarted;

    private String algorithm = DEFAULT_ALGORITHM;

    /**
     * Creates a session key based on bytes provided from an underlying
     * <code>SecureRandom</code>
     *
     * @return The created key
     */
    public String createSessionKey() {
        byte[] keyBytes = new byte[keyLength];
        synchronized (secureRandom) {
            secureRandom.nextBytes(keyBytes);
        }
        return bytesToSessionKey(keyBytes);
    }

    /**
     * Sets the number of <i>bytes</i> used when forming keys created by this factory.
     * A hex encoding is applied to the generated key bytes such that the number of
     * <i>characters</i> in keys created by this factory is twice the key length
     *
     * @param keyLength  The number of bytes employed in keys created by this factory
     */
    public void setKeyLength(int keyLength) {
        if (keyLength < MINIMUM_KEY_LENGTH) {
            throw new IllegalArgumentException("Key length must be >= "
                    + MINIMUM_KEY_LENGTH);
        }
        if (isStarted) { // sanity check
            throw new IllegalStateException("Key factory started");
        }
        this.keyLength = keyLength;
    }

    /**
     * Sets the algorithm employed by the underlying <code>SecureRandom</code>.
     * The default is <code>SHA1PRNG</code>
     *
     * @param algorithm The algorithm to be employed
     */
    public void setAlgorithm(String algorithm) {
        this.algorithm = algorithm;
    }

    /**
     * Starts this factory.
     */
    public void start() {
        isStarted = true;
        if (LOG.isDebugEnabled())
            LOG.debug("Attempting to obtain SecureRandom using algorithim: "
                    + algorithm);
        try {
            secureRandom = SecureRandom.getInstance(algorithm);
            LOG.info("Using algorithm: " + algorithm);
        } catch (NoSuchAlgorithmException e) {
            LOG.warn("Failed to obtain secure random with algorithm: "
                    + algorithm + ". Resorting to default");
        }
        secureRandom = new SecureRandom();
        secureRandom.nextBytes(new byte[keyLength]); // seed
    }

    private static String bytesToSessionKey(byte[] bytes) {
        char[] keyChars = new char[bytes.length * 2];
        for (int i = 0; i < bytes.length; ++i) {
            byte b1 = (byte) ((bytes[i] & 0xf0) >> 4);
            byte b2 = (byte) (bytes[i] & 0x0f);
            keyChars[2 * i] = toKeyStringChar(b1);
            keyChars[2 * i + 1] = toKeyStringChar(b2);
        }
        return new String(keyChars);
    }

    private static final char toKeyStringChar(byte b) {
        if (b < 10) {
            return (char) ('0' + b);
        } else {
            return (char) ('A' + b - 10);
        }
    }

}