org.openhie.openempi.util.SessionGenerator.java Source code

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Here is the source code for org.openhie.openempi.util.SessionGenerator.java

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/**
 *
 * Copyright (C) 2002-2012 "SYSNET International, Inc."
 * support@sysnetint.com [http://www.sysnetint.com]
 *
 * This file is part of OpenEMPI.
 *
 * OpenEMPI is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
package org.openhie.openempi.util;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Random;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Generates unique, secure session IDs. This class is based on the algorithm used to generate session IDs in the Apache Tomcat server
 * with small modifications.
 * 
 * @author <a href="mailto:odysseas@sysnetint.com">Odysseas Pentakalos</a>
 */
public class SessionGenerator {
    protected static final Log log = LogFactory.getLog(SessionGenerator.class);

    protected static final String DEFAULT_ALGORITHM = "MD5";
    protected static int sessionIdLength = 16;
    protected static int duplicates = 0;
    protected static MessageDigest digest;
    protected static Random random;
    protected static java.util.Map<String, String> sessions = new ConcurrentHashMap<String, String>();

    public static synchronized String generateSessionId() {

        byte random[] = new byte[16];
        String result = null;

        StringBuffer buffer = new StringBuffer();
        do {
            int resultLenBytes = 0;
            if (result != null) {
                buffer = new StringBuffer();
                duplicates++;
            }

            while (resultLenBytes < SessionGenerator.sessionIdLength) {
                getRandomBytes(random);
                random = getDigest().digest(random);
                for (int j = 0; j < random.length && resultLenBytes < SessionGenerator.sessionIdLength; j++) {
                    byte b1 = (byte) ((random[j] & 0xf0) >> 4);
                    byte b2 = (byte) (random[j] & 0x0f);
                    if (b1 < 10)
                        buffer.append((char) ('0' + b1));
                    else
                        buffer.append((char) ('A' + (b1 - 10)));
                    if (b2 < 10)
                        buffer.append((char) ('0' + b2));
                    else
                        buffer.append((char) ('A' + (b2 - 10)));
                    resultLenBytes++;
                }
            }

            result = buffer.toString();
        } while (sessions.containsKey(result));
        return (result);
    }

    protected static synchronized MessageDigest getDigest() {

        if (SessionGenerator.digest == null) {
            try {
                digest = MessageDigest.getInstance(DEFAULT_ALGORITHM);
            } catch (NoSuchAlgorithmException e) {
                log.error("Unable to obtain a message digest algorithm: " + e, e);
                throw new RuntimeException("Unable to obtain a message digest algorithm: " + e.getMessage());
            }
        }
        return digest;
    }

    protected static void getRandomBytes(byte bytes[]) {
        getRandom().nextBytes(bytes);
    }

    public static Random getRandom() {
        if (random == null) {
            // Calculate the new random number generator seed
            long seed = System.currentTimeMillis();

            random = new java.util.Random();
            random.setSeed(seed);
        }
        return (SessionGenerator.random);
    }
}