/*
 * =============================================================================
 * 
 *   Copyright (c) 2007-2010, The JASYPT team (http://www.jasypt.org)
 * 
 *   Licensed 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.jasypt.digest;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.Provider;

import org.jasypt.commons.CommonUtils;
import org.jasypt.digest.config.DigesterConfig;
import org.jasypt.exceptions.AlreadyInitializedException;
import org.jasypt.exceptions.EncryptionInitializationException;
import org.jasypt.exceptions.EncryptionOperationNotPossibleException;
import org.jasypt.salt.RandomSaltGenerator;
import org.jasypt.salt.SaltGenerator;

 Standard implementation of the ByteDigester interface. This class lets the user specify the algorithm (and provider) to be used for creating digests, the size of the salt to be applied, the number of times the hash function will be applied (iterations) and the salt generator to be used. 

This class is thread-safe.



Configuration


The algorithm, provider, salt size iterations and salt generator 
can take values in any of these ways:

  
Using its default values.
  
Setting a DigesterConfig 
      object which provides new 
      configuration values.
  
Calling the corresponding setAlgorithm(...), setProvider(...),
      setProviderName(...),
      setSaltSizeBytes(...), setIterations(...)
      or setSaltGenerator(...) methods.

And the actual values to be used for initialization will be established
by applying the following priorities:

  
First, the default values are considered.
  
Then, if a DigesterConfig 
      object has been set with
      setConfig, the non-null values returned by its
      getX methods override the default values.
  
Finally, if the corresponding setX method has been called
      on the digester itself for any of the configuration parameters, the 
      values set by these calls override all of the above.




Initialization


Before it is ready to create digests, an object of this class has to be
initialized. Initialization happens:

  
When initialize is called.
  
When digest or matches are called for the
      first time, if initialize has not been called before.

Once a digester has been initialized, trying to
change its configuration (algorithm, provider, salt size, iterations or 
salt generator)
will result in an AlreadyInitializedException being thrown.



Usage


A digester may be used in two different ways:

  
For creating digests, by calling the digest method.
  
For matching digests, this is, checking whether a digest
      corresponds adequately to a digest (as in password checking) or not, by
      calling the matches method.
 

The steps taken for creating digests are:

  
A salt of the specified size is generated (see SaltGenerator). If salt size is zero, no salt will be used.
  
The salt bytes are added to the message.
  
The hash function is applied to the salt and message altogether, 
      and then to the
      results of the function itself, as many times as specified
      (iterations).
  
If specified by the salt generator (see SaltGenerator.includePlainSaltInEncryptionResults()), the undigested salt and the final result of the hash
      function are concatenated and returned as a result.

Put schematically in bytes:

  

    DIGEST = |S|..(ssb)..|S|X|X|X|...|X|
      

        
S: salt bytes (plain, not digested). (OPTIONAL).
        
ssb: salt size in bytes.
        
X: bytes resulting from hashing (see below).
      
  
  

    |X|X|X|...|X| = 
    H(H(H(..(it)..H(Z|Z|Z|...|Z|))))
    

      
H: Hash function (algorithm).
      
it: Number of iterations.
      
Z: Input for hashing (see below).
 
    
  
  

    |Z|Z|Z|...|Z| =
    |S|..(ssb)..|S|M|M|M...|M|
    

        
S: salt bytes (plain, not digested).
        
ssb: salt size in bytes.
        
M: message bytes.
    
  

If a random salt generator is used, two digests created for the same 
message will always be different
(except in the case of random salt coincidence).
Because of this, in this case the result of the digest method 
will contain both the undigested salt and the digest of the 
(salt + message), so that another digest operation can be performed with 
the same salt on a different message to check if both messages match 
(all of which will be managed automatically by the matches method).

     
To learn more about the mechanisms involved in digest creation, read
PKCS #5: Password-Based Cryptography Standard.

Author:
Daniel Fernández
Since:
1.0/**
 * <p>
 * Standard implementation of the {@link ByteDigester} interface.
 * This class lets the user specify the algorithm (and provider) to be used for 
 * creating digests, the size of the salt to be applied, 
 * the number of times the hash function will be applied (iterations) and
 * the salt generator to be used.
 * </p>
 * <p>
 * This class is <i>thread-safe</i>.
 * </p>
 * <p>
 * <br/><b><u>Configuration</u></b>
 * </p>
 * <p>
 * The algorithm, provider, salt size iterations and salt generator 
 * can take values in any of these ways:
 * <ul>
 *   <li>Using its default values.</li>
 *   <li>Setting a <tt>{@link org.jasypt.digest.config.DigesterConfig}</tt> 
 *       object which provides new 
 *       configuration values.</li>
 *   <li>Calling the corresponding <tt>setAlgorithm(...)</tt>, <tt>setProvider(...)</tt>,
 *       <tt>setProviderName(...)</tt>,
 *       <tt>setSaltSizeBytes(...)</tt>, <tt>setIterations(...)</tt>
 *       or <tt>setSaltGenerator(...)</tt> methods.</li>
 * </ul>
 * And the actual values to be used for initialization will be established
 * by applying the following priorities:
 * <ol>
 *   <li>First, the default values are considered.</li>
 *   <li>Then, if a <tt>{@link org.jasypt.digest.config.DigesterConfig}</tt> 
 *       object has been set with
 *       <tt>setConfig</tt>, the non-null values returned by its
 *       <tt>getX</tt> methods override the default values.</li>
 *   <li>Finally, if the corresponding <tt>setX</tt> method has been called
 *       on the digester itself for any of the configuration parameters, the 
 *       values set by these calls override all of the above.</li>
 * </ol>
 * </p>
 * 
 * <p>
 * <br/><b><u>Initialization</u></b>
 * </p>
 * <p>
 * Before it is ready to create digests, an object of this class has to be
 * <i>initialized</i>. Initialization happens:
 * <ul>
 *   <li>When <tt>initialize</tt> is called.</li>
 *   <li>When <tt>digest</tt> or <tt>matches</tt> are called for the
 *       first time, if <tt>initialize</tt> has not been called before.</li>
 * </ul>
 * Once a digester has been initialized, trying to
 * change its configuration (algorithm, provider, salt size, iterations or 
 * salt generator)
 * will result in an <tt>AlreadyInitializedException</tt> being thrown.
 * </p>
 * 
 * <p>
 * <br/><b><u>Usage</u></b>
 * </p>
 * <p>
 * A digester may be used in two different ways:
 * <ul>
 *   <li>For <i>creating digests</i>, by calling the <tt>digest</tt> method.</li>
 *   <li>For <i>matching digests</i>, this is, checking whether a digest
 *       corresponds adequately to a digest (as in password checking) or not, by
 *       calling the <tt>matches</tt> method.</li> 
 * </ul>
 * The steps taken for creating digests are:
 * <ol>
 *   <li>A salt of the specified size is generated (see 
 *       {@link SaltGenerator}). If salt size is zero, no salt will be
 *       used.</li>
 *   <li>The salt bytes are added to the message.</li>
 *   <li>The hash function is applied to the salt and message altogether, 
 *       and then to the
 *       results of the function itself, as many times as specified
 *       (iterations).</li>
 *   <li>If specified by the salt generator (see 
 *       {@link org.jasypt.salt.SaltGenerator#includePlainSaltInEncryptionResults()}), 
 *       the <i>undigested</i> salt and the final result of the hash
 *       function are concatenated and returned as a result.</li>
 * </ol>
 * Put schematically in bytes:
 * <ul>
 *   <li>
 *     DIGEST = <tt>|<b>S</b>|..(ssb)..|<b>S</b>|<b>X</b>|<b>X</b>|<b>X</b>|...|<b>X</b>|</tt>
 *       <ul>
 *         <li><tt><b>S</b></tt>: salt bytes (plain, not digested). <i>(OPTIONAL)</i>.</li>
 *         <li><tt>ssb</tt>: salt size in bytes.</li>
 *         <li><tt><b>X</b></tt>: bytes resulting from hashing (see below).</li>
 *       </ul>
 *   </li>
 *   <li>
 *     <tt>|<b>X</b>|<b>X</b>|<b>X</b>|...|<b>X</b>|</tt> = 
 *     <tt><i>H</i>(<i>H</i>(<i>H</i>(..(it)..<i>H</i>(<b>Z</b>|<b>Z</b>|<b>Z</b>|...|<b>Z</b>|))))</tt>
 *     <ul>
 *       <li><tt><i>H</i></tt>: Hash function (algorithm).</li>
 *       <li><tt>it</tt>: Number of iterations.</li>
 *       <li><tt><b>Z</b></tt>: Input for hashing (see below).</li> 
 *     </ul>
 *   </li>
 *   <li>
 *     <tt>|<b>Z</b>|<b>Z</b>|<b>Z</b>|...|<b>Z</b>|</tt> =
 *     <tt>|<b>S</b>|..(ssb)..|<b>S</b>|<b>M</b>|<b>M</b>|<b>M</b>...|<b>M</b>|</tt>
 *     <ul>
 *         <li><tt><b>S</b></tt>: salt bytes (plain, not digested).</li>
 *         <li><tt>ssb</tt>: salt size in bytes.</li>
 *         <li><tt><b>M</b></tt>: message bytes.</li>
 *     </ul>
 *   </li>
 * </ul>
 * <b>If a random salt generator is used, two digests created for the same 
 * message will always be different
 * (except in the case of random salt coincidence).</b>
 * Because of this, in this case the result of the <tt>digest</tt> method 
 * will contain both the <i>undigested</i> salt and the digest of the 
 * (salt + message), so that another digest operation can be performed with 
 * the same salt on a different message to check if both messages match 
 * (all of which will be managed automatically by the <tt>matches</tt> method).
 * </p>
 * <p>     
 * To learn more about the mechanisms involved in digest creation, read
 * <a href="http://www.rsasecurity.com/rsalabs/node.asp?id=2127" 
 * target="_blank">PKCS &#035;5: Password-Based Cryptography Standard</a>.
 * </p>
 * 
 * @since 1.0
 * 
 * @author Daniel Fern&aacute;ndez
 * 
 */
public final class StandardByteDigester implements ByteDigester {

    
Default digest algorithm will be MD5 
/**
     * Default digest algorithm will be MD5 
     */
    public static final String DEFAULT_ALGORITHM = "MD5";
    
The minimum recommended size for salt is 8 bytes 
/**
     * The minimum recommended size for salt is 8 bytes 
     */
    public static final int DEFAULT_SALT_SIZE_BYTES = 8;
    
The minimum recommended iterations for hashing are 1000
/**
     * The minimum recommended iterations for hashing are 1000
     */
    public static final int DEFAULT_ITERATIONS = 1000;

    // Algorithm to be used for hashing
    private String algorithm = DEFAULT_ALGORITHM;
    // Size of salt to be applied
    private int saltSizeBytes = DEFAULT_SALT_SIZE_BYTES;
    // Number of hash iterations to be applied
    private int iterations = DEFAULT_ITERATIONS;
    // SaltGenerator to be used. Initialization of a salt generator is costly,
    // and so default value will be applied only in initialize(), if it finally
    // becomes necessary.
    private SaltGenerator saltGenerator = null;
    // Name of the java.security.Provider which will be asked for the selected
    // algorithm
    private String providerName = null;
    // java.security.Provider instance which will be asked for the selected
    // algorithm
    private Provider provider = null;
    // Whether salt bytes will be appended after message before digesting or
    // inserted before it (which is the default).
    private boolean invertPositionOfSaltInMessageBeforeDigesting = false;
    // Whether plain (unhashed) salt bytes will be appended after the digest
    // operation result or inserted before it (which is the default).
    private boolean invertPositionOfPlainSaltInEncryptionResults = false;
    // Whether digest matching operations will allow matching digests with a
    // salt size different to the one configured in the "saltSizeBytes" property.
    private boolean useLenientSaltSizeCheck = false;
    
    
    /*
     * Config: this object can set a configuration by bringing the values in 
     * whichever way the developer wants (it only has to implement the 
     * DigesterConfig interface).
     * 
     * Calls to setX methods OVERRIDE the values brought by this config.
     */
    private DigesterConfig config = null;

    /*
     * Set of booleans which indicate whether the config or default values
     * have to be overriden because of the setX methods having been
     * called.
     */
    private boolean algorithmSet = false;
    private boolean saltSizeBytesSet = false;
    private boolean iterationsSet = false;
    private boolean saltGeneratorSet = false;
    private boolean providerNameSet = false;
    private boolean providerSet = false;
    private boolean invertPositionOfSaltInMessageBeforeDigestingSet = false;
    private boolean invertPositionOfPlainSaltInEncryptionResultsSet = false;
    private boolean useLenientSaltSizeCheckSet = false;

    /*
     * Flag which indicates whether the digester has been initialized or not.
     * 
     * Once initialized, no further modifications to its configuration will
     * be allowed.
     */
    private boolean initialized = false;

    /*
     * If the salt size is set to a value higher than zero, this flag will
     * indicate that the salt mecanism has to be used.
     */
    private boolean useSalt = true;

    /*
     * MessageDigest to be used. 
     * 
     * IMPORTANT: MessageDigest is not a thread-safe class, and thus any
     * use of this variable will have to be adequately synchronized. 
     */
    private MessageDigest md = null;
    
    /*
     * Length of the result digest for the specified algorithm.
     * This might be zero if this operation is not supported by the 
     * algorithm provider and the implementation is not cloneable. 
     */
    private int digestLengthBytes = 0;
    
    

    
    
Creates a new instance of StandardByteDigester.
/**
     * Creates a new instance of <tt>StandardByteDigester</tt>.
     */
    public StandardByteDigester() {
        super();
    }
    
    
    

Sets a DigesterConfig object 
for the digester. If this config
object is set, it will be asked values for:


  
Algorithm
  
Security Provider (or provider name)
  
Salt size
  
Hashing iterations
  
Salt generator
  
Location of the salt in relation to the encrypted message 
      (default: before)


The non-null values it returns will override the default ones, 
and will be overriden by any values specified with a setX
method.

Params:
config – the DigesterConfig object to be used as the 
              source for configuration parameters./**
     * <p>
     * Sets a <tt>{@link org.jasypt.digest.config.DigesterConfig}</tt> object 
     * for the digester. If this config
     * object is set, it will be asked values for:
     * </p>
     * 
     * <ul>
     *   <li>Algorithm</li>
     *   <li>Security Provider (or provider name)</li>
     *   <li>Salt size</li>
     *   <li>Hashing iterations</li>
     *   <li>Salt generator</li>
     *   <li>Location of the salt in relation to the encrypted message 
     *       (default: before)</li>
     * </ul>
     * 
     * <p>
     * The non-null values it returns will override the default ones, 
     * <i>and will be overriden by any values specified with a <tt>setX</tt>
     * method</i>.
     * </p>
     * 
     * @param config the <tt>DigesterConfig</tt> object to be used as the 
     *               source for configuration parameters.
     */
    public synchronized void setConfig(final DigesterConfig config) {
        CommonUtils.validateNotNull(config, "Config cannot be set null");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.config = config;
    }
    
    
    

Sets the algorithm to be used for digesting, like MD5 
or SHA-1.


This algorithm has to be supported by your security infrastructure, and
it should be allowed as an algorithm for creating
java.security.MessageDigest instances.

 If you are specifying a security provider with setProvider(Provider) or setProviderName(String), this algorithm should be supported by your specified provider. 

If you are not specifying a provider, you will be able to use those
algorithms provided by the default security provider of your JVM vendor.
For valid names in the Sun JVM, see Java 
        Cryptography Architecture API Specification & 
        Reference.

Params:
algorithm – the name of the algorithm to be used./**
     * <p>
     * Sets the algorithm to be used for digesting, like <tt>MD5</tt> 
     * or <tt>SHA-1</tt>.
     * </p>
     * <p>
     * This algorithm has to be supported by your security infrastructure, and
     * it should be allowed as an algorithm for creating
     * java.security.MessageDigest instances.
     * </p>
     * <p>
     * If you are specifying a security provider with {@link #setProvider(Provider)} or
     * {@link #setProviderName(String)}, this algorithm should be
     * supported by your specified provider.
     * </p>
     * <p>
     * If you are not specifying a provider, you will be able to use those
     * algorithms provided by the default security provider of your JVM vendor.
     * For valid names in the Sun JVM, see <a target="_blank" 
     *         href="http://java.sun.com/j2se/1.5.0/docs/guide/security/CryptoSpec.html#AppA">Java 
     *         Cryptography Architecture API Specification & 
     *         Reference</a>.
     * </p>
     * 
     * @param algorithm the name of the algorithm to be used.
     */
    public synchronized void setAlgorithm(final String algorithm) {
        CommonUtils.validateNotEmpty(algorithm, "Algorithm cannot be empty");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.algorithm = algorithm;
        this.algorithmSet = true;
    }
    
    
    

Sets the size of the salt to be used to compute the digest.
This mechanism is explained in 
PKCS #5: Password-Based Cryptography Standard.


If salt size is set to zero, then no salt will be used.

Params:
saltSizeBytes – the size of the salt to be used, in bytes./**
     * <p>
     * Sets the size of the salt to be used to compute the digest.
     * This mechanism is explained in 
     * <a href="http://www.rsasecurity.com/rsalabs/node.asp?id=2127" 
     * target="_blank">PKCS &#035;5: Password-Based Cryptography Standard</a>.
     * </p>
     * 
     * <p>
     * If salt size is set to zero, then no salt will be used.
     * </p>
     * 
     * @param saltSizeBytes the size of the salt to be used, in bytes.
     */
    public synchronized void setSaltSizeBytes(final int saltSizeBytes) {
        CommonUtils.validateIsTrue(saltSizeBytes >= 0, "Salt size in bytes must be non-negative");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.saltSizeBytes = saltSizeBytes;
        this.useSalt = (saltSizeBytes > 0);
        this.saltSizeBytesSet = true;
    }

    
    

Set the number of times the hash function will be applied recursively.


The hash function will be applied to its own results as many times as 
specified: h(h(...h(x)...))


This mechanism is explained in 
PKCS #5: Password-Based Cryptography Standard.

Params:
iterations – the number of iterations./**
     * <p>
     * Set the number of times the hash function will be applied recursively.
     * <br/>
     * The hash function will be applied to its own results as many times as 
     * specified: <i>h(h(...h(x)...))</i>
     * </p>
     * <p>
     * This mechanism is explained in 
     * <a href="http://www.rsasecurity.com/rsalabs/node.asp?id=2127" 
     * target="_blank">PKCS &#035;5: Password-Based Cryptography Standard</a>.
     * </p>
     * 
     * @param iterations the number of iterations.
     */
    public synchronized void setIterations(final int iterations) {
        CommonUtils.validateIsTrue(iterations > 0, "Number of iterations must be greater than zero");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.iterations = iterations;
        this.iterationsSet = true;
    }

    
    
 Sets the salt generator to be used. If no salt generator is specified, an instance of RandomSaltGenerator will be used. 
Params:
saltGenerator – the salt generator to be used.
Since:
1.2/**
     * <p>
     * Sets the salt generator to be used. If no salt generator is specified,
     * an instance of {@link org.jasypt.salt.RandomSaltGenerator} will be used. 
     * </p>
     * 
     * @since 1.2
     * 
     * @param saltGenerator the salt generator to be used.
     */
    public synchronized void setSaltGenerator(final SaltGenerator saltGenerator) {
        CommonUtils.validateNotNull(saltGenerator, "Salt generator cannot be set null");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.saltGenerator = saltGenerator;
        this.saltGeneratorSet = true;
    }
    
    
    

Sets the name of the security provider to be asked for the
digest algorithm. This security provider has to be registered beforehand
at the JVM security framework. 

 The provider can also be set with the setProvider(Provider) method, in which case it will not be necessary neither registering the provider beforehand, nor calling this setProviderName(String) method to specify a provider name. 
 Note that a call to setProvider(Provider) overrides any value set by this method. 

If no provider name / provider is explicitly set, the default JVM
provider will be used.

Params:
providerName – the name of the security provider to be asked
                    for the digest algorithm.
Since:
1.3/**
     * <p>
     * Sets the name of the security provider to be asked for the
     * digest algorithm. This security provider has to be registered beforehand
     * at the JVM security framework. 
     * </p>
     * <p>
     * The provider can also be set with the {@link #setProvider(Provider)}
     * method, in which case it will not be necessary neither registering
     * the provider beforehand,
     * nor calling this {@link #setProviderName(String)} method to specify
     * a provider name.
     * </p>
     * <p>
     * Note that a call to {@link #setProvider(Provider)} overrides any value 
     * set by this method.
     * </p>
     * <p>
     * If no provider name / provider is explicitly set, the default JVM
     * provider will be used.
     * </p>
     * 
     * @since 1.3
     * 
     * @param providerName the name of the security provider to be asked
     *                     for the digest algorithm.
     */
    public synchronized void setProviderName(final String providerName) {
        CommonUtils.validateNotNull(providerName, "Provider name cannot be set null");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.providerName = providerName;
        this.providerNameSet = true;
    }
    
    
    

Sets the security provider to be asked for the digest algorithm.
The provider does not have to be registered at the security 
infrastructure beforehand, and its being used here will not result in
its being registered.

 If this method is called, calling setProviderName(String) becomes unnecessary. 

If no provider name / provider is explicitly set, the default JVM
provider will be used.

Params:
provider – the provider to be asked for the chosen algorithm
Since:
1.3/**
     * <p>
     * Sets the security provider to be asked for the digest algorithm.
     * The provider does not have to be registered at the security 
     * infrastructure beforehand, and its being used here will not result in
     * its being registered.
     * </p>
     * <p>
     * If this method is called, calling {@link #setProviderName(String)}
     * becomes unnecessary.
     * </p>
     * <p>
     * If no provider name / provider is explicitly set, the default JVM
     * provider will be used.
     * </p>
     * 
     * @since 1.3
     * 
     * @param provider the provider to be asked for the chosen algorithm
     */
    public synchronized void setProvider(final Provider provider) {
        CommonUtils.validateNotNull(provider, "Provider cannot be set null");
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.provider = provider;
        this.providerSet = true;
    }
    
    
    

Whether the salt bytes are to be appended after the 
message ones before performing the digest operation on the whole. The 
default behaviour is to insert those bytes before the message bytes, but 
setting this configuration item to true allows compatibility 
with some external systems and specifications (e.g. LDAP {SSHA}).


If this parameter is not explicitly set, the default behaviour 
(insertion of salt before message) will be applied.

Params:
invertPositionOfSaltInMessageBeforeDigesting – 
       whether salt will be appended after the message before applying 
       the digest operation on the whole, instead of inserted before it
       (which is the default).
Since:
1.7/**
     * <p>
     * Whether the salt bytes are to be appended after the 
     * message ones before performing the digest operation on the whole. The 
     * default behaviour is to insert those bytes before the message bytes, but 
     * setting this configuration item to <tt>true</tt> allows compatibility 
     * with some external systems and specifications (e.g. LDAP {SSHA}).
     * </p>
     * <p>
     * If this parameter is not explicitly set, the default behaviour 
     * (insertion of salt before message) will be applied.
     * </p>
     * 
     * @since 1.7
     * 
     * @param invertPositionOfSaltInMessageBeforeDigesting
     *        whether salt will be appended after the message before applying 
     *        the digest operation on the whole, instead of inserted before it
     *        (which is the default).
     */
    public synchronized void setInvertPositionOfSaltInMessageBeforeDigesting(
            final boolean invertPositionOfSaltInMessageBeforeDigesting) {
        
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.invertPositionOfSaltInMessageBeforeDigesting = invertPositionOfSaltInMessageBeforeDigesting;
        this.invertPositionOfSaltInMessageBeforeDigestingSet = true;
        
    }
    
    
    

Whether the plain (not hashed) salt bytes are to 
be appended after the digest operation result bytes. The default behaviour is 
to insert them before the digest result, but setting this configuration 
item to true allows compatibility with some external systems
and specifications (e.g. LDAP {SSHA}).


If this parameter is not explicitly set, the default behaviour 
(insertion of plain salt before digest result) will be applied.

Params:
invertPositionOfPlainSaltInEncryptionResults – 
       whether plain salt will be appended after the digest operation 
       result instead of inserted before it (which is the 
       default).
Since:
1.7/**
     * <p>
     * Whether the plain (not hashed) salt bytes are to 
     * be appended after the digest operation result bytes. The default behaviour is 
     * to insert them before the digest result, but setting this configuration 
     * item to <tt>true</tt> allows compatibility with some external systems
     * and specifications (e.g. LDAP {SSHA}).
     * </p>
     * <p>
     * If this parameter is not explicitly set, the default behaviour 
     * (insertion of plain salt before digest result) will be applied.
     * </p>
     * 
     * @since 1.7
     * 
     * @param invertPositionOfPlainSaltInEncryptionResults
     *        whether plain salt will be appended after the digest operation 
     *        result instead of inserted before it (which is the 
     *        default).
     */
    public synchronized void setInvertPositionOfPlainSaltInEncryptionResults(
            final boolean invertPositionOfPlainSaltInEncryptionResults) {
        
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.invertPositionOfPlainSaltInEncryptionResults = invertPositionOfPlainSaltInEncryptionResults;
        this.invertPositionOfPlainSaltInEncryptionResultsSet = true;
        
    }

    
    
    

Whether digest matching operations will allow matching
digests with a salt size different to the one configured in the "saltSizeBytes"
property. This is possible because digest algorithms will produce a fixed-size 
result, so the remaining bytes from the hashed input will be considered salt.


This will allow the digester to match digests produced in environments which do not
establish a fixed salt size as standard (for example, SSHA password encryption
in LDAP systems).  


The value of this property will not affect the creation of digests, 
which will always have a salt of the size established by the "saltSizeBytes" 
property. It will only affect digest matching.  


Setting this property to true is not compatible with SaltGenerator implementations which return false for their SaltGenerator.includePlainSaltInEncryptionResults() property. 

Also, be aware that some algorithms or algorithm providers might not support
knowing the size of the digests beforehand, which is also incompatible with
a lenient behaviour.


If this parameter is not explicitly set, the default behaviour 
(NOT lenient) will be applied.

Params:
useLenientSaltSizeCheck – whether the digester will allow matching of 
       digests with different salt sizes than established or not (default 
       is false).
Since:
1.7/**
     * <p>
     * Whether digest matching operations will allow matching
     * digests with a salt size different to the one configured in the "saltSizeBytes"
     * property. This is possible because digest algorithms will produce a fixed-size 
     * result, so the remaining bytes from the hashed input will be considered salt.
     * </p>
     * <p>
     * This will allow the digester to match digests produced in environments which do not
     * establish a fixed salt size as standard (for example, SSHA password encryption
     * in LDAP systems).  
     * </p>
     * <p>
     * The value of this property will <b>not</b> affect the creation of digests, 
     * which will always have a salt of the size established by the "saltSizeBytes" 
     * property. It will only affect digest matching.  
     * </p>
     * <p>
     * Setting this property to <tt>true</tt> is not compatible with {@link SaltGenerator}
     * implementations which return false for their 
     * {@link SaltGenerator#includePlainSaltInEncryptionResults()} property. 
     * </p>
     * <p>
     * Also, be aware that some algorithms or algorithm providers might not support
     * knowing the size of the digests beforehand, which is also incompatible with
     * a lenient behaviour.
     * </p>
     * <p>
     * If this parameter is not explicitly set, the default behaviour 
     * (NOT lenient) will be applied.
     * </p>
     * 
     * @since 1.7
     * 
     * @param useLenientSaltSizeCheck whether the digester will allow matching of 
     *        digests with different salt sizes than established or not (default 
     *        is false).
     */
    public synchronized void setUseLenientSaltSizeCheck(final boolean useLenientSaltSizeCheck) {
        
        if (isInitialized()) {
            throw new AlreadyInitializedException();
        }
        this.useLenientSaltSizeCheck = useLenientSaltSizeCheck;
        this.useLenientSaltSizeCheckSet = true;
        
    }


    
    
    
    
    /*
     * Clone this digester.
     */
    StandardByteDigester cloneDigester() {
        
        // Check initialization
        if (!isInitialized()) {
            initialize();
        }
        
        final StandardByteDigester cloned = new StandardByteDigester();
        if (CommonUtils.isNotEmpty(this.algorithm)) {
            cloned.setAlgorithm(this.algorithm);
        }
        cloned.setInvertPositionOfPlainSaltInEncryptionResults(this.invertPositionOfPlainSaltInEncryptionResults);
        cloned.setInvertPositionOfSaltInMessageBeforeDigesting(this.invertPositionOfSaltInMessageBeforeDigesting);
        cloned.setIterations(this.iterations);
        if (this.provider != null) {
            cloned.setProvider(this.provider);
        }
        if (this.providerName != null) {
            cloned.setProviderName(this.providerName);
        }
        if (this.saltGenerator != null) {
            cloned.setSaltGenerator(this.saltGenerator);
        }
        cloned.setSaltSizeBytes(this.saltSizeBytes);
        cloned.setUseLenientSaltSizeCheck(this.useLenientSaltSizeCheck);
        
        return cloned;
        
    }
    

    

    

  Returns true if the digester has already been initialized, false if
  not.
 
  Initialization happens:


  
When initialize is called.
  
When digest or matches are called for the
      first time, if initialize has not been called before.


  Once a digester has been initialized, trying to
  change its configuration (algorithm, provider, salt size, iterations
  or salt generator) will
  result in an AlreadyInitializedException being thrown.

Returns:
true if the digester has already been initialized, false if
  not./**
     * <p>
     *   Returns true if the digester has already been initialized, false if
     *   not.<br/> 
     *   Initialization happens:
     * </p>
     * <ul>
     *   <li>When <tt>initialize</tt> is called.</li>
     *   <li>When <tt>digest</tt> or <tt>matches</tt> are called for the
     *       first time, if <tt>initialize</tt> has not been called before.</li>
     * </ul>
     * <p>
     *   Once a digester has been initialized, trying to
     *   change its configuration (algorithm, provider, salt size, iterations
     *   or salt generator) will
     *   result in an <tt>AlreadyInitializedException</tt> being thrown.
     * </p>
     * 
     * @return true if the digester has already been initialized, false if
     *   not.
     */
    public boolean isInitialized() {
        return this.initialized;
    }
    

    

Initialize the digester.


This operation will consist in determining the actual configuration 
values to be used, and then initializing the digester with them.


These values are decided by applying the following priorities:


  
First, the default values are considered.
  
Then, if a 
      DigesterConfig 
      object has been set with
      setConfig, the non-null values returned by its
      getX methods override the default values.
  
Finally, if the corresponding setX method has been called
      on the digester itself for any of the configuration parameters, the 
      values set by these calls override all of the above.


  Once a digester has been initialized, trying to
  change its configuration will result in an 
  AlreadyInitializedException being thrown.

Throws:
EncryptionInitializationException – if initialization could not
        be correctly done (for example, if the digest algorithm chosen
        cannot be used)./**
     * <p>
     * Initialize the digester.
     * </p>
     * <p>
     * This operation will consist in determining the actual configuration 
     * values to be used, and then initializing the digester with them.
     * <br/>
     * These values are decided by applying the following priorities:
     * </p>
     * <ol>
     *   <li>First, the default values are considered.</li>
     *   <li>Then, if a 
     *       <tt>{@link org.jasypt.digest.config.DigesterConfig}</tt> 
     *       object has been set with
     *       <tt>setConfig</tt>, the non-null values returned by its
     *       <tt>getX</tt> methods override the default values.</li>
     *   <li>Finally, if the corresponding <tt>setX</tt> method has been called
     *       on the digester itself for any of the configuration parameters, the 
     *       values set by these calls override all of the above.</li>
     * </ol>
     * <p>
     *   Once a digester has been initialized, trying to
     *   change its configuration will result in an 
     *   <tt>AlreadyInitializedException</tt> being thrown.
     * </p>
     * 
     * @throws EncryptionInitializationException if initialization could not
     *         be correctly done (for example, if the digest algorithm chosen
     *         cannot be used).
     *
     */
    public synchronized void initialize() {
        
        // Double-check to avoid synchronization issues
        if (!this.initialized) {

            /*
             * If a DigesterConfig object has been set, we need to 
             * consider the values it returns (if, for each value, the
             * corresponding "setX" method has not been called).
             */
            if (this.config != null) {
                
                final String configAlgorithm = this.config.getAlgorithm();
                if (configAlgorithm != null) {
                    CommonUtils.validateNotEmpty(configAlgorithm, "Algorithm cannot be empty");
                }
                
                final Integer configSaltSizeBytes = this.config.getSaltSizeBytes();
                if (configSaltSizeBytes != null) {
                    CommonUtils.validateIsTrue(configSaltSizeBytes.intValue() >= 0, 
                            "Salt size in bytes must be non-negative");
                }
                
                final Integer configIterations = this.config.getIterations();
                if (configIterations != null) {
                    CommonUtils.validateIsTrue(configIterations.intValue() > 0, 
                            "Number of iterations must be greater than zero");
                }
                
                final SaltGenerator configSaltGenerator = this.config.getSaltGenerator();
                
                final String configProviderName = this.config.getProviderName();
                if (configProviderName != null) {
                    CommonUtils.validateNotEmpty(configProviderName, "Provider name cannot be empty");
                }
                
                final Provider configProvider = this.config.getProvider();
                
                final Boolean configInvertPositionOfSaltInMessageBeforeDigesting =
                    this.config.getInvertPositionOfSaltInMessageBeforeDigesting();

                final Boolean configInvertPositionOfPlainSaltInEncryptionResults =
                    this.config.getInvertPositionOfPlainSaltInEncryptionResults();
                
                final Boolean configUseLenientSaltSizeCheck =
                    this.config.getUseLenientSaltSizeCheck();
                

                this.algorithm = 
                    ((this.algorithmSet) || (configAlgorithm == null))?
                            this.algorithm : configAlgorithm;
                this.saltSizeBytes = 
                    ((this.saltSizeBytesSet) || (configSaltSizeBytes == null))?
                            this.saltSizeBytes : configSaltSizeBytes.intValue();
                this.iterations = 
                    ((this.iterationsSet) || (configIterations == null))?
                            this.iterations : configIterations.intValue();
                this.saltGenerator = 
                    ((this.saltGeneratorSet) || (configSaltGenerator == null))?
                            this.saltGenerator : configSaltGenerator;
                this.providerName = 
                    ((this.providerNameSet) || (configProviderName == null))?
                            this.providerName : configProviderName;
                this.provider = 
                    ((this.providerSet) || (configProvider == null))?
                            this.provider : configProvider;
                this.invertPositionOfSaltInMessageBeforeDigesting =
                    ((this.invertPositionOfSaltInMessageBeforeDigestingSet) || (configInvertPositionOfSaltInMessageBeforeDigesting == null))?
                            this.invertPositionOfSaltInMessageBeforeDigesting : configInvertPositionOfSaltInMessageBeforeDigesting.booleanValue();
                this.invertPositionOfPlainSaltInEncryptionResults =
                    ((this.invertPositionOfPlainSaltInEncryptionResultsSet) || (configInvertPositionOfPlainSaltInEncryptionResults == null))?
                            this.invertPositionOfPlainSaltInEncryptionResults : configInvertPositionOfPlainSaltInEncryptionResults.booleanValue();
                this.useLenientSaltSizeCheck =
                    ((this.useLenientSaltSizeCheckSet) || (configUseLenientSaltSizeCheck == null))?
                            this.useLenientSaltSizeCheck : configUseLenientSaltSizeCheck.booleanValue();
                
            }
            
            /*
             * If the digester was not set a salt generator in any way,
             * it is time to apply its default value.
             */
            if (this.saltGenerator == null) {
                this.saltGenerator = new RandomSaltGenerator();
            }
            
            
            /*
             * Test compatibility of salt generator with salt size checking
             * behaviour
             */
            if (this.useLenientSaltSizeCheck) {
                if (!this.saltGenerator.includePlainSaltInEncryptionResults()) {
                    throw new EncryptionInitializationException(
                            "The configured Salt Generator (" + 
                            this.saltGenerator.getClass().getName() + 
                            ") does not include plain salt " +
                            "in encryption results, which is not compatible" +
                            "with setting the salt size checking behaviour to \"lenient\".");
                }
            }
            
            /*
             * MessageDigest is initialized the usual way, and the digester
             * is marked as "initialized" so that configuration cannot be
             * changed in the future.
             */
            try {
                if (this.provider != null) {
                    this.md = 
                        MessageDigest.getInstance(
                                this.algorithm, 
                                this.provider);
                } else if (this.providerName != null) {
                    this.md = 
                        MessageDigest.getInstance(
                                this.algorithm, 
                                this.providerName);
                } else {
                    this.md = MessageDigest.getInstance(this.algorithm);
                }
            } catch (NoSuchAlgorithmException e) {
                throw new EncryptionInitializationException(e);
            } catch (NoSuchProviderException e) {
                throw new EncryptionInitializationException(e);
            }
            
            
            /*
             * Store the digest length (algorithm-dependent) and check
             * the operation is supported by the provider.
             */
            this.digestLengthBytes = this.md.getDigestLength();
            if (this.digestLengthBytes <= 0) {
                throw new EncryptionInitializationException(
                        "The configured algorithm (" + 
                        this.algorithm + ") or its provider do  " +
                        "not allow knowing the digest length beforehand " +
                        "(getDigestLength() operation), which is not compatible" +
                        "with setting the salt size checking behaviour to \"lenient\".");
            }
            
            this.initialized = true;
            
        }
        
    }
    

    

Performs a digest operation on a byte array message.


The steps taken for creating the digest are:

  
A salt of the specified size is generated (see SaltGenerator).
  
The salt bytes are added to the message.
  
The hash function is applied to the salt and message altogether, 
      and then to the
      results of the function itself, as many times as specified
      (iterations).
  
If specified by the salt generator (see SaltGenerator.includePlainSaltInEncryptionResults()), the undigested salt and the final result of the hash
      function are concatenated and returned as a result.

Put schematically in bytes:

  

    DIGEST = |S|..(ssb)..|S|X|X|X|...|X|
      

        
S: salt bytes (plain, not digested). (OPTIONAL).
        
ssb: salt size in bytes.
        
X: bytes resulting from hashing (see below).
      
  
  

    |X|X|X|...|X| = 
    H(H(H(..(it)..H(Z|Z|Z|...|Z|))))
    

      
H: Hash function (algorithm).
      
it: Number of iterations.
      
Z: Input for hashing (see below).
 
    
  
  

    |Z|Z|Z|...|Z| =
    |S|..(ssb)..|S|M|M|M...|M|
    

        
S: salt bytes (plain, not digested).
        
ssb: salt size in bytes.
        
M: message bytes.
    
  



If a random salt generator is used, two digests created for the same 
message will always be different
(except in the case of random salt coincidence).
Because of this, in this case the result of the digest method 
will contain both the undigested salt and the digest of the 
(salt + message), so that another digest operation can be performed 
with the same salt on a different message to check if both messages 
match (all of which will be managed automatically by the 
matches method).

Params:
message – the byte array to be digested
Throws:
EncryptionOperationNotPossibleException – if the digest operation
        fails, ommitting any further information about the cause for
        security reasons.
EncryptionInitializationException – if initialization could not
        be correctly done (for example, if the digest algorithm chosen
        cannot be used).
Returns:
the digest result/**
     * <p>
     * Performs a digest operation on a byte array message.
     * </p>
     * <p>
     * The steps taken for creating the digest are:
     * <ol>
     *   <li>A salt of the specified size is generated (see 
     *       {@link SaltGenerator}).</li>
     *   <li>The salt bytes are added to the message.</li>
     *   <li>The hash function is applied to the salt and message altogether, 
     *       and then to the
     *       results of the function itself, as many times as specified
     *       (iterations).</li>
     *   <li>If specified by the salt generator (see 
     *       {@link org.jasypt.salt.SaltGenerator#includePlainSaltInEncryptionResults()}), 
     *       the <i>undigested</i> salt and the final result of the hash
     *       function are concatenated and returned as a result.</li>
     * </ol>
     * Put schematically in bytes:
     * <ul>
     *   <li>
     *     DIGEST = <tt>|<b>S</b>|..(ssb)..|<b>S</b>|<b>X</b>|<b>X</b>|<b>X</b>|...|<b>X</b>|</tt>
     *       <ul>
     *         <li><tt><b>S</b></tt>: salt bytes (plain, not digested). <i>(OPTIONAL)</i>.</li>
     *         <li><tt>ssb</tt>: salt size in bytes.</li>
     *         <li><tt><b>X</b></tt>: bytes resulting from hashing (see below).</li>
     *       </ul>
     *   </li>
     *   <li>
     *     <tt>|<b>X</b>|<b>X</b>|<b>X</b>|...|<b>X</b>|</tt> = 
     *     <tt><i>H</i>(<i>H</i>(<i>H</i>(..(it)..<i>H</i>(<b>Z</b>|<b>Z</b>|<b>Z</b>|...|<b>Z</b>|))))</tt>
     *     <ul>
     *       <li><tt><i>H</i></tt>: Hash function (algorithm).</li>
     *       <li><tt>it</tt>: Number of iterations.</li>
     *       <li><tt><b>Z</b></tt>: Input for hashing (see below).</li> 
     *     </ul>
     *   </li>
     *   <li>
     *     <tt>|<b>Z</b>|<b>Z</b>|<b>Z</b>|...|<b>Z</b>|</tt> =
     *     <tt>|<b>S</b>|..(ssb)..|<b>S</b>|<b>M</b>|<b>M</b>|<b>M</b>...|<b>M</b>|</tt>
     *     <ul>
     *         <li><tt><b>S</b></tt>: salt bytes (plain, not digested).</li>
     *         <li><tt>ssb</tt>: salt size in bytes.</li>
     *         <li><tt><b>M</b></tt>: message bytes.</li>
     *     </ul>
     *   </li>
     * </ul>
     * </p>
     * <p>
     * <b>If a random salt generator is used, two digests created for the same 
     * message will always be different
     * (except in the case of random salt coincidence).</b>
     * Because of this, in this case the result of the <tt>digest</tt> method 
     * will contain both the <i>undigested</i> salt and the digest of the 
     * (salt + message), so that another digest operation can be performed 
     * with the same salt on a different message to check if both messages 
     * match (all of which will be managed automatically by the 
     * <tt>matches</tt> method).
     * </p>
     * 
     * @param message the byte array to be digested 
     * @return the digest result
     * @throws EncryptionOperationNotPossibleException if the digest operation
     *         fails, ommitting any further information about the cause for
     *         security reasons.
     * @throws EncryptionInitializationException if initialization could not
     *         be correctly done (for example, if the digest algorithm chosen
     *         cannot be used).
     *         
     */
    public byte[] digest(byte[] message) {
        
        if (message == null) {
            return null;
        }

        // Check initialization
        if (!isInitialized()) {
            initialize();
        }
        
        // Create salt
        byte[] salt = null;
        if (this.useSalt) {
            salt = this.saltGenerator.generateSalt(this.saltSizeBytes);
        }

        // Create digest
        return digest(message, salt);
        
    }

    
    /*
     * This method truly performs the digest operation, assuming that a salt
     * has already been created (if needed) and the digester has already been
     * initialized.
     */
    private byte[] digest(final byte[] message, final byte[] salt) {
        
        try {

            byte[] digest = null;
            
            synchronized (this.md) {
                
                this.md.reset();
                
                if (salt != null) {
                    
                    if (!this.invertPositionOfSaltInMessageBeforeDigesting) {
                        
                        // The salt bytes are added before the message to be digested
                        this.md.update(salt);
                        this.md.update(message);
                        
                    } else {
                        
                        // The salt bytes are appended after the message to be digested
                        this.md.update(message);
                        this.md.update(salt);
                        
                    }
                    
                } else {
                    
                    //No salt to be added
                    this.md.update(message);
                    
                }
                
                digest = this.md.digest();
                for (int i = 0; i < (this.iterations - 1); i++) {
                    this.md.reset();
                    digest = this.md.digest(digest);
                }
                
            }

            // Finally we build an array containing both the unhashed (plain) salt
            // and the digest of the (salt + message). This is done only
            // if the salt generator we are using specifies to do so.
            if (this.saltGenerator.includePlainSaltInEncryptionResults() && salt != null) {

                if (!this.invertPositionOfPlainSaltInEncryptionResults) {
                    
                    // Insert unhashed salt before the hashing result (default behaviour)
                    return CommonUtils.appendArrays(salt, digest);
                    
                }
                    
                // Append unhashed salt after the hashing result
                return CommonUtils.appendArrays(digest, salt);
                
            }
            
            return digest;
            
        } catch (Exception e) {
            // If digest fails, it is more secure not to return any information
            // about the cause in nested exceptions. Simply fail.
            throw new EncryptionOperationNotPossibleException();
        }
        
    }
    
    
    

Checks a message against a given digest.


This method tells whether a message corresponds to a specific digest
or not by getting the salt with which the digest was created and
applying it to a digest operation performed on the message. If 
new and existing digest match, the message is said to match the digest.


This method will be used, for instance, for password checking in
authentication processes.


A null message will only match a null digest.

Params:
message – the message to be compared to the digest.
digest – the digest.
Throws:
EncryptionOperationNotPossibleException – if the digest matching
        operation fails, ommitting any further information about the 
        cause for security reasons.
EncryptionInitializationException – if initialization could not
        be correctly done (for example, if the digest algorithm chosen
        cannot be used).
Returns:
true if the specified message matches the digest, false
        if not./**
     * <p>
     * Checks a message against a given digest.
     * </p>
     * <p>
     * This method tells whether a message corresponds to a specific digest
     * or not by getting the salt with which the digest was created and
     * applying it to a digest operation performed on the message. If 
     * new and existing digest match, the message is said to match the digest.
     * </p>
     * <p>
     * This method will be used, for instance, for password checking in
     * authentication processes.
     * </p>
     * <p>
     * A null message will only match a null digest.
     * </p>
     * 
     * @param message the message to be compared to the digest.
     * @param digest the digest. 
     * @return true if the specified message matches the digest, false
     *         if not.
     * @throws EncryptionOperationNotPossibleException if the digest matching
     *         operation fails, ommitting any further information about the 
     *         cause for security reasons.
     * @throws EncryptionInitializationException if initialization could not
     *         be correctly done (for example, if the digest algorithm chosen
     *         cannot be used).
     */
    public boolean matches(final byte[] message, final byte[] digest) {

        if (message == null) {
            return (digest == null);
        } else if (digest == null) {
            return false;
        }
        
        // Check initialization
        if (!isInitialized()) {
            initialize();
        }
            
        try {

            // If we are using a salt, extract it to use it.
            byte[] salt = null;
            if (this.useSalt) {
                // If we are using a salt generator which specifies the salt
                // to be included into the digest itself, get it from there.
                // If not, the salt is supposed to be fixed and thus the
                // salt generator can be safely asked for it again.
                if (this.saltGenerator.includePlainSaltInEncryptionResults()) {
                    
                    // Compute size figures and perform length checks
                    int digestSaltSize = this.saltSizeBytes;
                    if (this.digestLengthBytes > 0) {
                        if (this.useLenientSaltSizeCheck) {
                            if (digest.length < this.digestLengthBytes) {
                                throw new EncryptionOperationNotPossibleException();
                            }
                            digestSaltSize = digest.length - this.digestLengthBytes;
                        } else {
                            if (digest.length != (this.digestLengthBytes + this.saltSizeBytes)) {
                                throw new EncryptionOperationNotPossibleException();
                            }
                        }
                    } else {
                        // Salt size check behaviour cannot be set to lenient
                        if (digest.length < this.saltSizeBytes) {
                            throw new EncryptionOperationNotPossibleException();
                        }
                    }
                    
                    if (!this.invertPositionOfPlainSaltInEncryptionResults) {
                        salt = new byte[digestSaltSize];
                        System.arraycopy(digest, 0, salt, 0, digestSaltSize);
                    } else {
                        salt = new byte[digestSaltSize];
                        System.arraycopy(digest, digest.length - digestSaltSize, salt, 0, digestSaltSize);
                    }
                    
                } else {
                    salt = this.saltGenerator.generateSalt(this.saltSizeBytes);
                }
            }
            
            // Digest the message with the extracted digest.
            final byte[] encryptedMessage = digest(message, salt);
            
            // If, using the same salt, digests match, then messages too. 
            return (digestsAreEqual(encryptedMessage, digest));
        
        } catch (Exception e) {
            // If digest fails, it is more secure not to return any information
            // about the cause in nested exceptions. Simply fail.
            throw new EncryptionOperationNotPossibleException();
        }
        
    }

    
    // Time-constant comparison of byte arrays
    private static boolean digestsAreEqual(byte[] a, byte[] b) {

        if (a == null || b == null) {
            return false;
        }

        final int aLen = a.length;
        if (b.length != aLen) {
            return false;
        }

        int match = 0;
        for (int i = 0; i < aLen; i++) {
            match |= a[i] ^ b[i];
        }
        
        return (match == 0);
        
    }
    
    
    
}