/*
 * @(#)MessageDigestSpi.java	1.13 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.security;

import java.util.*;
import java.lang.*;
import java.io.IOException;
import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.io.InputStream;
import java.io.ByteArrayInputStream;

/**
 * This class defines the <i>Service Provider Interface</i> (<b>SPI</b>)
 * for the <code>MessageDigest</code> class, which provides the functionality
 * of a message digest algorithm, such as MD5 or SHA. Message digests are
 * secure one-way hash functions that take arbitrary-sized data and output a
 * fixed-length hash value.
 *
 * <p> All the abstract methods in this class must be implemented by a
 * cryptographic service provider who wishes to supply the implementation
 * of a particular message digest algorithm.
 *
 * <p> Implementations are free to implement the Cloneable interface.
 *
 * @author Benjamin Renaud 
 *
 * @version 1.13, 01/23/03
 *
 * @see MessageDigest
 */

public abstract class MessageDigestSpi {

    /**
     * Returns the digest length in bytes.
     *
     * <p>This concrete method has been added to this previously-defined
     * abstract class. (For backwards compatibility, it cannot be abstract.)
     * 
     * <p>The default behavior is to return 0.
     * 
     * <p>This method may be overridden by a provider to return the digest
     * length.
     *
     * @return the digest length in bytes.
     *
     * @since 1.2
     */
    protected int engineGetDigestLength() {
	return 0;
    }

    /**
     * Updates the digest using the specified byte.
     *
     * @param input the byte to use for the update.
     */
    protected abstract void engineUpdate(byte input);

    /**
     * Updates the digest using the specified array of bytes,    
     * starting at the specified offset.
     *
     * @param input the array of bytes to use for the update.
     *
     * @param offset the offset to start from in the array of bytes.
     *
     * @param len the number of bytes to use, starting at 
     * <code>offset</code>.
     */
    protected abstract void engineUpdate(byte[] input, int offset, int len);

    /**
     * Completes the hash computation by performing final
     * operations such as padding. Once <code>engineDigest</code> has 
     * been called, the engine should be reset (see 
     * {@link #engineReset() engineReset}).  
     * Resetting is the responsibility of the
     * engine implementor.
     *
     * @return the array of bytes for the resulting hash value.  
     */
    protected abstract byte[] engineDigest();

    /**
     * Completes the hash computation by performing final
     * operations such as padding. Once <code>engineDigest</code> has
     * been called, the engine should be reset (see 
     * {@link #engineReset() engineReset}).  
     * Resetting is the responsibility of the
     * engine implementor.
     *
     * This method should be abstract, but we leave it concrete for
     * binary compatibility.  Knowledgeable providers should override this
     * method.
     *
     * @param buf the output buffer in which to store the digest
     *
     * @param offset offset to start from in the output buffer
     *
     * @param len number of bytes within buf allotted for the digest.
     * Both this default implementation and the SUN provider do not
     * return partial digests.  The presence of this parameter is solely
     * for consistency in our API's.  If the value of this parameter is less
     * than the actual digest length, the method will throw a DigestException.
     * This parameter is ignored if its value is greater than or equal to
     * the actual digest length.
     *
     * @return the length of the digest stored in the output buffer.
     * 
     * @exception DigestException if an error occurs.
     *
     * @since 1.2
     */
    protected int engineDigest(byte[] buf, int offset, int len)
						throws DigestException {

	byte[] digest = engineDigest();
	if (len < digest.length)
		throw new DigestException("partial digests not returned");
	if (buf.length - offset < digest.length)
		throw new DigestException("insufficient space in the output "
					  + "buffer to store the digest");
	System.arraycopy(digest, 0, buf, offset, digest.length);
	return digest.length;
    }

    /**
     * Resets the digest for further use.
     */
    protected abstract void engineReset();    

    /**    
     * Returns a clone if the implementation is cloneable.    
     * 
     * @return a clone if the implementation is cloneable.
     *
     * @exception CloneNotSupportedException if this is called on an
     * implementation that does not support <code>Cloneable</code>.
     */
    public Object clone() throws CloneNotSupportedException {
	if (this instanceof Cloneable) {
	    return super.clone();
	} else {
	    throw new CloneNotSupportedException();
	}
    }
}