/* ====================================================================
   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.poi.hssf.record.crypto;

import java.io.ByteArrayOutputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.util.HexDump;
import org.apache.poi.util.LittleEndianOutputStream;

public final class Biff8EncryptionKey {
	// these two constants coincidentally have the same value
	private static final int KEY_DIGEST_LENGTH = 5;
	private static final int PASSWORD_HASH_NUMBER_OF_BYTES_USED = 5;

	private final byte[] _keyDigest;

	/**
	 * Create using the default password and a specified docId
	 * @param docId 16 bytes
	 */
	public static Biff8EncryptionKey create(byte[] docId) {
		return new Biff8EncryptionKey(createKeyDigest("VelvetSweatshop", docId));
	}
	public static Biff8EncryptionKey create(String password, byte[] docIdData) {
		return new Biff8EncryptionKey(createKeyDigest(password, docIdData));
	}

	Biff8EncryptionKey(byte[] keyDigest) {
		if (keyDigest.length != KEY_DIGEST_LENGTH) {
			throw new IllegalArgumentException("Expected 5 byte key digest, but got " + HexDump.toHex(keyDigest));
		}
		_keyDigest = keyDigest;
	}

	static byte[] createKeyDigest(String password, byte[] docIdData) {
		check16Bytes(docIdData, "docId");
		int nChars = Math.min(password.length(), 16);
		byte[] passwordData = new byte[nChars*2];
		for (int i=0; i<nChars; i++) {
			char ch = password.charAt(i);
			passwordData[i*2+0] = (byte) ((ch << 0) & 0xFF);
			passwordData[i*2+1] = (byte) ((ch << 8) & 0xFF);
		}

		byte[] kd;
		MessageDigest md5;
		try {
			md5 = MessageDigest.getInstance("MD5");
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException(e);
		}

		md5.update(passwordData);
		byte[] passwordHash = md5.digest();
		md5.reset();

		for (int i=0; i<16; i++) {
			md5.update(passwordHash, 0, PASSWORD_HASH_NUMBER_OF_BYTES_USED);
			md5.update(docIdData, 0, docIdData.length);
		}
		kd = md5.digest();
		byte[] result = new byte[KEY_DIGEST_LENGTH];
		System.arraycopy(kd, 0, result, 0, KEY_DIGEST_LENGTH);
		return result;
	}

	/**
	 * @return <code>true</code> if the keyDigest is compatible with the specified saltData and saltHash
	 */
	public boolean validate(byte[] saltData, byte[] saltHash) {
		check16Bytes(saltData, "saltData");
		check16Bytes(saltHash, "saltHash");

		// validation uses the RC4 for block zero
		RC4 rc4 = createRC4(0);
		byte[] saltDataPrime = (byte[]) saltData.clone();
		rc4.encrypt(saltDataPrime);

		byte[] saltHashPrime = (byte[]) saltHash.clone();
		rc4.encrypt(saltHashPrime);

		MessageDigest md5;
		try {
			md5 = MessageDigest.getInstance("MD5");
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException(e);
		}
		md5.update(saltDataPrime);
		byte[] finalSaltResult = md5.digest();

		if (false) { // set true to see a valid saltHash value
			byte[] saltHashThatWouldWork = xor(saltHash, xor(saltHashPrime, finalSaltResult));
			System.out.println(HexDump.toHex(saltHashThatWouldWork));
		}

		return Arrays.equals(saltHashPrime, finalSaltResult);
	}

	private static byte[] xor(byte[] a, byte[] b) {
		byte[] c = new byte[a.length];
		for (int i = 0; i < c.length; i++) {
			c[i] = (byte) (a[i] ^ b[i]);
		}
		return c;
	}
	private static void check16Bytes(byte[] data, String argName) {
		if (data.length != 16) {
			throw new IllegalArgumentException("Expected 16 byte " + argName + ", but got " + HexDump.toHex(data));
		}
	}

	/**
	 * The {@link RC4} instance needs to be changed every 1024 bytes.
	 * @param keyBlockNo used to seed the newly created {@link RC4}
	 */
	RC4 createRC4(int keyBlockNo) {
		MessageDigest md5;
		try {
			md5 = MessageDigest.getInstance("MD5");
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException(e);
		}

		md5.update(_keyDigest);
		ByteArrayOutputStream baos = new ByteArrayOutputStream(4);
		new LittleEndianOutputStream(baos).writeInt(keyBlockNo);
		md5.update(baos.toByteArray());

		byte[] digest = md5.digest();
		return new RC4(digest);
	}


	/**
	 * Stores the BIFF8 encryption/decryption password for the current thread.  This has been done
	 * using a {@link ThreadLocal} in order to avoid further overloading the various public APIs
	 * (e.g. {@link HSSFWorkbook}) that need this functionality.
	 */
	private static final ThreadLocal _userPasswordTLS = new ThreadLocal();

	/**
	 * Sets the BIFF8 encryption/decryption password for the current thread.
	 *
	 * @param password pass <code>null</code> to clear user password (and use default)
	 */
	public static void setCurrentUserPassword(String password) {
		_userPasswordTLS.set(password);
	}

	/**
	 * @return the BIFF8 encryption/decryption password for the current thread.
	 * <code>null</code> if it is currently unset.
	 */
	public static String getCurrentUserPassword() {
		return (String) _userPasswordTLS.get();
	}
}
