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/* ====================================================================
   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.poifs.crypt;

import java.io.IOException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.GeneralSecurityException;
import java.security.NoSuchAlgorithmException;
import org.apache.poi.poifs.filesystem.NPOIFSFileSystem;
import org.apache.poi.poifs.filesystem.POIFSFileSystem;
import org.apache.poi.poifs.filesystem.DirectoryNode;
import org.apache.poi.EncryptedDocumentException;
import org.apache.poi.util.LittleEndian;

public abstract class Decryptor {
    public static final String DEFAULT_PASSWORD="VelvetSweatshop";

    /**
     * Return a stream with decrypted data.
     * 
     * Use {@link #getLength()} to get the size of that data that can be safely read from the stream.
     * Just reading to the end of the input stream is not sufficient because there are
     * normally padding bytes that must be discarded
     * 
     *
     * @param dir the node to read from
     * @return decrypted stream
     */
    public abstract InputStream getDataStream(DirectoryNode dir)
        throws IOException, GeneralSecurityException;

    public abstract boolean verifyPassword(String password)
        throws GeneralSecurityException;

    /**
     * Returns the length of the encytpted data that can be safely read with
     * {@link #getDataStream(org.apache.poi.poifs.filesystem.DirectoryNode)}.
     * Just reading to the end of the input stream is not sufficient because there are
     * normally padding bytes that must be discarded
     *
     * 
     *    The length variable is initialized in {@link #getDataStream(org.apache.poi.poifs.filesystem.DirectoryNode)},
     *    an attempt to call getLength() prior to getDataStream() will result in IllegalStateException.
     * 
     *
     * @return length of the encrypted data
     * @throws IllegalStateException if {@link #getDataStream(org.apache.poi.poifs.filesystem.DirectoryNode)}
     * was not called
     */
    public abstract long getLength();

    public static Decryptor getInstance(EncryptionInfo info) {
        int major = info.getVersionMajor();
        int minor = info.getVersionMinor();

        if (major == 4 && minor == 4)
            return new AgileDecryptor(info);
        else if (minor == 2 && (major == 3 || major == 4))
            return new EcmaDecryptor(info);
        else
            throw new EncryptedDocumentException("Unsupported version");
    }

    public InputStream getDataStream(NPOIFSFileSystem fs) throws IOException, GeneralSecurityException {
        return getDataStream(fs.getRoot());
    }

    public InputStream getDataStream(POIFSFileSystem fs) throws IOException, GeneralSecurityException {
        return getDataStream(fs.getRoot());
    }

    protected static int getBlockSize(int algorithm) {
        switch (algorithm) {
        case EncryptionHeader.ALGORITHM_AES_128: return 16;
        case EncryptionHeader.ALGORITHM_AES_192: return 24;
        case EncryptionHeader.ALGORITHM_AES_256: return 32;
        }
        throw new EncryptedDocumentException("Unknown block size");
    }

    protected byte[] hashPassword(EncryptionInfo info,
                                  String password) throws NoSuchAlgorithmException {
        MessageDigest sha1 = MessageDigest.getInstance("SHA-1");
        byte[] bytes;
        try {
            bytes = password.getBytes("UTF-16LE");
        } catch (UnsupportedEncodingException e) {
            throw new EncryptedDocumentException("UTF16 not supported");
        }

        sha1.update(info.getVerifier().getSalt());
        byte[] hash = sha1.digest(bytes);
        byte[] iterator = new byte[4];

        for (int i = 0; i < info.getVerifier().getSpinCount(); i++) {
            sha1.reset();
            LittleEndian.putInt(iterator, 0, i);
            sha1.update(iterator);
            hash = sha1.digest(hash);
        }

        return hash;
    }
}