EcmaDecryptor.java

/* ====================================================================
   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.
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package org.apache.poi.poifs.crypt;

import java.io.IOException;
import java.io.InputStream;
import java.security.GeneralSecurityException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;

import org.apache.poi.poifs.filesystem.DirectoryNode;
import org.apache.poi.poifs.filesystem.DocumentInputStream;
import org.apache.poi.util.LittleEndian;

/**
 *  @author Maxim Valyanskiy
 *  @author Gary King
 */
public class EcmaDecryptor extends Decryptor {
    private final EncryptionInfo info;
    private byte[] passwordHash;
    private long _length = -1;

    public EcmaDecryptor(EncryptionInfo info) {
        this.info = info;
    }

    private byte[] generateKey(int block) throws NoSuchAlgorithmException {
        MessageDigest sha1 = MessageDigest.getInstance("SHA-1");

        sha1.update(passwordHash);
        byte[] blockValue = new byte[4];
        LittleEndian.putInt(blockValue, 0, block);
        byte[] finalHash = sha1.digest(blockValue);

        int requiredKeyLength = info.getHeader().getKeySize()/8;

        byte[] buff = new byte[64];

        Arrays.fill(buff, (byte) 0x36);

        for (int i=0; i<finalHash.length; i++) {
            buff[i] = (byte) (buff[i] ^ finalHash[i]);
        }

        sha1.reset();
        byte[] x1 = sha1.digest(buff);

        Arrays.fill(buff, (byte) 0x5c);
        for (int i=0; i<finalHash.length; i++) {
            buff[i] = (byte) (buff[i] ^ finalHash[i]);
        }

        sha1.reset();
        byte[] x2 = sha1.digest(buff);

        byte[] x3 = new byte[x1.length + x2.length];
        System.arraycopy(x1, 0, x3, 0, x1.length);
        System.arraycopy(x2, 0, x3, x1.length, x2.length);

        return truncateOrPad(x3, requiredKeyLength);
    }

    public boolean verifyPassword(String password) throws GeneralSecurityException {
        passwordHash = hashPassword(info, password);

        Cipher cipher = getCipher();

        byte[] verifier = cipher.doFinal(info.getVerifier().getVerifier());

        MessageDigest sha1 = MessageDigest.getInstance("SHA-1");
        byte[] calcVerifierHash = sha1.digest(verifier);

        byte[] verifierHash = truncateOrPad(cipher.doFinal(info.getVerifier().getVerifierHash()), calcVerifierHash.length);

        return Arrays.equals(calcVerifierHash, verifierHash);
    }

    /**
     * Returns a byte array of the requested length,
     *  truncated or zero padded as needed.
     * Behaves like Arrays.copyOf in Java 1.6
     */
    private byte[] truncateOrPad(byte[] source, int length) {
       byte[] result = new byte[length];
       System.arraycopy(source, 0, result, 0, Math.min(length, source.length));
       if(length > source.length) {
          for(int i=source.length; i<length; i++) {
             result[i] = 0;
          }
       }
       return result;
    }

    private Cipher getCipher() throws GeneralSecurityException {
        byte[] key = generateKey(0);
        Cipher cipher = Cipher.getInstance("AES/ECB/NoPadding");
        SecretKey skey = new SecretKeySpec(key, "AES");
        cipher.init(Cipher.DECRYPT_MODE, skey);

        return cipher;
    }

    public InputStream getDataStream(DirectoryNode dir) throws IOException, GeneralSecurityException {
        DocumentInputStream dis = dir.createDocumentInputStream("EncryptedPackage");

        _length = dis.readLong();

        return new CipherInputStream(dis, getCipher());
    }

    public long getLength(){
        if(_length == -1) throw new IllegalStateException("EcmaDecryptor.getDataStream() was not called");
        return _length;
    }
}