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The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.
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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.standard;

import static org.apache.poi.poifs.crypt.EncryptionInfo.flagAES;
import static org.apache.poi.poifs.crypt.EncryptionInfo.flagCryptoAPI;

import java.io.IOException;
import java.io.InputStream;

import org.apache.poi.hssf.record.RecordInputStream;
import org.apache.poi.poifs.crypt.ChainingMode;
import org.apache.poi.poifs.crypt.CipherAlgorithm;
import org.apache.poi.poifs.crypt.CipherProvider;
import org.apache.poi.poifs.crypt.EncryptionHeader;
import org.apache.poi.poifs.crypt.HashAlgorithm;
import org.apache.poi.util.LittleEndianByteArrayOutputStream;
import org.apache.poi.util.LittleEndianConsts;
import org.apache.poi.util.LittleEndianInput;
import org.apache.poi.util.LittleEndianOutput;
import org.apache.poi.util.StringUtil;

public class StandardEncryptionHeader extends EncryptionHeader implements EncryptionRecord {

    protected StandardEncryptionHeader(StandardEncryptionHeader other) {
        super(other);
    }

    protected StandardEncryptionHeader(LittleEndianInput is) throws IOException {
        setFlags(is.readInt());
        setSizeExtra(is.readInt());
        setCipherAlgorithm(CipherAlgorithm.fromEcmaId(is.readInt()));
        setHashAlgorithm(HashAlgorithm.fromEcmaId(is.readInt()));
        int keySize = is.readInt();
        if (keySize == 0) {
            // for the sake of inheritance of the cryptoAPI classes
            // see 2.3.5.1 RC4 CryptoAPI Encryption Header
            // If set to 0x00000000, it MUST be interpreted as 0x00000028 bits.
            keySize = 0x28;
        }
        setKeySize(keySize);
        setBlockSize(getKeySize());
        setCipherProvider(CipherProvider.fromEcmaId(is.readInt()));

        is.readLong(); // skip reserved

        // CSPName may not always be specified
        // In some cases, the salt value of the EncryptionVerifier is the next chunk of data
        if (is instanceof RecordInputStream) {
            ((RecordInputStream)is).mark(LittleEndianConsts.INT_SIZE+1);
        } else {
            ((InputStream)is).mark(LittleEndianConsts.INT_SIZE+1);
        }
        int checkForSalt = is.readInt();
        if (is instanceof RecordInputStream) {
            ((RecordInputStream)is).reset();
        } else {
            ((InputStream)is).reset();
        }

        if (checkForSalt == 16) {
            setCspName("");
        } else {
            StringBuilder builder = new StringBuilder();
            while (true) {
                char c = (char) is.readShort();
                if (c == 0) {
                    break;
                }
                builder.append(c);
            }
            setCspName(builder.toString());
        }

        setChainingMode(ChainingMode.ecb);
        setKeySalt(null);
    }

    protected StandardEncryptionHeader(CipherAlgorithm cipherAlgorithm, HashAlgorithm hashAlgorithm, int keyBits, int blockSize, ChainingMode chainingMode) {
        setCipherAlgorithm(cipherAlgorithm);
        setHashAlgorithm(hashAlgorithm);
        setKeySize(keyBits);
        setBlockSize(blockSize);
        setCipherProvider(cipherAlgorithm.provider);
        setFlags(flagCryptoAPI.setBoolean(0, true)
                | flagAES.setBoolean(0, cipherAlgorithm.provider == CipherProvider.aes));
        // see http://msdn.microsoft.com/en-us/library/windows/desktop/bb931357(v=vs.85).aspx for a full list
        // setCspName("Microsoft Enhanced RSA and AES Cryptographic Provider");
    }

    /**
     * serializes the header
     */
    @Override
    public void write(LittleEndianByteArrayOutputStream bos) {
        int startIdx = bos.getWriteIndex();
        LittleEndianOutput sizeOutput = bos.createDelayedOutput(LittleEndianConsts.INT_SIZE);
        bos.writeInt(getFlags());
        bos.writeInt(0); // size extra
        bos.writeInt(getCipherAlgorithm().ecmaId);
        bos.writeInt(getHashAlgorithm().ecmaId);
        bos.writeInt(getKeySize());
        bos.writeInt(getCipherProvider().ecmaId);
        bos.writeInt(0); // reserved1
        bos.writeInt(0); // reserved2
        String cspName = getCspName();
        if (cspName == null) {
            cspName = getCipherProvider().cipherProviderName;
        }
        bos.write(StringUtil.getToUnicodeLE(cspName));
        bos.writeShort(0);
        int headerSize = bos.getWriteIndex()-startIdx-LittleEndianConsts.INT_SIZE;
        sizeOutput.writeInt(headerSize);
    }

    @Override
    public StandardEncryptionHeader copy() {
        return new StandardEncryptionHeader(this);
    }
}