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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8.
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package org.bouncycastle.util.encoders;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import org.bouncycastle.util.Strings;

/**
 * Utility class for converting Base64 data to bytes and back again.
 */
public class Base64
{
    private static final Encoder encoder = new Base64Encoder();

    public static String toBase64String(
        byte[] data)
    {
        return toBase64String(data, 0, data.length);
    }

    public static String toBase64String(
        byte[] data,
        int off,
        int length)
    {
        byte[] encoded = encode(data, off, length);
        return Strings.fromByteArray(encoded);
    }

    /**
     * encode the input data producing a base 64 encoded byte array.
     *
     * @return a byte array containing the base 64 encoded data.
     */
    public static byte[] encode(
        byte[] data)
    {
        return encode(data, 0, data.length);
    }

    /**
     * encode the input data producing a base 64 encoded byte array.
     *
     * @return a byte array containing the base 64 encoded data.
     */
    public static byte[] encode(
        byte[] data,
        int off,
        int length)
    {
        int len = (length + 2) / 3 * 4;
        ByteArrayOutputStream bOut = new ByteArrayOutputStream(len);

        try
        {
            encoder.encode(data, off, length, bOut);
        }
        catch (Exception e)
        {
            throw new EncoderException("exception encoding base64 string: " + e.getMessage(), e);
        }

        return bOut.toByteArray();
    }

    /**
     * Encode the byte data to base 64 writing it to the given output stream.
     *
     * @return the number of bytes produced.
     */
    public static int encode(
        byte[] data,
        OutputStream out)
        throws IOException
    {
        return encoder.encode(data, 0, data.length, out);
    }

    /**
     * Encode the byte data to base 64 writing it to the given output stream.
     *
     * @return the number of bytes produced.
     */
    public static int encode(
        byte[] data,
        int off,
        int length,
        OutputStream out)
        throws IOException
    {
        return encoder.encode(data, off, length, out);
    }

    /**
     * decode the base 64 encoded input data. It is assumed the input data is valid.
     *
     * @return a byte array representing the decoded data.
     */
    public static byte[] decode(
        byte[] data)
    {
        int len = data.length / 4 * 3;
        ByteArrayOutputStream bOut = new ByteArrayOutputStream(len);

        try
        {
            encoder.decode(data, 0, data.length, bOut);
        }
        catch (Exception e)
        {
            throw new DecoderException("unable to decode base64 data: " + e.getMessage(), e);
        }

        return bOut.toByteArray();
    }

    /**
     * decode the base 64 encoded String data - whitespace will be ignored.
     *
     * @return a byte array representing the decoded data.
     */
    public static byte[] decode(
        String data)
    {
        int len = data.length() / 4 * 3;
        ByteArrayOutputStream bOut = new ByteArrayOutputStream(len);

        try
        {
            encoder.decode(data, bOut);
        }
        catch (Exception e)
        {
            throw new DecoderException("unable to decode base64 string: " + e.getMessage(), e);
        }

        return bOut.toByteArray();
    }

    /**
     * decode the base 64 encoded String data writing it to the given output stream,
     * whitespace characters will be ignored.
     *
     * @return the number of bytes produced.
     */
    public static int decode(
        String data,
        OutputStream out)
        throws IOException
    {
        return encoder.decode(data, out);
    }

    /**
     * Decode to an output stream;
     *
     * @param base64Data       The source data.
     * @param start            Start position.
     * @param length           the length.
     * @param out The output stream to write to.
     */
    public static int decode(byte[] base64Data, int start, int length, OutputStream out)
    {
        try
        {
           return encoder.decode(base64Data, start, length, out);
        }
        catch (Exception e)
        {
            throw new DecoderException("unable to decode base64 data: " + e.getMessage(), e);
        }

    }
}