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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for Android. Android ships with a stripped-down version of Bouncy Castle - this causes classloader collisions if you try to add an alternative (updated/complete) Bouncy Castle jar. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5.

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package org.spongycastle.util.encoders;

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

/**
 * Convert binary data to and from UrlBase64 encoding.  This is identical to
 * Base64 encoding, except that the padding character is "." and the other 
 * non-alphanumeric characters are "-" and "_" instead of "+" and "/".
 * 
 * The purpose of UrlBase64 encoding is to provide a compact encoding of binary
 * data that is safe for use as an URL parameter. Base64 encoding does not
 * produce encoded values that are safe for use in URLs, since "/" can be 
 * interpreted as a path delimiter; "+" is the encoded form of a space; and
 * "=" is used to separate a name from the corresponding value in an URL 
 * parameter.
 */
public class UrlBase64
{
    private static final Encoder encoder = new UrlBase64Encoder();
    
    /**
     * Encode the input data producing a URL safe base 64 encoded byte array.
     *
     * @return a byte array containing the URL safe base 64 encoded data.
     */
    public static byte[] encode(
        byte[]    data)
    {
        ByteArrayOutputStream    bOut = new ByteArrayOutputStream();
        
        try
        {
            encoder.encode(data, 0, data.length, bOut);
        }
        catch (IOException e)
        {
            throw new RuntimeException("exception encoding URL safe base64 string: " + e);
        }
        
        return bOut.toByteArray();
    }

    /**
     * Encode the byte data 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);
    }
    
    /**
     * Decode the URL safe base 64 encoded input data - white space will be ignored.
     *
     * @return a byte array representing the decoded data.
     */
    public static byte[] decode(
        byte[]    data)
    {
        ByteArrayOutputStream    bOut = new ByteArrayOutputStream();
        
        try
        {
            encoder.decode(data, 0, data.length, bOut);
        }
        catch (IOException e)
        {
            throw new RuntimeException("exception decoding URL safe base64 string: " + e);
        }
        
        return bOut.toByteArray();
    }
    
    /**
     * decode the URL safe base 64 encoded byte data writing it to the given output stream,
     * whitespace characters will be ignored.
     *
     * @return the number of bytes produced.
     */
    public static int decode(
        byte[]                data,
        OutputStream    out)
        throws IOException
    {
        return encoder.decode(data, 0, data.length, out);
    }
    
    /**
     * decode the URL safe base 64 encoded String data - whitespace will be ignored.
     *
     * @return a byte array representing the decoded data.
     */
    public static byte[] decode(
        String    data)
    {
        ByteArrayOutputStream    bOut = new ByteArrayOutputStream();
        
        try
        {
            encoder.decode(data, bOut);
        }
        catch (IOException e)
        {
            throw new RuntimeException("exception decoding URL safe base64 string: " + e);
        }
        
        return bOut.toByteArray();
    }
    
    /**
     * Decode the URL safe 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);
    }
}