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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 1.7.
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package org.bouncycastle.util.encoders;

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

public class HexEncoder
    implements Encoder
{
    protected final byte[] encodingTable =
        {
            (byte)'0', (byte)'1', (byte)'2', (byte)'3', (byte)'4', (byte)'5', (byte)'6', (byte)'7',
            (byte)'8', (byte)'9', (byte)'a', (byte)'b', (byte)'c', (byte)'d', (byte)'e', (byte)'f'
        };
    
    /*
     * set up the decoding table.
     */
    protected final byte[] decodingTable = new byte[128];

    protected void initialiseDecodingTable()
    {
        for (int i = 0; i < decodingTable.length; i++)
        {
            decodingTable[i] = (byte)0xff;
        }

        for (int i = 0; i < encodingTable.length; i++)
        {
            decodingTable[encodingTable[i]] = (byte)i;
        }
        
        decodingTable['A'] = decodingTable['a'];
        decodingTable['B'] = decodingTable['b'];
        decodingTable['C'] = decodingTable['c'];
        decodingTable['D'] = decodingTable['d'];
        decodingTable['E'] = decodingTable['e'];
        decodingTable['F'] = decodingTable['f'];
    }
    
    public HexEncoder()
    {
        initialiseDecodingTable();
    }
    
    /**
     * encode the input data producing a Hex output stream.
     *
     * @return the number of bytes produced.
     */
    public int encode(
        byte[]                data,
        int                    off,
        int                    length,
        OutputStream    out) 
        throws IOException
    {        
        for (int i = off; i < (off + length); i++)
        {
            int    v = data[i] & 0xff;

            out.write(encodingTable[(v >>> 4)]);
            out.write(encodingTable[v & 0xf]);
        }

        return length * 2;
    }

    private static boolean ignore(
        char    c)
    {
        return c == '\n' || c =='\r' || c == '\t' || c == ' ';
    }

    /**
     * decode the Hex encoded byte data writing it to the given output stream,
     * whitespace characters will be ignored.
     *
     * @return the number of bytes produced.
     */
    public int decode(
        byte[]          data,
        int             off,
        int             length,
        OutputStream    out)
        throws IOException
    {
        byte    b1, b2;
        int     outLen = 0;
        
        int     end = off + length;
        
        while (end > off)
        {
            if (!ignore((char)data[end - 1]))
            {
                break;
            }
            
            end--;
        }
        
        int i = off;
        while (i < end)
        {
            while (i < end && ignore((char)data[i]))
            {
                i++;
            }
            
            b1 = decodingTable[data[i++]];
            
            while (i < end && ignore((char)data[i]))
            {
                i++;
            }
            
            b2 = decodingTable[data[i++]];

            if ((b1 | b2) < 0)
            {
                throw new IOException("invalid characters encountered in Hex data");
            }

            out.write((b1 << 4) | b2);
            
            outLen++;
        }

        return outLen;
    }
    
    /**
     * decode the Hex encoded String data writing it to the given output stream,
     * whitespace characters will be ignored.
     *
     * @return the number of bytes produced.
     */
    public int decode(
        String          data,
        OutputStream    out)
        throws IOException
    {
        byte    b1, b2;
        int     length = 0;
        
        int     end = data.length();
        
        while (end > 0)
        {
            if (!ignore(data.charAt(end - 1)))
            {
                break;
            }
            
            end--;
        }
        
        int i = 0;
        while (i < end)
        {
            while (i < end && ignore(data.charAt(i)))
            {
                i++;
            }
            
            b1 = decodingTable[data.charAt(i++)];
            
            while (i < end && ignore(data.charAt(i)))
            {
                i++;
            }
            
            b2 = decodingTable[data.charAt(i++)];

            if ((b1 | b2) < 0)
            {
                throw new IOException("invalid characters encountered in Hex string");
            }

            out.write((b1 << 4) | b2);
            
            length++;
        }

        return length;
    }
}