http://www.bouncycastle.org/java.html: 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. 
package org.bouncycastle.crypto.io;

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

import org.bouncycastle.crypto.Signer;

public class SignerOutputStream
    extends FilterOutputStream
{
    protected Signer signer;

    public SignerOutputStream(
        OutputStream    stream,
        Signer          signer)
    {
        super(stream);
        this.signer = signer;
    }

    public void write(int b)
        throws IOException
    {
        signer.update((byte)b);
        out.write(b);
    }

    public void write(
        byte[] b,
        int off,
        int len)
        throws IOException
    {
        signer.update(b, off, len);
        out.write(b, off, len);
    }

    public Signer getSigner()
    {
        return signer;
    }
}