http://www.bouncycastle.org/java.html: The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.
  • The Legion of the Bouncy Castle Inc. 
package org.bouncycastle.operator.bc;

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

import org.bouncycastle.crypto.CryptoException;
import org.bouncycastle.crypto.Signer;

public class BcSignerOutputStream
    extends OutputStream
{
    private Signer sig;

    BcSignerOutputStream(Signer sig)
    {
        this.sig = sig;
    }

    public void write(byte[] bytes, int off, int len)
        throws IOException
    {
        sig.update(bytes, off, len);
    }

    public void write(byte[] bytes)
        throws IOException
    {
        sig.update(bytes, 0, bytes.length);
    }

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

    byte[] getSignature()
        throws CryptoException
    {
        return sig.generateSignature();
    }

    boolean verify(byte[] expected)
    {
        return sig.verifySignature(expected);
    }
}