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;

import java.io.OutputStream;

import org.bouncycastle.asn1.x509.AlgorithmIdentifier;


General interface for an operator that is able to create a signature from
a stream of output.

/**
 * General interface for an operator that is able to create a signature from
 * a stream of output.
 */
public interface ContentSigner
{
    
Return the algorithm identifier describing the signature
algorithm and parameters this signer generates.

Returns:algorithm oid and parameters.
/**
     * Return the algorithm identifier describing the signature
     * algorithm and parameters this signer generates.
     *
     * @return algorithm oid and parameters.
     */
    AlgorithmIdentifier getAlgorithmIdentifier();

    
Returns a stream that will accept data for the purpose of calculating
a signature. Use org.bouncycastle.util.io.TeeOutputStream if you want to accumulate
the data on the fly as well.

Returns:an OutputStream
/**
     * Returns a stream that will accept data for the purpose of calculating
     * a signature. Use org.bouncycastle.util.io.TeeOutputStream if you want to accumulate
     * the data on the fly as well.
     *
     * @return an OutputStream
     */
    OutputStream getOutputStream();

    
Returns a signature based on the current data written to the stream, since the
start or the last call to getSignature().

Returns:bytes representing the signature.
/**
     * Returns a signature based on the current data written to the stream, since the
     * start or the last call to getSignature().
     *
     * @return bytes representing the signature.
     */
    byte[] getSignature();
}