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 to JDK 1.8.
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package org.bouncycastle.pqc.crypto;

import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;



Implements the sign and verify functions for a Signature Scheme using a hash function to allow processing of large messages.


 This class can be used with algorithms where the state associated with the private key changes as each signature is
 generated. Calling getUpdatedPrivateKey() will recover the private key that can be used to initialize a signer
 next time around.



/**
 * Implements the sign and verify functions for a Signature Scheme using a hash function to allow processing of large messages.
 * <p>
 *  This class can be used with algorithms where the state associated with the private key changes as each signature is
 *  generated. Calling getUpdatedPrivateKey() will recover the private key that can be used to initialize a signer
 *  next time around.
 * </p>
 */
public class DigestingStateAwareMessageSigner
    extends DigestingMessageSigner
{
    private final StateAwareMessageSigner signer;

    public DigestingStateAwareMessageSigner(StateAwareMessageSigner messSigner, Digest messDigest)
    {
        super(messSigner, messDigest);

        this.signer = messSigner;
    }

    
Return the current version of the private key with the updated state.


Note: calling this method will effectively disable the Signer from being used for further
 signature generation without another call to init().



Returns:an updated private key object, which can be used for later signature generation.
/**
     * Return the current version of the private key with the updated state.
     * <p>
     * <b>Note:</b> calling this method will effectively disable the Signer from being used for further
     *  signature generation without another call to init().
     * </p>
     * @return an updated private key object, which can be used for later signature generation.
     */
    public AsymmetricKeyParameter getUpdatedPrivateKey()
    {
        return signer.getUpdatedPrivateKey();
    }
}