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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.pqc.crypto;

import org.spongycastle.crypto.Digest;
import org.spongycastle.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.
 * 
 */
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().
     * 
     * @return an updated private key object, which can be used for later signature generation.
     */
    public AsymmetricKeyParameter getUpdatedPrivateKey()
    {
        return signer.getUpdatedPrivateKey();
    }
}