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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 and up.

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package org.bouncycastle.crypto;

import java.math.BigInteger;

/**
 * interface for classes implementing algorithms modeled similar to the Digital Signature Alorithm.
 */
public interface DSA
{
    /**
     * initialise the signer for signature generation or signature
     * verification.
     *
     * @param forSigning true if we are generating a signature, false
     * otherwise.
     * @param param key parameters for signature generation.
     */
    public void init(boolean forSigning, CipherParameters param);

    /**
     * sign the passed in message (usually the output of a hash function).
     *
     * @param message the message to be signed.
     * @return two big integers representing the r and s values respectively.
     */
    public BigInteger[] generateSignature(byte[] message);

    /**
     * verify the message message against the signature values r and s.
     *
     * @param message the message that was supposed to have been signed.
     * @param r the r signature value.
     * @param s the s signature value.
     */
    public boolean verifySignature(byte[] message, BigInteger  r, BigInteger s);
}