import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.MessageDigest;
import java.security.SecureRandom;
import java.security.Security;
import java.security.Signature;
import javax.crypto.Cipher;
import org.bouncycastle.asn1.ASN1InputStream;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.ASN1Sequence;
import org.bouncycastle.asn1.util.ASN1Dump;
public class MainClass {
public static void main(String[] args) throws Exception {
Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA", "BC");
keyGen.initialize(512, new SecureRandom());
KeyPair keyPair = keyGen.generateKeyPair();
Signature signature = Signature.getInstance("SHA256withRSA", "BC");
signature.initSign(keyPair.getPrivate());
byte[] message = "abc".getBytes();
signature.update(message);
byte[] sigBytes = signature.sign();
Cipher cipher = Cipher.getInstance("RSA/None/PKCS1Padding", "BC");
cipher.init(Cipher.DECRYPT_MODE, keyPair.getPublic());
byte[] decSig = cipher.doFinal(sigBytes);
ASN1InputStream aIn = new ASN1InputStream(decSig);
ASN1Sequence seq = (ASN1Sequence) aIn.readObject();
System.out.println(ASN1Dump.dumpAsString(seq));
MessageDigest hash = MessageDigest.getInstance("SHA-256", "BC");
hash.update(message);
ASN1OctetString sigHash = (ASN1OctetString) seq.getObjectAt(1);
System.out.println(MessageDigest.isEqual(hash.digest(), sigHash.getOctets()));
}
}