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Using the CertificateFactory Class

import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.InputStream; import java.math.BigInteger; import java.security.InvalidKeyException; import java.security.KeyPair; import java.security.KeyPairGenerator; import java.security.NoSuchProviderException; import java.security.SecureRandom; import java.security.Security; import java.security.SignatureException; import java.security.cert.CertificateFactory; import java.security.cert.X509Certificate; import java.util.Date; import javax.security.auth.x500.X500Principal; import org.bouncycastle.x509.X509V1CertificateGenerator; public class MainClass {   public static void main(String[] args) throws Exception {     Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());     KeyPair pair = generateRSAKeyPair();     ByteArrayOutputStream bOut = new ByteArrayOutputStream();     bOut.write(generateV1Certificate(pair).getEncoded());     bOut.close();     InputStream in = new ByteArrayInputStream(bOut.toByteArray());     CertificateFactory fact = CertificateFactory.getInstance("X.509", "BC");     X509Certificate x509Cert = (X509Certificate) fact.generateCertificate(in);     System.out.println("issuer: " + x509Cert.getIssuerX500Principal());   }   public static X509Certificate generateV1Certificate(KeyPair pair) throws InvalidKeyException,       NoSuchProviderException, SignatureException {     Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());     X509V1CertificateGenerator certGen = new X509V1CertificateGenerator();     certGen.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));     certGen.setIssuerDN(new X500Principal("CN=Test Certificate"));     certGen.setNotBefore(new Date(System.currentTimeMillis() - 10000));     certGen.setNotAfter(new Date(System.currentTimeMillis() + 10000));     certGen.setSubjectDN(new X500Principal("CN=Test Certificate"));     certGen.setPublicKey(pair.getPublic());     certGen.setSignatureAlgorithm("SHA256WithRSAEncryption");     return certGen.generateX509Certificate(pair.getPrivate(), "BC");   }   public static KeyPair generateRSAKeyPair() throws Exception {     KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA", "BC");     kpGen.initialize(1024, new SecureRandom());     return kpGen.generateKeyPair();   } }