convert X.509 certificate into PEM-formatted file with SpongyCastle

SpongyCastleSample.java

    static {
        Security.insertProviderAt(new org.spongycastle.jce.provider.BouncyCastleProvider(), 1);
 
    }
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        try {
            final KeyPair pair = RSAPemGenerator.generateRSAKeyPair();
            X509V1CertificateGenerator certGenerator = new X509V1CertificateGenerator();
            X509Principal dnName = new X509Principal("CN=John Doe");
 
            certGenerator.setSerialNumber(BigInteger.valueOf(System.currentTimeMillis()));
            certGenerator.setSubjectDN(dnName);
            certGenerator.setIssuerDN(dnName);
            // yesterday
            Date validityBeginDate = new Date(System.currentTimeMillis() - 24 * 60 * 60 * 1000);
            // in 2 years
            Date validityEndDate = new Date(System.currentTimeMillis() + 2 * 365 * 24 * 60 * 60 * 1000);
 
            certGenerator.setNotBefore(validityBeginDate);
            certGenerator.setNotAfter(validityEndDate);
            certGenerator.setPublicKey(pair.getPublic());
            certGenerator.setSignatureAlgorithm("SHA256WithRSAEncryption");
 
            final X509Certificate cert = certGenerator.generate(pair.getPrivate());
            Log.d("main", cert.toString());
 
            // write it as PEM file
            PemWriter writer = new PemWriter(new PrintWriter(new File(getExternalCacheDir() + "/cert.pem")));
 
            // X.509 cert version
            PemObjectGenerator generator = new PemObjectGenerator() {
                @Override
                public PemObject generate() throws PemGenerationException {
                    try {
                        return new PemObject(cert.getType(), cert.getTBSCertificate());
                    } catch (CertificateEncodingException e) {
                        e.printStackTrace();
                        return null;
                    }
                }
            };
          }
            writer.writeObject(generator);
            writer.flush();
            writer.close();
 
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    
 
// result
$ cat cert.pem
-----BEGIN X.509-----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-----END X.509-----