The following solution works for generating a self signed certificate using Spongy Castle (Bouncy Castle) on Android. I've tested the code with Android 10 (Q) and Android Pie.
This code is a modified version of Netty's io.netty.handler.ssl.util.SelfSignedCertificate. The original version requires Bouncy Castle; which does not seem to be present by default on Android 10 resulting in a java.lang.NoClassDefFoundError: org.spongycastle.jce.provider.BouncyCastleProvider. Hence, I had to copy over the code and modify it to get it working with Spongy Castle.
build.gradle
dependencies {
implementation 'com.madgag.spongycastle:bcpkix-jdk15on:1.58.0.0'
}
SelfSignedCertificate.java
import android.util.Base64;
import android.util.Log;
import org.spongycastle.asn1.x500.X500Name;
import org.spongycastle.cert.X509CertificateHolder;
import org.spongycastle.cert.X509v3CertificateBuilder;
import org.spongycastle.cert.jcajce.JcaX509CertificateConverter;
import org.spongycastle.cert.jcajce.JcaX509v3CertificateBuilder;
import org.spongycastle.jce.provider.BouncyCastleProvider;
import org.spongycastle.operator.ContentSigner;
import org.spongycastle.operator.jcajce.JcaContentSignerBuilder;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.math.BigInteger;
import java.nio.charset.Charset;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.Provider;
import java.security.SecureRandom;
import java.security.cert.CertificateEncodingException;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.Date;
public final class SelfSignedCertificate {
private static final String TAG = SelfSignedCertificate.class.getSimpleName();
/**
* Current time minus 1 year, just in case software clock goes back due to time synchronization
*/
private static final Date DEFAULT_NOT_BEFORE = new Date(System.currentTimeMillis() - 86400000L * 365);
/**
* The maximum possible value in X.509 specification: 9999-12-31 23:59:59
*/
private static final Date DEFAULT_NOT_AFTER = new Date(253402300799000L);
/**
* FIPS 140-2 encryption requires the key length to be 2048 bits or greater.
* Let's use that as a sane default but allow the default to be set dynamically
* for those that need more stringent security requirements.
*/
private static final int DEFAULT_KEY_LENGTH_BITS = 2048;
/**
* FQDN to use if none is specified.
*/
private static final String DEFAULT_FQDN = "example.com";
/**
* 7-bit ASCII, as known as ISO646-US or the Basic Latin block of the
* Unicode character set
*/
private static final Charset US_ASCII = Charset.forName("US-ASCII");
private static final Provider provider = new BouncyCastleProvider();
private final File certificate;
private final File privateKey;
private final X509Certificate cert;
private final PrivateKey key;
/**
* Creates a new instance.
*/
public SelfSignedCertificate() throws CertificateException {
this(DEFAULT_NOT_BEFORE, DEFAULT_NOT_AFTER);
}
/**
* Creates a new instance.
*
* @param notBefore Certificate is not valid before this time
* @param notAfter Certificate is not valid after this time
*/
public SelfSignedCertificate(Date notBefore, Date notAfter) throws CertificateException {
this("example.com", notBefore, notAfter);
}
/**
* Creates a new instance.
*
* @param fqdn a fully qualified domain name
*/
public SelfSignedCertificate(String fqdn) throws CertificateException {
this(fqdn, DEFAULT_NOT_BEFORE, DEFAULT_NOT_AFTER);
}
/**
* Creates a new instance.
*
* @param fqdn a fully qualified domain name
* @param notBefore Certificate is not valid before this time
* @param notAfter Certificate is not valid after this time
*/
public SelfSignedCertificate(String fqdn, Date notBefore, Date notAfter) throws CertificateException {
// Bypass entropy collection by using insecure random generator.
// We just want to generate it without any delay because it's for testing purposes only.
this(fqdn, new SecureRandom(), DEFAULT_KEY_LENGTH_BITS, notBefore, notAfter);
}
/**
* Creates a new instance.
*
* @param fqdn a fully qualified domain name
* @param random the {@link java.security.SecureRandom} to use
* @param bits the number of bits of the generated private key
*/
public SelfSignedCertificate(String fqdn, SecureRandom random, int bits) throws CertificateException {
this(fqdn, random, bits, DEFAULT_NOT_BEFORE, DEFAULT_NOT_AFTER);
}
/**
* Creates a new instance.
*
* @param fqdn a fully qualified domain name
* @param random the {@link java.security.SecureRandom} to use
* @param bits the number of bits of the generated private key
* @param notBefore Certificate is not valid before this time
* @param notAfter Certificate is not valid after this time
*/
public SelfSignedCertificate(String fqdn, SecureRandom random, int bits, Date notBefore, Date notAfter)
throws CertificateException {
// Generate an RSA key pair.
final KeyPair keypair;
try {
KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(bits, random);
keypair = keyGen.generateKeyPair();
} catch (NoSuchAlgorithmException e) {
// Should not reach here because every Java implementation must have RSA key pair generator.
throw new Error(e);
}
String[] paths;
try {
// Try Bouncy Castle if the current JVM didn't have sun.security.x509.
paths = generateCertificate(fqdn, keypair, random, notBefore, notAfter);
} catch (Throwable t2) {
Log.d(TAG, "Failed to generate a self-signed X.509 certificate using Bouncy Castle:", t2);
throw new CertificateException("No provider succeeded to generate a self-signed certificate. See debug log for the root cause.", t2);
}
certificate = new File(paths[0]);
privateKey = new File(paths[1]);
key = keypair.getPrivate();
FileInputStream certificateInput = null;
try {
certificateInput = new FileInputStream(certificate);
cert = (X509Certificate) CertificateFactory.getInstance("X509").generateCertificate(certificateInput);
} catch (Exception e) {
throw new CertificateEncodingException(e);
} finally {
if (certificateInput != null) {
try {
certificateInput.close();
} catch (IOException e) {
Log.w(TAG, "Failed to close a file: " + certificate, e);
}
}
}
}
/**
* Returns the generated X.509 certificate file in PEM format.
*/
public File certificate() {
return certificate;
}
/**
* Returns the generated RSA private key file in PEM format.
*/
public File privateKey() {
return privateKey;
}
/**
* Returns the generated X.509 certificate.
*/
public X509Certificate cert() {
return cert;
}
/**
* Returns the generated RSA private key.
*/
public PrivateKey key() {
return key;
}
/**
* Deletes the generated X.509 certificate file and RSA private key file.
*/
public void delete() {
safeDelete(certificate);
safeDelete(privateKey);
}
private static String[] generateCertificate(String fqdn, KeyPair keypair, SecureRandom random, Date notBefore, Date notAfter)
throws Exception {
PrivateKey key = keypair.getPrivate();
// Prepare the information required for generating an X.509 certificate.
X500Name owner = new X500Name("CN=" + fqdn);
X509v3CertificateBuilder builder = new JcaX509v3CertificateBuilder(
owner, new BigInteger(64, random), notBefore, notAfter, owner, keypair.getPublic());
ContentSigner signer = new JcaContentSignerBuilder("SHA256WithRSAEncryption").build(key);
X509CertificateHolder certHolder = builder.build(signer);
X509Certificate cert = new JcaX509CertificateConverter().setProvider(provider).getCertificate(certHolder);
cert.verify(keypair.getPublic());
return newSelfSignedCertificate(fqdn, key, cert);
}
private static String[] newSelfSignedCertificate(String fqdn, PrivateKey key, X509Certificate cert) throws IOException, CertificateEncodingException {
String keyText = "-----BEGIN PRIVATE KEY-----\n" + Base64.encodeToString(key.getEncoded(), Base64.DEFAULT) + "\n-----END PRIVATE KEY-----\n";
File keyFile = File.createTempFile("keyutil_" + fqdn + '_', ".key");
keyFile.deleteOnExit();
OutputStream keyOut = new FileOutputStream(keyFile);
try {
keyOut.write(keyText.getBytes(US_ASCII));
keyOut.close();
keyOut = null;
} finally {
if (keyOut != null) {
safeClose(keyFile, keyOut);
safeDelete(keyFile);
}
}
String certText = "-----BEGIN CERTIFICATE-----\n" + Base64.encodeToString(cert.getEncoded(), Base64.DEFAULT) + "\n-----END CERTIFICATE-----\n";
File certFile = File.createTempFile("keyutil_" + fqdn + '_', ".crt");
certFile.deleteOnExit();
OutputStream certOut = new FileOutputStream(certFile);
try {
certOut.write(certText.getBytes(US_ASCII));
certOut.close();
certOut = null;
} finally {
if (certOut != null) {
safeClose(certFile, certOut);
safeDelete(certFile);
safeDelete(keyFile);
}
}
return new String[]{certFile.getPath(), keyFile.getPath()};
}
private static void safeDelete(File certFile) {
if (!certFile.delete()) {
Log.w(TAG, "Failed to delete a file: " + certFile);
}
}
private static void safeClose(File keyFile, OutputStream keyOut) {
try {
keyOut.close();
} catch (IOException e) {
Log.w(TAG, "Failed to close a file: " + keyFile, e);
}
}
}
Usage
private SslContext getSslContext() throws CertificateException, SSLException {
SelfSignedCertificate ssc = new SelfSignedCertificate(BuildConfig.APPLICATION_ID);
return SslContextBuilder.forServer(ssc.certificate(), ssc.privateKey()).protocols("TLSv1.2").build();
}
I pass this SslContext to create the ChannelPipeline for starting a Netty server with HTTPS support, but you may use the generated certificate any way you like.
