How to use keystore in Java to store private key?
NOTE: This code is for demonstration purposes only. Private keys must be encrypted when you store them on disk. Do not use it as is.
You can do something like this:
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(2048);
KeyPair kp = kpg.genKeyPair();
KeyFactory fact = KeyFactory.getInstance("RSA");
RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(),
RSAPublicKeySpec.class);
saveToFile(PUBLIC_KEY_FILE,
pub.getModulus(), pub.getPublicExponent());
RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(),
RSAPrivateKeySpec.class);
saveToFile(PRIVATE_KEY_FILE,
priv.getModulus(), priv.getPrivateExponent());
The save function:
private static void saveToFile(String fileName,
BigInteger mod, BigInteger exp)
throws SomeException {
ObjectOutputStream oout = new ObjectOutputStream(
new BufferedOutputStream(new FileOutputStream(fileName)));
try {
oout.writeObject(mod);
oout.writeObject(exp);
} catch (Exception e) {
throw new SomeException(e);
} finally {
oout.close();
}
}
And read the same way back:
private static PublicKey readPublicKey() throws SomeException {
InputStream in = new FileInputStream(PUBLIC_KEY_FILE);
ObjectInputStream oin =
new ObjectInputStream(new BufferedInputStream(in));
try {
BigInteger m = (BigInteger) oin.readObject();
BigInteger e = (BigInteger) oin.readObject();
RSAPublicKeySpec keySpec = new RSAPublicKeySpec(m, e);
KeyFactory fact = KeyFactory.getInstance("RSA");
PublicKey pubKey = fact.generatePublic(keySpec);
return pubKey;
} catch (Exception e) {
throw new SomeException(e);
} finally {
oin.close();
}
}
Reading private key is similar.
This block of code will generate and store a KeyPair on the AndroidKeyStore. (NOTE: Exception catches omitted)
KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
keyStore.load(null);
String alias = "my_key"; // replace as required or get it as a function argument
int nBefore = keyStore.size(); // debugging variable to help convince yourself this works
// Create the keys if necessary
if (!keyStore.containsAlias(alias)) {
Calendar notBefore = Calendar.getInstance();
Calendar notAfter = Calendar.getInstance();
notAfter.add(Calendar.YEAR, 1);
KeyPairGeneratorSpec spec = new KeyPairGeneratorSpec.Builder(this)
.setAlias(alias)
.setKeyType("RSA")
.setKeySize(2048)
.setSubject(new X500Principal("CN=test"))
.setSerialNumber(BigInteger.ONE)
.setStartDate(notBefore.getTime())
.setEndDate(notAfter.getTime())
.build();
KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");
generator.initialize(spec);
KeyPair keyPair = generator.generateKeyPair();
}
int nAfter = keyStore.size();
Log.v(TAG, "Before = " + nBefore + " After = " + nAfter);
// Retrieve the keys
KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)keyStore.getEntry(alias, null);
RSAPrivateKey privateKey = (RSAPrivateKey) privateKeyEntry.getPrivateKey();
RSAPublicKey publicKey = (RSAPublicKey) privateKeyEntry.getCertificate().getPublicKey();
Log.v(TAG, "private key = " + privateKey.toString());
Log.v(TAG, "public key = " + publicKey.toString());