Sample code for signing a client secret for FamilySearch oauth2 confidential client

SampleOAuth2ClientSecret.java

KeyStore ks = KeyStore.getInstance("JKS");
char[] password = "mypassword".toCharArray();
InputStream is = Thread.currentThread().getContextClassLoader().getResourceAsStream("mykeystore.jks");
ks.load(is, password);
is.close();
PrivateKey privateKey = (PrivateKey)ks.getKey("mykey", password);

// Create the secret with the private key
Security.addProvider(new BouncyCastleProvider());
long timestamp = System.currentTimeMillis();
String sTimestamp = Long.toString(timestamp);
byte[] bytesTimestampUtf8Unencrypted = sTimestamp.getBytes(Charset.forName("UTF-8"));
Cipher cipher = Cipher.getInstance(privateKey.getAlgorithm());
cipher.init(Cipher.ENCRYPT_MODE, privateKey);
byte[] bytesTimestampUtf8Encrypted = cipher.doFinal(bytesTimestampUtf8Unencrypted);
String encoded = (new BASE64Encoder()).encodeBuffer(bytesTimestampUtf8Encrypted);
String secret = URLEncoder.encode(encoded, "UTF-8");
form.setClient_secret(secret);
SimpleDateFormat formatter = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss.SSSS");
form.setClient_secret_time_stamp(formatter.format(new Date(timestamp)));