java-otp

java-otp is a Java library for generating HOTP (RFC 4226) or TOTP (RFC 6238) one-time passwords.

java-otp requires Java 8 or newer.

You can download java-otp as a jar file (it has no dependencies) from the GitHub releases page and add it to your project's classpath. If you're using Maven (or something that understands Maven dependencies) to build your project, you can add java-otp as a dependency:

<dependency>
  <groupId>com.eatthepath</groupId>
  <artifactId>java-otp</artifactId>
  <version>1.0.0</version>
</dependency>

The latest API docs are available at https://jchambers.github.io/java-otp/apidocs/latest/.

To demonstrate generating one-time passwords, we'll focus on the TOTP algorithm. To create a TOTP generator with a default password length (6 digits), time step (30 seconds), and HMAC algorithm (HMAC-SHA1):

final TimeBasedOneTimePasswordGenerator totp = new TimeBasedOneTimePasswordGenerator();

To actually generate time-based one-time passwords, you'll need a key and a timestamp. Secure key management is beyond the scope of this document; for the purposes of an example, though, we'll generate a random key:

final SecretKey key;
{
    final KeyGenerator keyGenerator = KeyGenerator.getInstance(totp.getAlgorithm());

    // Key length should match the length of the HMAC output (160 bits for SHA-1, 256 bits
    // for SHA-256, and 512 bits for SHA-512). Note that while Mac#getMacLength() returns a
    // length in _bytes,_ KeyGenerator#init(int) takes a key length in _bits._
    final int macLengthInBytes = Mac.getInstance(totp.getAlgorithm()).getMacLength();
    keyGenerator.init(macLengthInBytes * 8);

    key = keyGenerator.generateKey();
}

Armed with a key, we can deterministically generate one-time passwords for any timestamp:

final Instant now = Instant.now();
final Instant later = now.plus(totp.getTimeStep());

System.out.println("Current password: " + totp.generateOneTimePasswordString(key, now));
System.out.println("Future password:  " + totp.generateOneTimePasswordString(key, later));

…which produces (for one randomly-generated key):

Current password: 164092
Future password:  046148

To validate a one-time password:

// In a real-world scenario, this might come from an HTTP request or some other remote channel
final int userSuppliedOneTimePassword = 164092;

if (totp.validateOneTimePassword(key, now, userSuppliedOneTimePassword)) {
    System.out.println("Password was correct");
} else {
    System.out.println("Password was incorrect");
}

One-time password generators are thread-safe and reusable. Generally, applications should treat one-time password generator instances as long-lived resources (as opposed to creating new generators for each password-generation call).

java-otp is published under the MIT License.