Android JWT: JSON Web Token for Android

AJWT aims to be the easiest to use and understand library for creating and verifying JSON Web Tokens (JWTs) on Android. This project is derived from the JJWT Library for the JVM. All credit for the implementation goes to those guys, this project simply converts their work so it can be used on Android.

JJWT is a 'clean room' implementation based solely on the JWT, JWS, JWE and JWA RFC draft specifications.

Installation

Use your favorite Maven-compatible build tool to pull the dependency (and its transitive dependencies) from Maven Central.

Maven:

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt</artifactId>
    <version>0.4</version>
</dependency>

Gradle:

dependencies {
    compile 'io.jsonwebtoken:jjwt:0.4'
}

Note: JJWT depends on Jackson 2.x. If you're already using an older version of Jackson in your app, read this

Usage

Most complexity is hidden behind a convenient and readable builder-based fluent interface, great for relying on IDE auto-completion to write code quickly. Here's an example:

import io.jsonwebtoken.Jwts;
import static io.jsonwebtoken.SignatureAlgorithm.*;

//Let's create a random signing key for testing:
Random random = new SecureRandom();
byte[] key = new byte[64];
random.nextBytes(key);

String compact = Jwts.builder().setSubject("Joe").signWith(HS256, key).compact();

How easy was that!?

Now let's verify the JWT (you should always discard JWTs that don't match an expected signature):

assert Jwts.parser().setSigningKey(key).parseClaimsJws(compact).getBody().getSubject().equals("Joe");

You have to love one-line code snippets!

But what if signature validation failed? You can catch SignatureException and react accordingly:

try {

    Jwts.parser().setSigningKey(key).parse(compactJwt);

    //OK, we can trust this JWT

} catch (SignatureException e) {

    //don't trust the JWT!
}

Supported Features

• Creating and parsing plaintext compact JWTs

• Creating, parsing and verifying digitally signed compact JWTs (aka JWSs) with the following algorithms:

  • HS256: HMAC using SHA-384
  • HS384: HMAC using SHA-384
  • HS512: HMAC using SHA-512
  • RS256: RSASSA-PKCS-v1_5 using SHA-256
  • RS384: RSASSA-PKCS-v1_5 using SHA-384
  • RS512: RSASSA-PKCS-v1_5 using SHA-512
  • PS256: RSASSA-PSS using SHA-256 and MGF1 with SHA-256
  • PS384: RSASSA-PSS using SHA-384 and MGF1 with SHA-384
  • PS512: RSASSA-PSS using SHA-512 and MGF1 with SHA-512

Currently Unsupported Features

• Non-compact serialization and parsing.
• Elliptic Curve signature algorithms ES256, ES384 and ES512.
• JWE (Encryption for JWT)

These feature sets will be implemented in a future release when possible. Community contributions are welcome!

Release Notes

0.4

• Issue 8: Add ability to find signing key by inspecting the JWS values before verifying the signature.

This is a handy little feature. If you need to parse a signed JWT (a JWS) and you don't know which signing key was used to sign it, you can now use the new SigningKeyResolver concept.

A SigningKeyresolver can inspect the JWS header and body (Claims or String) before the JWS signature is verified. By inspecting the data, you can find the key and return it, and the parser will use the returned key to validate the signature. For example:

SigningKeyResolver resolver = new MySigningKeyResolver();

Jws<Claims> jws = Jwts.parser().setSigningKeyResolver(resolver).parseClaimsJws(compact);

The signature is still validated, and the JWT instance will still not be returned if the jwt string is invalid, as expected. You just get to 'see' the JWT data for key discovery before the parser validates. Nice.

This of course requires that you put some sort of information in the JWS when you create it so that your SigningKeyResolver implementation can look at it later and look up the key. The standard way to do this is to use the JWS kid ('key id') field, for example:

Jwts.builder().setHeaderParam("kid", your_signing_key_id_NOT_THE_SECRET).build();

You could of course set any other header parameter or claims parameter instead of setting kid if you want - that's just the default field reserved for signing key identification. If you can locate the signing key based on other information in the header or claims, you don't need to set the kid field - just make sure your resolver implementation knows how to look up the key.

Finally, a nice SigningKeyResolverAdapter is provided to allow you to write quick and simple subclasses or anonymous classes instead of having to implement the SigningKeyResolver interface directly. For example:

Jws<Claims> jws = Jwts.parser().setSigningKeyResolver(new SigningKeyResolverAdapter() {
        @Override
        public byte[] resolveSigningKeyBytes(JwsHeader header, Claims claims) {
            //inspect the header or claims, lookup and return the signing key
            String keyId = header.getKeyId(); //or any other field that you need to inspect
            return getSigningKey(keyId); //implement me
        }})
    .parseClaimsJws(compact);

0.3

• Issue 6: Parsing an expired Claims JWT or JWS (as determined by the exp claims field) will now throw an ExpiredJwtException.
• Issue 7: Parsing a premature Claims JWT or JWS (as determined by the nbf claims field) will now throw a PrematureJwtException.

0.2

More convenient Claims building

This release adds convenience methods to the JwtBuilder interface so you can set claims directly on the builder without having to create a separate Claims instance/builder, reducing the amount of code you have to write. For example, this:

Claims claims = Jwts.claims().setSubject("Joe");

String compactJwt = Jwts.builder().setClaims(claims).signWith(HS256, key).compact();

can now be written as:

String compactJwt = Jwts.builder().setSubject("Joe").signWith(HS256, key).compact();

A Claims instance based on the specified claims will be created and set as the JWT's payload automatically.

Type-safe handling for JWT and JWS with generics

The following < 0.2 code produced a JWT as expected:

Jwt jwt = Jwts.parser().setSigningKey(key).parse(compact);

But you couldn't easily determine if the jwt was a JWT or JWS instance or if the body was a Claims instance or a plaintext String without resorting to a bunch of yucky instanceof checks. In 0.2, we introduce the JwtHandler when you don't know the exact format of the compact JWT string ahead of time, and parsing convenience methods when you do.

JwtHandler

If you do not know the format of the compact JWT string at the time you try to parse it, you can determine what type it is after parsing by providing a JwtHandler instance to the JwtParser with the new parse(String compactJwt, JwtHandler handler) method. For example:

T returnVal = Jwts.parser().setSigningKey(key).parse(compact, new JwtHandler<T>() {
    @Override
    public T onPlaintextJwt(Jwt<Header, String> jwt) {
        //the JWT parsed was an unsigned plaintext JWT
        //inspect it, then return an instance of T (see returnVal above)
    }
    
    @Override
    public T onClaimsJwt(Jwt<Header, Claims> jwt) {
        //the JWT parsed was an unsigned Claims JWT
        //inspect it, then return an instance of T (see returnVal above)
    }

    @Override
    public T onPlaintextJws(Jws<String> jws) {
        //the JWT parsed was a signed plaintext JWS
        //inspect it, then return an instance of T (see returnVal above)
    }

    @Override
    public T onClaimsJws(Jws<Claims> jws) {
        //the JWT parsed was a signed Claims JWS
        //inspect it, then return an instance of T (see returnVal above)
    }
});

Of course, if you know you'll only have to parse a subset of the above, you can use the JwtHandlerAdapter and implement only the methods you need. For example:

T returnVal = Jwts.parser().setSigningKey(key).parse(plaintextJwt, new JwtHandlerAdapter<Jwt<Header, T>>() {
    @Override
    public T onPlaintextJws(Jws<String> jws) {
        //the JWT parsed was a signed plaintext JWS
        //inspect it, then return an instance of T (see returnVal above)
    }

    @Override
    public T onClaimsJws(Jws<Claims> jws) {
        //the JWT parsed was a signed Claims JWS
        //inspect it, then return an instance of T (see returnVal above)
    }
});

Known Type convenience parse methods

If, unlike above, you are confident of the compact string format and know which type of JWT or JWS it will produce, you can just use one of the 4 new convenience parsing methods to get exactly the type of JWT or JWS you know exists. For example:

//for a known plaintext jwt string:
Jwt<Header,String> jwt = Jwts.parser().parsePlaintextJwt(compact);

//for a known Claims JWT string:
Jwt<Header,Claims> jwt = Jwts.parser().parseClaimsJwt(compact);

//for a known signed plaintext JWT (aka a plaintext JWS):
Jws<String> jws = Jwts.parser().setSigningKey(key).parsePlaintextJws(compact);

//for a known signed Claims JWT (aka a Claims JWS):
Jws<Claims> jws = Jwts.parser().setSigningKey(key).parseClaimsJws(compact);

Already using an older Jackson dependency?

JJWT depends on Jackson 2.4.x (or later). If you are already using a Jackson version in your own application less than 2.x, for example 1.9.x, you will likely see runtime errors. To avoid this, you should change your project build configuration to explicitly point to a 2.x version of Jackson. For example:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.4.2</version>
</dependency>