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nJwt - JWTs for Node.js

"Nin-Jot" /ˈnɪn.dʒɑt/

nJwt is the cleanest JSON Web Token (JWT) library for Node.js developers. nJwt removes all the complexities around JWTs, and gives you a simple, intuitive API, that allows you to securely make and use JWTs in your applications without needing to read rfc7519.

Creating Secure, Signed JWTs

JWTs expect "claims", they are a set of assertions about who the user is and what they can do. The most common use case for JWTs is to declare the "scope" of the access token, which is a list of things that the holder of the token (the user) is allowed to do.

JWTs should be signed, otherwise you can't verify that they were created by you. Our library expects that you give us a highly random signing key for signing tokens. We use the HS256 algorithm by default, and the byte length of the signing key should match that of the signing algorithm, to ensure cryptographic security.

While the library will accept strings for signing keys, we suggest you use a Buffer instead. Using buffers makes it easy to do other operations, like convert your signing key to Base64URL encoding, if you need to transmit your key to other systems.

While the claims are completely up to you, we do recommend setting the "Subject" and "Audience" fields.

JWTs commonly contain the iat, nbf and exp claims, which declare the time the token was issued, activation date and when it expires. Our library will create these for you (except nbf), with a default expiration of 1 hour. nbf is optional.

Here is a simple example that shows you how to create a secure byte string for your signing key, and then use that key to sign a JWT with some claims that you provide:

var nJwt = require('njwt');
var secureRandom = require('secure-random');

var signingKey = secureRandom(256, {type: 'Buffer'}); // Create a highly random byte array of 256 bytes

var claims = {
  iss: "http://myapp.com/",  // The URL of your service
  sub: "users/user1234",    // The UID of the user in your system
  scope: "self, admins"
}

var jwt = nJwt.create(claims,signingKey);

Once you have created the JWT, you can look at its internal structure by logging it to the console. This is our internal representation of the token, this is not what you'll send to your end user:

console.log(jwt);

{
  "header": {
    "typ": "JWT",
    "alg": "HS256"
  },
  "body": {
    "jti": "c84280e6-0021-4e69-ad76-7a3fdd3d4ede",
    "iat": 1434660338,
    "exp": 1434663938,
    "nbf": 1434663938,
    "iss": "http://myapp.com/",
    "sub": "users/user1234",
    "scope": ["self","admins"]
  }
}

Our library has added the jti field for you, this is a random ID that will be unique for every token. You can use this if you want to create a database of tokens that have been issued to the user.

When you are ready to give the token to your end user, you need to compact it. This will turn it into a Base64 URL encoded string, making it safe to pass around in browsers without any unexpected formatting applied to it.

var token = jwt.compact();
console.log(token);

eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJmb28iOiJiYXIiLCJpYXQiOjE0MzQ0Nzk4ODN9.HQyx15jWm1upqsrKSf89X_iP0sg7N46a9pqBVGPMYdiqZeuU_ZZOdU-zizHJoIHMIJxtEWzpSMaVubJW0AJsTqjqQf6GoJ4cmFAfmfUFXmMC4Xv5oc4UqvGizpoLjfZedd834PcwbS-WskZcL4pVNmBIGRtDXkoU1j2X1P5M_sNJ9lYZ5vITyqe4MYJovQzNdQziUNhcMI5wkXncV7XzGInBeQsPquASWVG4gb3Y--k1P3xWA4Df3rKeEQBbInDKXczvDpfIlTojx4Ch8OM8vXWWNxW-mIQrV31wRrS9XtNoig7irx8N0MzokiYKrQ8WP_ezPicHvVPIHhz-InOw

This is the JWT that the client application will retain, and use for authentication.

Your server application will also need to persist the signing key that was used to sign the token, and when the client tries to use this token for authentication, you will need to use the same signing key for verification.

The Buffer needs to be converted to a string so that it can be persisted in a database, and you can do so like this:

var base64SigningKey = signingKey.toString('base64');

If you are going to use multiple signing keys, it is common practice to create a random ID which identifies the key, and store that ID with the key in your database. When you create JWTs, set the kid field of the header to be this ID. Then when verifying JWTs, this kid field will tell you which signing key should be used for verification.

Verifying Signed JWTs

The end user will use their JWT to authenticate themselves with your service. When they present the JWT, you want to check the token to ensure that it's valid. This library does the following checks when you call the verify method:

• It was created by you (by verifying the signature, using the secret signing key)
• It hasn't been modified (e.g. some claims were maliciously added)
• It hasn't expired
• It is active

To verify a previously issued token, use the verify method. You must give it the same signing key that you are using to create tokens:

nJwt.verify(token,signingKey,function(err,verifiedJwt){
  if(err){
    console.log(err); // Token has expired, has been tampered with, etc
  }else{
    console.log(verifiedJwt); // Will contain the header and body
  }
});

If validation fails you can look at err.message to understand the problem. If the header and body of the JWT were parse-able (not not verifiable) they will be provided as objects at err.parsedHeader and err.parsedBody.

You can also use verify synchronously, in which case the errors will be thrown:

try{
  verifiedJwt = nJwt.verify(token,signingKey);
}catch(e){
  console.log(e);
}

Changing the algorithm

If you want to change the algorithm from the default HS256, you can do so by passing it as a third argument to the create or verify methods:

var jwt = nJwt.create(claims,signingKey,'HS512');

nJwt.verify(token,signingKey, 'HS512');

See the table below for a list of supported algorithms. If using RSA key pairs, the public key will be the signing key parameter.

Customizing the token

While we've chosen secure, sensible defaults for you, you may need to change it up.

Claims

If you need to provide custom claims, simply supply them to the create method or add them manually to the claims body after the JWT is created. These two examples create the same claims body:

var claims = {
  scope: 'admins'
}

var jwt = nJwt.create(claims,secret);

jwt.body.scope = 'admins';

jwt.setClaim('otherClaim', 'value');

Headers

You can manually modify headers object, or use the setHeader() method:

var jwt = nJwt.create({}, keyMap.kid_a);

jwt.headers.myClaim = 'foo';

jwt.setHeader('kid', 'kid_a');

Using a key resolver

If your application is using multiple signing keys, nJwt provides a handy little feature that allows you to resolve which signing key should be used to verify a token.

To do this, you first need to manually create a verifier instance, using nJwt.createVerifier(), and then provide your key resolution function to the withKeyResolver() method:

var keyMap = {
  kid_a: '<secure signing key>',
  kid_b: '<secure signing key>'
};

function myKeyResolver(kid, cb) {
  var key = keyMap[kid];

  if (key) {
    return cb(null, key);
  }

  cb(new Error('Unknown kid'));
}

var tokenA = nJwt.create({}, keyMap.kid_a).setHeader('kid', 'kid_a').compact();

var tokenB = nJwt.create({}, 'foo').setHeader('kid', 'bar').compact();

var verifier = nJwt.createVerifier().withKeyResolver(myKeyResolver);

// synchronously

try {

  // This will pass and print the result

  var parsedJwt = verifier.verify(tokenA);
  console.log(parsedJwt);

} catch(e) {
  console.log(e);
}

// asynchronously

verifier.verify(tokenB, function(err, verifiedJwt) {
  if (err) {
    return console.log(err);  // This error with "'Error while resolving signing key for kid "bar"'"
  }

  console.log(verifiedJwt);
});

Expiration Claim

A convenience method is supplied for modifying the exp claim. You can modify the exp claim by passing a Date object, or a millisecond value, to the setExpiration method:

var jwt = nJwt.create(claims,secret);

jwt.setExpiration(new Date('2015-07-01')); // A specific date
jwt.setExpiration(new Date().getTime() + (60*60*1000)); // One hour from now
jwt.setExpiration(); // Remove the exp claim

NotBefore Claim

A convenience method is supplied for modifying the nbf claim. You can modify the nbf claim by passing a Date object, or a millisecond value, to the setNotBefore method:

var jwt = nJwt.create(claims,secret);

jwt.setNotBefore(new Date('2015-07-01')); // token is active from this date
jwt.setNotBefore(new Date().getTime() + (60*60*1000)); // One hour from now
jwt.setNotBefore(); // Remove the exp claim

Supported Algorithms

Unsupported features

The following features are not yet supported by this library:

• Encrypting the JWT (aka JWE)

TypeScript usage

This package includes TypeScript definitions for library interface. They can be used as follows:

import { Jwt, create } from 'njwt';
import crypto = require('crypto');

const signingKey: Buffer = crypto.randomBytes(256); // Create a highly random byte array of 256 bytes
const claims = {
  iss: 'http://myapp.com/',  // The URL of your service
};
const jwt: Jwt = create(claims, signingKey);