Validating a JWT in node.js

https://stackoverflow.com/questions/8867456

16-04-2021
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質問

I'm trying to parse and validate a JWT token in node.js based on this sample (authored in .NET): https://github.com/liveservices/LiveSDK/blob/master/Samples/Asp.net/AuthenticationTokenSample/JsonWebToken.cs

Here is my node js javascript that validates the token:

var validateSignature = function(key, claims, envelope, signature) {
    var hasher = crypto.createHash('sha256');
    hasher.update(key + "JWTSig");
    var key = hasher.digest('binary');
    var hmac = crypto.createHmac('sha256', key);
    hmac.update(envelope + '.' + claims);
    var out = hmac.digest('base64');
    console.log(out);
    console.log(signature);
    console.log(out === signature);
}

Now, the very weird thing is - it almost works. Here's the output of the three console.log statements:

pEwNPJ+LUHBdvNx631UzdyVhPFUOvFY8jG3x/cP81FE=
pEwNPJ-LUHBdvNx631UzdyVhPFUOvFY8jG3x_cP81FE
false

It seems suspicious to me that the hashes are both the same except for the +-/_=

Anybody spot my mistake? Something to do with my base64 encoding.

UPDATE

I played some more and there seems to be something funky going on with base64 encoding here. The following code in node js:

console.log(signature);
var b = new Buffer(signature, 'base64');
console.log(b.toString('base64'));

yields:

pEwNPJ-LUHBdvNx631UzdyVhPFUOvFY8jG3x_cP81FE
pEwNPJLUHBdvNx631UzdyVhPFUOvFY8jG3xcP81F

Which seems very odd, right?

解決

Thanks to Timothy Meade for commenting and pushing me in the right direction.

Node's Buffer type generates standard Base64 with +, / and =

There is a URL safe base64 encoding as mentioned here: http://en.wikipedia.org/wiki/Base64

It replaces + with -, / with _ and = is optional. The token that is passed on the QueryString (d'uh) is a URL safe version. Hence the difference.

Code was fixed by a simple:

out = out.replace('+','-').replace('/','_').replace('=','');

他のヒント

I wrote this library a while ago, I guess you can use some of the code. It is supposed to run in both node.js and in a modern browser.

JWT library for javascript

This is not the exact method that you were trying to use, but I believe it is the preferred way to validate a JWT in NodeJS. Note that I am using the NPM base64url library to convert between base64Url (the default encoding for a JWT) and base64 (what NodeJS expects for the verification function).

Also note, you need a public and private keypair to sign and verify respectively. I have included the private and public keys that were used to sign and verify this JWT at the bottom of this post.

const base64 = require('base64url');
const crypto = require('crypto');
const verifyFunction = crypto.createVerify('RSA-SHA256');
const fs = require('fs');

// The sample JWT from https://jwt.io/
const JWT = 'eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWUsImlhdCI6MTUxNjIzOTAyMn0.POstGetfAytaZS82wHcjoTyoqhMyxXiWdR7Nn7A29DNSl0EiXLdwJ6xC6AfgZWF1bOsS_TuYI3OG85AmiExREkrS6tDfTQ2B3WXlrr-wp5AokiRbz3_oB4OxG-W9KcEEbDRcZc0nH3L7LzYptiy1PtAylQGxHTWZXtGz4ht0bAecBgmpdgXMguEIcoqPJ1n3pIWk_dUZegpqx0Lka21H6XxUTxiy8OcaarA8zdnPUnV6AmNP3ecFawIFYdvJB_cm-GvpCSbr8G8y_Mllj8f4x9nBH8pQux89_6gUY618iYv7tuPWBFfEbLxtF2pZS6YC1aSfLQxeNe8djT9YjpvRZA';

// This just gets the value of the public key (same as the one at bottom of this post)
const PUB_KEY = fs.readFileSync(__dirname + '/id_rsa_pub.pem', 'utf8');

// Split the JWT by `.` to get each part
const jwtHeader = JWT.split('.')[0];
const jwtPayload = JWT.split('.')[1];
const jwtSignature = JWT.split('.')[2];

// We only need the first two pieces to verify
verifyFunction.write(jwtHeader + '.' + jwtPayload);
verifyFunction.end();

// IMPORTANT: NodeJS expects base64 format, not base64url format!
const jwtSignatureBase64 = base64.toBase64(jwtSignature);

// IMPORTANT: You need to specify that the `jwtSignatureBase64` data is base64 format, 
// otherwise, it will default to Buffer format and return false
const signatureIsValid = verifyFunction.verify(PUB_KEY, jwtSignatureBase64, 'base64');

console.log(signatureIsValid); // true

The keys below are from the example JWT mentioned here.

Private Key:

Public Key:

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