JWT (JSON Web Token) in PHP without using 3rd-party library. How to sign?

Question

There are a few libraries for implementing JSON Web Tokens (JWT) in PHP, such as php-jwt. I am writing my own, very small and simple class but cannot figure out why my sign

Answer 1

I solved it! I did not realize that the signature itself needs to be base64 encoded. In addition, I needed to set the last optional parameter of the hash_hmac function to $raw_output=true (see the docs. In short I needed to change my code from the original:

//build the signature
$key = 'secret';
$signature = hash_hmac('sha256',"$headers_encoded.$payload_encoded",$key);

//build and return the token
$token = "$headers_encoded.$payload_encoded.$signature";

To the corrected:

//build the signature
$key = 'secret';
$signature = hash_hmac('sha256',"$headers_encoded.$payload_encoded",$key,true);
$signature_encoded = base64url_encode($signature);

//build and return the token
$token = "$headers_encoded.$payload_encoded.$signature_encoded";
echo $token;

Answer 2

If you want to solve it using RS256 (instead of HS256 like OP) you can use it like this:

//build the headers
$headers = ['alg'=>'RS256','typ'=>'JWT'];
$headers_encoded = base64url_encode(json_encode($headers));

//build the payload
$payload = ['sub'=>'1234567890','name'=>'John Doe', 'admin'=>true];
$payload_encoded = base64url_encode(json_encode($payload));

//build the signature
$key = "-----BEGIN PRIVATE KEY----- ....";
openssl_sign("$headers_encoded.$payload_encoded", $signature, $key, 'sha256WithRSAEncryption'); 
$signature_encoded = base64url_encode($signature);

//build and return the token
$token = "$headers_encoded.$payload_encoded.$signature_encoded";
echo $token;

Took me way longer than I'd like to admit