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JSON Web Tokens without Firebase JWT


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Published: December, 28 2021

I have created a few tutorials on how to use Firebase JWT to create and use JSON Web Tokens, but I decided to try to do it without a framework, using just PHP. In this article, you can see how I created my own simple JWT generator.

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Creating Our Class

class JWT {

    private $headers;
    private $secret;

    public function __construct()
    {
        $this->headers = [
            'alg' => 'HS256', // we are using a SHA256 algorithm
            'typ' => 'JWT', // JWT type
            'iss' => 'jwt.local', // token issuer
            'aud' => 'example.com' // token audience
        ];
        $this->secret = 'thisIsASecret';
    }
}
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Generate A Token

Within this class, we will create our generate function. this function will generate our token to be used against our validator.

public function generate(array $payload): string
{
    $headers = $this->encode(json_encode($this->headers)); // encode headers
    $payload["exp"] = time() + 60; // add expiration to payload
    $payload = $this->encode(json_encode($payload)); // encode payload
    $signature = hash_hmac('SHA256', "$headers.$payload", $this->secret, true); // create SHA256 signature
    $signature = $this->encode($signature); // encode signature

    return "$headers.$payload.$signature";
}
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Within our generate function, we reference a encode function. This function will simply base64 encode our strings to be used by the token.

private function encode(string $str): string
{
    return rtrim(strtr(base64_encode($str), '+/', '-_'), '='); // base64 encode string
}
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Validate Our Token

In this function, we take the generated token, and validate the strings, the encoding, and finally the time. I have it set for 1 minute so the token will expire after 1 minute to ensure it is used for a purpose.

public function is_valid(string $jwt): bool
{
    $token = explode('.', $jwt); // explode token based on JWT breaks
    if (!isset($token[1]) && !isset($token[2])) {
        return false; // fails if the header and payload is not set
    }
    $headers = base64_decode($token[0]); // decode header, create variable
    $payload = base64_decode($token[1]); // decode payload, create variable
    $clientSignature = $token[2]; // create variable for signature

    if (!json_decode($payload)) {
        return false; // fails if payload does not decode
    }

    if ((json_decode($payload)->exp - time()) < 0) {
        return false; // fails if expiration is greater than 0, setup for 1 minute
    }

    if (isset(json_decode($payload)->iss)) {
        if (json_decode($headers)->iss != json_decode($payload)->iss) {
            return false; // fails if issuers are not the same
        }
    } else {
        return false; // fails if issuer is not set 
    }

    if (isset(json_decode($payload)->aud)) {
        if (json_decode($headers)->aud != json_decode($payload)->aud) {
            return false; // fails if audiences are not the same
        }
    } else {
        return false; // fails if audience is not set
    }

    $base64_header = $this->encode($headers);
    $base64_payload = $this->encode($payload);

    $signature = hash_hmac('SHA256', $base64_header . "." . $base64_payload, $this->secret, true);
    $base64_signature = $this->encode($signature);

    return ($base64_signature === $clientSignature);
}
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Putting It All Together

class JWT
{
    private $headers;

    private $secret;

    public function __construct()
    {
        $this->headers = [
            'alg' => 'HS256', // we are using a SHA256 algorithm
            'typ' => 'JWT', // JWT type
            'iss' => 'jwt.local', // token issuer
            'aud' => 'example.com' // token audience
        ];
        $this->secret = 'thisIsASecret'; // change this to your secret code
    }

    public function generate(array $payload): string
    {
        $headers = $this->encode(json_encode($this->headers)); // encode headers
        $payload["exp"] = time() + 60; // add expiration to payload
        $payload = $this->encode(json_encode($payload)); // encode payload
        $signature = hash_hmac('SHA256', "$headers.$payload", $this->secret, true); // create SHA256 signature
        $signature = $this->encode($signature); // encode signature

        return "$headers.$payload.$signature";
    }

    private function encode(string $str): string
    {
        return rtrim(strtr(base64_encode($str), '+/', '-_'), '='); // base64 encode string
    }

    public function is_valid(string $jwt): bool
    {
        $token = explode('.', $jwt); // explode token based on JWT breaks
        if (!isset($token[1]) && !isset($token[2])) {
            return false; // fails if the header and payload is not set
        }
        $headers = base64_decode($token[0]); // decode header, create variable
        $payload = base64_decode($token[1]); // decode payload, create variable
        $clientSignature = $token[2]; // create variable for signature

        if (!json_decode($payload)) {
            return false; // fails if payload does not decode
        }

        if ((json_decode($payload)->exp - time()) < 0) {
            return false; // fails if expiration is greater than 0, setup for 1 minute
        }

        if (isset(json_decode($payload)->iss)) {
            if (json_decode($headers)->iss != json_decode($payload)->iss) {
                return false; // fails if issuers are not the same
            }
        } else {
            return false; // fails if issuer is not set 
        }

        if (isset(json_decode($payload)->aud)) {
            if (json_decode($headers)->aud != json_decode($payload)->aud) {
                return false; // fails if audiences are not the same
            }
        } else {
            return false; // fails if audience is not set
        }

        $base64_header = $this->encode($headers);
        $base64_payload = $this->encode($payload);

        $signature = hash_hmac('SHA256', $base64_header . "." . $base64_payload, $this->secret, true);
        $base64_signature = $this->encode($signature);

        return ($base64_signature === $clientSignature);
    }
}
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Conclusion

It is a simple project, but works as intended. I have used Firebase JWT in the past but sometimes you just need a simple solution without having to import and entire library. I hope this helps for your next small project. You can download the class on my GitHub or subscribe on YouTube for more tutorials.


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