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The JSON Web Tokens (JWT) standard describes a concise way for verifiable data transfers. Each token includes a signature which allows the issuing party to check on the messages integrity.

In this post, youll learn what the JWT structure includes and ways to generate your personal tokens. JWTs certainly are a popular solution to secure APIs and authenticate user sessions because theyre simple and self-contained.

How JWTs Work

Probably the most common tasks in virtually any API is validating that users are who they claim to be. Authentications usually handled insurance firms your client include an API key with requests it sends to the server. The main element contains embedded information that identifies an individual. This still leaves one big question: how do the server validate that it issued the main element to begin with?

JWTs conveniently solve this issue with a secret to sign each token. The server can check a tokens valid by attempting to recompute the presented signature which consists of private secret. Any tampering may cause the verification to fail.

The JWT Format

JWTs are formed from three distinct components:

  • Header This consists of metadata concerning the token itself, like the signing algorithm that has been used.
  • Payload The tokens payload could be any arbitrary data highly relevant to your system. It might are the users ID and a summary of features they are able to connect to.
  • Signature The signature allows the tokens integrity to be validated later on. Its developed by signing the header and payload utilizing a secret value thats known and then the server.

These three components are joined with periods to create the JWT:


Each piece is encoded using Base-64. The entire token is really a string of text which can be easily consumed in programming environments and sent with HTTP requests.

Developing a JWT

The steps to produce a JWT could be implemented in every programming languages. This example uses PHP however the process will undoubtedly be similar is likely to system.

Begin by creating the header. This usually includes two fields, alg and typ:

  • alg The hashing algorithm that’ll be used to generate the signature. That is normally HMAC SHA256 (HS256).
  • typ The kind of token thats being generated. This will be JWT.

Heres the JSON that defines the header:

    "alg": "HS256",
    "typ": "JWT"

The header JSON must be Base64-encoded next:

$headerData = ["alg" => "HS256", "typ" => "JWT"];$header = base64_encode(json_encode($headerData));

Next define your tokens payload as another JSON object. That is application-specific. The example offers information on the authenticated user account, and also information regarding the token itself. exp, iat, and nbf are fields utilized by convention expressing the tokens expiry time, issued at time, rather than valid before (start) time. The payload must be Base64-encoded too.

$payloadData = [    "userId" => 1001,    "userName" => "demo",    "licensedFeatures" => ["todos", "calendar", "invoicing"],    "exp" => (time() + 900),    "iat" => time(),    "nbf" => time()];$payload = base64_encode(json_encode($payloadData));

All that remains would be to create the signature. To create this you first combine the header and payload right into a single string separated by way of a . character:

$headerAndPayload = "$header.$payload";

After that you must generate a distinctive secret to utilize as your signing key. The trick must be stored securely on your own server and must never be delivered to clients. Exposure of the value allows one to create valid tokens.

// PHP solution to generate 32 random characters$secret = bin2hex(openssl_random_pseudo_bytes(16));

You complete the procedure utilizing the secret to sign the combined header and payload string utilizing the hashing algorithm you indicated in the header. The output signature should be Base64-encoded just like the other components.

$signature = base64_encode(hash_hmac("sha256", $headerAndPayload, $secret, true));

Now youve got the header, payload, and signature as individual textual components. Join all of them as well as . separators to generate the JWT to send to your client:

$jwt = "$header.$payload.$signature";

Verifying Incoming JWTs

Your client application can determine the features open to an individual by decoding the tokens payload. Heres a good example in JavaScript:

const tokenComponents = jwt.split(".");const payload = token[1];const payloadDecoded = JSON.parse(atob(payload));// ["todos", "calendar", "invoicing"]console.log(payloadDecoded.licensedFeatures);

An attacker might realize this data is plain text and looks an easy task to modify. They might make an effort to convince the server theyve got an additional benefit feature by changing the tokens payload within their next request:

// Develop a new payload componentconst modifiedPayload = btoa(JSON.stringify(    ...payloadDecoded,    licensedFeatures:  ["todos", "calendar", "invoicing", "extraPremiumFeature"]));// Stitch the JWT back alongside the original header and signatureconst newJwt = `$token[0].$modifiedPayload.$token[2]`

The solution to the way the server defends against these attacks is based on the technique used to create the signature. The signature value considers the tokens header and payload. Modifying the payload, as in this example, means the signature is not any longer valid.

Server-side code verifies incoming JWTs by recomputing their signatures. The token has been tampered with if the signature sent by your client doesnt match the worthiness generated on the server.

$tamperedToken = $_POST["apiKey"];list($header, $payload, $signature) = $tamperedToken;// Determine the signature this token *shouldhave // once the server's secret can be used because the key$expectedSignature = hash_hmac("sha256", "$header.$payload", $secret, true);// The token has been tampered with because its // signature is incorrect for the info it offersif ($signature !== $expectedSignature)     http_response_code(403);// The signatures match - we generated this // token and may safely trust its dataelse     $user = fetchUserById($payload["userId"]);

Its impossible for an attacker to create a valid token without usage of the servers secret. This implies that accidental loss or deliberate rotation of the trick will immediately invalidate all previously issued tokens.

In a real-world situation, your authentication code also needs to inspect the expiration rather than before timestamps in the tokens payload. They are used to find out if the users session continues to be valid.

When to utilize JWTs

JWTs are generally useful for API authentication because theyre straightforward to implement on the server, an easy task to consume on your client, and easy to transmit across network boundaries. Despite their simplicity they will have good security because each token is signed utilizing the servers secret key.

JWTs certainly are a stateless mechanism which means you dont have to record information regarding issued tokens on your own server. You may get information regarding the client that displays a JWT from the tokens payload, rather than having to perform look-up in a database. These details could be safely trusted once youve verified the tokens signature.

Using JWTs is an excellent choice once you have to exchange information between two parties minus the threat of tampering. You can find weak spots to understand though: the complete system will undoubtedly be compromised if your servers secret key is leaked, or your signature verification code includes a bug. Because of this many developers opt for an open-source library to implement JWT generation and validation. Options are for sale to all popular programming languages. They get rid of the threat of oversight once you verify tokens yourself.


The JWT standard is really a data exchange format which includes built-in integrity verification. JWTs are generally used to secure interactions between API servers and client applications. The server can trust incoming tokens if its in a position to reproduce their signatures. This enables actions to be safely performed using information obtained from the tokens payload.

JWTs are convenient however they do involve some drawbacks. A JWTs Base64-encoded textual representation can easily become large if youve got greater than a couple of payload fields. This may become an unacceptable overhead whenever your client must send the JWT with each request.

The statelessness of JWTs is another potential downside too: once theyre issued, tokens are immutable and can be used as-is until they expire. Clients that use JWT payloads to find out a users permissions or licensed features will have to get yourself a new token from the backend whenever their assignations change.

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