JWT Encoder

Tool Overview

This tool helps you build and sign JSON Web Tokens (JWT) directly in your browser. It lets you edit the header and payload as JSON, choose a signing algorithm, provide a secret, and instantly see the final token string.

JWTs are widely used for authentication and authorization. A wrong header, a broken payload, or a weak secret can cause serious security problems or unexpected behavior. Manually crafting JWTs by hand is slow and error-prone, especially when you must base64url encode and sign the content.

The JWT Encoder automates these steps. It validates your JSON, forces the header algorithm to match your selected algorithm, uses the Web Crypto API to compute an HMAC signature, and shows the token in the standard header.payload.signature form. It also offers AI-based security analysis of your payload so you can understand possible risks and improvements.

This tool is designed for developers, testers, and security-focused users with at least beginner to intermediate technical skills who work with JWT-based systems.

Background & Concept Explanation

A JSON Web Token is a compact data format used to pass claims between parties. It has three parts: a header, a payload, and a signature. The header describes the type of token and the algorithm used. The payload holds the claims, such as user ID or roles. The signature verifies that the token has not been changed.

Each part is a JSON object. For transport, each part is encoded using Base64URL (a URL-safe variant of Base64). The header and payload are encoded separately and then joined with a dot. The result of these two segments is signed using an algorithm such as HS256, HS384, or HS512. These names represent HMAC signatures with SHA-256, SHA-384, or SHA-512 hashes. A related operation involves encoding data in Base64 as part of a similar workflow.

To create a valid token, you must construct valid JSON, encode it correctly, select the right algorithm, and provide a matching secret key. If any part is wrong, clients or servers may reject the token, or worse, accept an insecure configuration. Doing this by hand, especially the encoding and signing steps, is tedious and easy to get wrong.

The JWT Encoder makes these steps explicit and safe. It provides separate areas for header, payload, and secret. It limits input sizes to reasonable bounds. It checks that header and payload are valid JSON before attempting to encode. It then uses your selected algorithm and secret to sign the token in a standard-compliant way using the browser’s cryptographic capabilities.

Because all signing work happens in the browser, your secret never leaves your machine. This is important when experimenting with real or test secrets and when doing security research or debugging existing JWT flows.

Key Features

• Editable header JSON: A dedicated editor area lets you view and change the JWT header as JSON. The default header includes the alg and typ fields. The tool enforces valid JSON and shows errors if the structure is invalid.
• Editable payload JSON: A separate editor area holds the JWT payload. You can write any valid JSON object, including common claims such as sub, name, iat, or custom fields. Validation ensures that malformed JSON is flagged quickly.
• Algorithm selection: A group of buttons lets you choose between HS256, HS384, and HS512 algorithms. The selected algorithm is stored in state and also written back into the header’s alg field so both stay consistent.
• Secret key input: A dedicated input field holds the HMAC secret used for signing. The tool enforces that a non-empty secret is required before encoding and shows a clear error if it is missing.
• Input size limits: To prevent browser slowdowns, the tool caps the header at 50KB, the payload at 100KB, and the secret at 10KB. Character counters help you understand how close you are to these limits.
• Automatic token generation: Any time the header, payload, secret, or algorithm changes, the tool re-validates the JSON and, if valid, recomputes the token by calling the signing function.
• Color-coded token preview: The output panel shows the encoded JWT with each section in a different color. This makes it easy to see where the header, payload, and signature start and end.
• Copy helpers: Buttons allow you to copy the full token, the header JSON, or the payload JSON to the clipboard. Visual feedback shows when content has been copied successfully.
• Preset payloads: For quick testing, small preset buttons populate the payload with example structures such as a standard auth payload, a Firebase-like payload, or an empty object.
• Error feedback for encoding issues: If JSON is invalid, the secret is empty, or signing fails, the tool clears the token output and shows a short error message describing what went wrong.
• Local-only signing: The signing logic uses the Web Crypto API in the browser to import the secret, sign the data, and compute the signature. No secret material is sent to a server.
• AI-powered security analysis: An AI Analysis button sends the payload and selected algorithm to a backend service. The response includes a numeric score, a list of risks, and suggestions, which appear in a dedicated security panel.
• Clear-all command: A Clear All button resets header, payload, secret, errors, token, and analysis back to starting values so you can begin a new experiment.

Common Use Cases

Testing authentication flows: When building or debugging a login system that uses JWTs, you can use this tool to generate tokens with specific claims and see how your server responds.

Learning JWT structure: By editing header and payload fields and seeing how the encoded token changes, beginners can build an intuition for how JWTs are put together. For adjacent tasks, base64 encoder operations addresses a complementary step.

Creating tokens for temporary access: In development environments, you might need to generate tokens that represent certain users or roles. This tool lets you do that without writing custom code.

Assessing payload security: The AI-based analysis can highlight potential risks such as sensitive information in payloads, missing expiration claims, or unclear audience settings. This is useful during design and review sessions.

Verifying secret and algorithm behavior: By changing algorithms and secrets and observing how the token changes, you can confirm that your understanding of HMAC signing is correct before deploying changes in your apps.

How to Use This Tool (Step-by-Step)

1. Review the default header: The Header editor starts with a basic JSON object that includes alg and typ. Make sure this structure matches your needs, but know that the algorithm field will always be updated to match your selected algorithm.
2. Choose an algorithm: In the Algorithm section, click HS256, HS384, or HS512. The chosen option becomes highlighted. This selection controls the hash used during signing.
3. Enter or adjust the payload: In the Payload editor, type or paste a JSON object with your claims. You can also click one of the preset buttons to load a standard example and edit it.
4. Provide a secret key: In the Secret field, type a strong HMAC secret string. This key will never leave your browser but is required to sign the token.
5. Check for validation errors: If the header or payload is not valid JSON, or the secret is empty, an error message appears near the affected field. Fix these issues until there are no errors.
6. Inspect the token output: Once everything is valid, the Token panel shows the full JWT. Each section (header, payload, signature) has its own color, and you can see the character length as well.
7. Copy the token: Use the Copy Token button at the top to copy the entire JWT to your clipboard. You can then paste it into your application, testing tools, or decoders.
8. Copy header or payload (optional): Use the small copy icons next to the Header or Payload labels to copy just those JSON sections for reuse elsewhere.
9. Run security analysis (optional): Click AI Analysis to send the payload and algorithm to the backend AI service. After a short wait, review the security score, risk list, and recommendations in the Security Analysis panel.
10. Reset when needed: When you are done with a scenario, click Clear All to return the tool to its initial state and start again.

Calculations & Logic

The JWT Encoder uses a helper function to convert strings to Base64URL. It first encodes the input as UTF-8, then applies Base64 encoding, and finally replaces characters and trims padding so the result is URL-safe. This function is used for both the header and the payload.

Before signing, the tool parses the header and payload JSON strings into objects. It validates that they are real objects, not other types. The header’s alg property is overwritten with the selected algorithm to avoid mismatches. Both objects are then JSON-stringified and base64url encoded. When working with related formats, decoding JSON Web Tokens can be a useful part of the process.

The encoded header and payload strings are joined with a dot to form the signing input. The signing function then maps the chosen algorithm (HS256, HS384, HS512) to the corresponding Web Crypto hash name (SHA-256, SHA-384, SHA-512).

Using TextEncoder, the secret string and signing input are converted into byte arrays. The secret bytes are imported into the Web Crypto API as an HMAC key with the selected hash function. The data bytes are then signed using crypto.subtle.sign, producing a raw binary signature.

This binary signature is turned into a Base64 string, which is then converted into Base64URL by replacing characters and removing padding. The final token is the concatenation of header, payload, and signature, each separated by dots.

Validation logic ensures that no signing is attempted if the Web Crypto API is missing, header or payload are not objects, or the secret is empty. In these cases, the function throws a descriptive error that surfaces in the user interface.

JSON validation uses a small helper that calls JSON.parse inside a try/catch. If parsing succeeds, the JSON is considered valid; otherwise it is marked invalid and an error message is displayed near the affected field. In some workflows, encoding HTML entities is a relevant follow-up operation.

Reference Tables or Scales

Algorithm	Hash function	Typical use
HS256	SHA-256	Common choice for HMAC JWTs
HS384	SHA-384	Stronger hash for higher security margins
HS512	SHA-512	Highest hash strength among listed options

Tips, Limitations & Best Practices

Use strong secrets: For HS-based algorithms, the security of the token depends heavily on the strength of the secret. Use long, random values rather than short words or common phrases.

Keep secrets local: Do not paste production secrets into this tool in shared or untrusted environments. While signing happens in your browser, good practice is to treat all secrets with care.

Always include expiration: Include an exp claim in your payload and make sure it is reasonable. Tokens without expiration are risky because they may be valid forever if leaked.

Validate tokens on the server: This tool is good for creating and inspecting tokens, but real applications must validate tokens server-side, including signature, issuer, audience, and expiration.

Check AI analysis before changing designs: Use the security score and suggestions as guidance. Review them with your team and confirm they align with your system’s requirements before changing live configurations. For related processing needs, encoding URL components handles a complementary task.

Do not use in place of full security reviews: The tool’s checks and AI feedback are helpful for learning and early design, but they are not a substitute for formal security assessments.

Watch input sizes: Very large payloads can make tokens unwieldy. Keep payloads compact and store large data elsewhere. The tool’s limits give you a sense of safe practical sizes.

Use a decoder to verify results: After creating a token, you can use a separate decoder tool to confirm that the header and payload look as you expect. This double-checks both sides of your workflow.

Remember environment differences: Some browsers or environments may not support the Web Crypto API. If you see an error about missing Web Crypto, use a modern browser for signing or sign tokens in backend code instead.

Separate test and production usage: Use different secrets, payload patterns, and even algorithms in test and production environments. The encoder is ideal for development and testing, but production tokens should be generated in controlled backend systems.