Creating and Signing a Token Using a Private JWK Encoded RSA Key

Creating and Signing a Token Using a Private JWK Encoded RSA Key

JSON Web Tokens (JWTs) are a widely used mechanism for representing claims securely between two parties. They consist of three parts: a header, a payload, and a signature. The signature ensures that the token has not been tampered with and that it was issued by a trusted source.

In this article, we will explore how to create and sign a JWT using a private JWK encoded RSA key.

Prerequisites

Before you begin, you will need the following:

• A private RSA key encoded in JWK format
• A library or framework that supports JWT creation and signing (e.g., PyJWT, JWT.io)

Step 1: Create a JWT Header

The JWT header contains information about the signing algorithm and the type of token. For signing with a private RSA key, we will use the “RS256” algorithm and set the token type to “JWT”:

{
  "alg": "RS256",
  "typ": "JWT"
}

Step 2: Create a JWT Payload

The JWT payload contains the claims that you want to assert. These claims can be any arbitrary data, but they are typically used to represent user information, roles, permissions, or other metadata.

For example, let’s create a payload that contains the user’s username, email, and role:

{
  "username": "alice",
  "email": "alice@example.com",
  "role": "admin"
}

Step 3: Encode the Header and Payload

The header and payload need to be encoded as Base64URL strings before they can be signed. Use a library or framework that supports Base64URL encoding.

Step 4: Sign the Token

To sign the token, you will need your private RSA key. Import it into your chosen library or framework.

Then, use the signing algorithm specified in the header (“RS256”) and your private key to sign the encoded header and payload. This will produce a digital signature.

Step 5: Create the JWT

Finally, combine the encoded header, encoded payload, and signature to create the complete JWT. The format is:

<encoded header>.<encoded payload>.<signature>

Example

Using PyJWT, here is an example of how to create and sign a JWT using a private JWK encoded RSA key:

import jwt

# Load the private key
private_key = jwt.utils.load_pem(open('private_key.pem').read())

# Create the header and payload
header = {'alg': 'RS256', 'typ': 'JWT'}
payload = {'username': 'alice', 'email': 'alice@example.com', 'role': 'admin'}

# Encode the header and payload
encoded_header = jwt.utils.base64url_encode(jwt.jwk_encode(header)).decode('utf-8')
encoded_payload = jwt.utils.base64url_encode(jwt.json_encode(payload)).decode('utf-8')

# Sign the token
signature = jwt.sign(encoded_header + '.' + encoded_payload, private_key, algorithm='RS256')

# Create the JWT
jwt = encoded_header + '.' + encoded_payload + '.' + signature

Conclusion

By following these steps, you can create and sign JWTs using a private JWK encoded RSA key. This allows you to securely assert claims and verify their authenticity. JWTs are a valuable tool for implementing authentication and authorization in modern applications.