JWSとJWK Set

Welcome! In this lesson, we'll dive into two crucial components for securing OpenID Connect: JSON Web Signatures (JWS) and JSON Web Key Sets (JWKS).

These mechanisms ensure that the ID Tokens you receive are authentic and haven't been tampered with.

Imagine receiving an ID Token that tells you who a user is. How do you know it's truly from the trusted Identity Provider and not a malicious actor?

This is where signatures come in. They provide:

• Authenticity: Verifying the sender's identity.
• Integrity: Ensuring the token hasn't been changed since it was signed.

A JSON Web Signature (JWS) is a compact, URL-safe representation of signed content. Think of it as a digital seal on your JSON data.

An ID Token is typically a type of JWS, meaning it has been digitally signed to prove its origin and integrity.

A JWS consists of three parts, separated by dots (.):

• Header: Describes the signing algorithm (e.g., RS256) and the key used.
• Payload: The actual data (e.g., the user claims in an ID Token).
• Signature: The cryptographic signature generated from the header, payload, and a secret key.

Example: HEADER.PAYLOAD.SIGNATURE

To verify a JWS signature, you need the correct public key. A JSON Web Key (JWK) is a standard way to represent cryptographic keys (like RSA or EC public keys) in a JSON format.

It makes it easy for systems to share and understand cryptographic keys.

A JWK contains several parameters to describe a key. Common ones include:

• kty (Key Type): e.g., RSA, EC
• use (Public Key Use): e.g., sig (for signing)
• alg (Algorithm): e.g., RS256
• kid (Key ID): A unique identifier for the key.
• Specific key components (e.g., n and e for an RSA public key).

An Identity Provider (IdP) might use several keys for signing ID Tokens, or rotate them over time. A JSON Web Key Set (JWKS) is a JSON object containing an array of JWKs.

It's the standard way for an IdP to publish all its public keys that clients can use for verification.

How does your application find the IdP's JWKS?

The OpenID Connect Discovery Endpoint (e.g., https://idp.com/.well-known/openid-configuration) provides a URL called jwks_uri.

Your application fetches the JWKS from this jwks_uri to get the public keys needed for verification.

Here's the typical process for a client to verify an ID Token:

1. Receive the ID Token (JWS).
2. Parse its header to find the kid (Key ID) and alg (algorithm).
3. Fetch the JWKS from the IdP's jwks_uri.
4. Locate the JWK in the JWKS that matches the kid and alg from the ID Token header.
5. Use the public key from the matched JWK to cryptographically verify the ID Token's signature.

Great job! You've learned about the core components for ID Token security:

• JWS: A signed JSON structure, ensuring token authenticity and integrity.
• JWK: A standard JSON format for cryptographic keys.
• JWKS: A set of JWKs published by an IdP, allowing clients to discover and retrieve public keys for signature verification.

These pieces work together to build trust in the identity information provided by OpenID Connect.