ID Token Structure & Signature
Deconstruct the ID Token (JWT) header, payload, and signature, understanding how it's signed and what information it contains.
ID Token Structure & Signature is a free OAuth2 & OpenID Connect Deep Dive lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the OAuth2 & OpenID Connect Deep Dive learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Meet the ID Token
In OpenID Connect (OIDC), the ID Token is a crucial piece of information. It's a security token that contains claims about the authentication of an end-user by an Authorization Server.
Think of it as a digital ID card for the user, issued after they successfully log in.
ID Tokens Are JWTs
ID Tokens are always formatted as JSON Web Tokens (JWTs). A JWT is a compact, URL-safe means of representing claims to be transferred between two parties.
Every JWT has three main parts, separated by dots:
- Header
- Payload
- Signature
The Header: What Algorithm?
The first part of an ID Token is the Header. It's a JSON object that describes the token itself, like what type of token it is and the algorithm used to sign it.
alg: The cryptographic algorithm used for signing (e.g., RS256, HS256).typ: The type of token, which is usually"JWT".
Header in Action
When you decode the base64url-encoded header, you'll see a JSON object like this. This tells you how the token was secured.
{
"alg": "RS256",
"typ": "JWT",
"kid": "someKeyId"
}The kid (Key ID) helps find the correct public key for verification.
Decoding Header Example
The header is base64url encoded. Here's how you might decode a JWT header string in Java. This helps reveal its content.
import java.util.Base64;
import java.nio.charset.StandardCharsets;
public class JwtDecoder {
public static void main(String[] args) {
String encodedHeader = "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCIsImtpZCI6InNvbWVLZXlJZCJ9";
byte[] decodedBytes = Base64.getUrlDecoder().decode(encodedHeader);
String decodedString = new String(decodedBytes, StandardCharsets.UTF_8);
System.out.println("Decoded Header:");
System.out.println(decodedString);
}
}The Payload: Claims About You
The second part is the Payload. This is where the actual "claims" about the user and the authentication event are stored. Claims are statements about an entity (typically the user).
Standard claims you'll often see include:
iss: Issuer (who issued the token).sub: Subject (unique identifier for the user).aud: Audience (who the token is for).exp: Expiration Time (when the token expires).iat: Issued At Time (when the token was issued).
Payload in Action
Similar to the header, the payload is also a base64url-encoded JSON object. It contains the identity information you need.
{
"iss": "https://example.com/auth",
"sub": "user123",
"aud": "myAppClientId",
"exp": 1678886400,
"iat": 1678882800,
"name": "Alice Wonderland"
}exp and iat are Unix timestamps.
The Signature: Ensuring Trust
The third and final part is the Signature. This is critical for security! It's used to verify that the token hasn't been tampered with and that it comes from a legitimate issuer.
The signature is created by taking the encoded header, the encoded payload, and a secret key or private key, and running them through the cryptographic algorithm specified in the header.
Verifying the Signature
When your application receives an ID Token, it uses the public key (provided by the Issuer) to verify the signature. It re-computes the signature using the header, payload, and the public key.
If the re-computed signature matches the token's signature, you can trust that:
- The token hasn't been altered.
- It was issued by the expected Authorization Server.
Quick Check on JWT
You've learned that ID Tokens are structured as JWTs, with three distinct parts. Each part plays a vital role in conveying and securing user identity information.
Recap: ID Token Anatomy
Great job! You now understand the fundamental structure of an ID Token.
- Header: Describes the token and signing algorithm.
- Payload: Contains identity claims about the user.
- Signature: Ensures the token's integrity and authenticity.
Each part is base64url encoded and separated by dots, forming a secure and verifiable digital identity for the user.
Frequently asked questions
Is the “ID Token Structure & Signature” lesson free?
Yes — the full text of “ID Token Structure & Signature” is free to read here on the web, and the OAuth2 & OpenID Connect Deep Dive course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the OAuth2 & OpenID Connect Deep Dive course, upgrade to CoddyKit PRO.
What will I learn in “ID Token Structure & Signature”?
Deconstruct the ID Token (JWT) header, payload, and signature, understanding how it's signed and what information it contains. You practise OAuth2 & OpenID Connect Deep Dive with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start OAuth2 & OpenID Connect Deep Dive?
No prior experience is required. OAuth2 & OpenID Connect Deep Dive on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “ID Token Structure & Signature” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this OAuth2 & OpenID Connect Deep Dive lesson?
Yes. Every OAuth2 & OpenID Connect Deep Dive lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- ID Token Structure & Signature
- JWS and JWK Sets
- Token Revocation & Introspection
- Validating Standard ID Token Claims