재생 공격 방지를 위한 nonce 사용

The nonce is a random value the client generates and includes in the authentication request. The OpenID Provider echoes it back inside the issued ID token. Matching them proves the token belongs to this request.

Without a nonce, an attacker who captures a valid ID token (for example, in the Implicit or Hybrid flow where tokens travel via the browser) could replay it into another session. The nonce ties the token to one specific request, defeating replay.

They are different tools:

• state — protects the OAuth2 authorization request/response against CSRF.
• nonce — protects the ID token against replay, validated inside the token itself.

Use both together in OIDC flows.

Create a high-entropy random value and store it bound to the user's session before redirecting.

Add the nonce to the authorization request alongside the usual parameters.

The ID token's payload includes the exact nonce you sent.

After validating the ID token's signature and claims, compare its nonce with the value stored in the session.

The nonce is mandatory in the Implicit and Hybrid flows because ID tokens are returned through the browser front channel. In the Authorization Code flow it is recommended and strongly encouraged.

Treat each nonce as single-use. Remove it from the session as soon as it is validated so the same value can never authorize a second token, closing replay windows.

Pitfalls to avoid:

• Using a predictable or reused nonce.
• Forgetting to compare it after validating the signature.
• Storing it client-side without integrity protection.
• Skipping it in front-channel flows.

The full lifecycle: generate nonce, store in session, send in auth request, receive it in the ID token, verify signature and claims, then compare and discard the nonce. Only then trust the authentication.

The nonce protects ID tokens from replay.

• Generate a random nonce, store it in session, send it in the auth request.
• The OP echoes it inside the ID token.
• Validate by comparing token nonce to session nonce, then discard it.
• Required in Implicit/Hybrid flows; recommended everywhere.