PKCE Implementation (Proof Key for Code Exchange)

#Solving the Public Client Problem#link

We previously discussed how the Authorization Code flow is secure because the token exchange happens on a back-channel. But what if there is no back-channel? For mobile apps or SPAs, the 'Client Secret' cannot be stored securely. This is where PKCE (pronounced 'pixie') comes in.

The Vulnerability: Auth Code Interception

In a standard flow on a mobile device, the Authorization Code is delivered via a custom URI scheme (e.g., `myapp://callback`). A malicious app on the same device could register that same scheme and intercept the code.

// 1. Generate a random 'Code Verifier'
const verifier = generateRandomString(64);

// 2. Create 'Code Challenge' (S256)
const challenge = base64UrlEncode(sha256(verifier));

// 3. Send challenge in the initial request
const authUrl = `...&code_challenge=${challenge}&code_challenge_method=S256`;

The client sends the *challenge* during the initial request. The server stores this challenge and associates it with the issued authorization code.

The Verification Step

The server hashes the provided `code_verifier` and checks if it matches the `code_challenge` sent earlier. If it matches, it proves the requester is the original client.

Production Implementation

Modern security standards (OAuth 2.1) mandate PKCE for all clients, even those with a backend. It adds a layer of defense-in-depth that protects against code injection and interception.

• ▪Always use S256 (SHA-256) for challenges
• ▪Never reuse a code verifier across sessions
• ▪Implement PKCE on the server-side to reject non-PKCE requests
• ▪Use secure random number generators for the verifier