OAuth2 Proof Key for Code Exchange (PKCE) — Complete Guide to RFC 7636

In this tutorial, you will learn about OAuth2 Proof Key for Code Exchange (PKCE). We cover key concepts, practical examples, and best practices to help you master this topic.

OAuth2 PKCE (Proof Key for Code Exchange, RFC 7636) prevents authorization code interception attacks by requiring the client to prove it initiated the authorization flow through a cryptographic challenge-verifier pair.

What You'll Learn

• PKCE flow and how it prevents code interception
• Code verifier generation and code challenge methods
• S256 vs plain challenge methods
• PKCE with mobile and SPA clients
• Server-side PKCE validation

Why It Matters

PKCE is mandatory for public clients (mobile apps, SPAs) and recommended for all OAuth2 flows. Without PKCE, an attacker who intercepts the authorization code can exchange it for tokens. DodaTech requires PKCE for all client types, eliminating authorization code interception as a threat vector.

sequenceDiagram
    participant Client as Mobile App
    participant Auth as Authorization Server

    Client->>Client: Generate code_verifier (random string)
    Client->>Client: Compute code_challenge = SHA256(verifier)
    Client->>Auth: Authorization Request + code_challenge (S256)
    Auth->>Client: Authorization code
    Client->>Auth: Token Request + code_verifier
    Auth->>Auth: Verify SHA256(verifier) == stored challenge
    Note over Auth: If verifier doesn't match the challenge, reject
    Auth-->>Client: Access + Refresh Tokens

What's Next

Apply PKCE to {{< ilink "OAuth" "OAuth2 Client Types" }} for public clients, and combine with {{< ilink "OAuth" "OAuth2 Pushed Authorization Requests (PAR)" }} for defense in depth.