PKCE Generator Tool

Introduction to PKCE

Proof Key for Code Exchange (PKCE, pronounced "pixie") is a crucial security extension to the OAuth 2.0 authorization code flow. Originally designed to secure OAuth flows in mobile applications, PKCE has become a best practice for all OAuth clients, including single-page applications and even confidential clients.

The Security Problem PKCE Solves

Traditional OAuth 2.0 authorization code flow has a vulnerability window where authorization codes can be intercepted. This is particularly problematic in:

• Mobile applications using custom URL schemes
• Single-page applications without secure backend storage
• Public clients that cannot securely store client secrets
• Environments where authorization codes might be logged or cached

How PKCE Works

PKCE adds an extra layer of security by introducing a dynamically generated secret that proves the client initiating the authorization request is the same client exchanging the authorization code for tokens.

The PKCE Flow:

1. Code Verifier Generation: Client generates a cryptographically random string (43-128 characters)
2. Code Challenge Creation: Client creates SHA256 hash of the code verifier and base64url-encodes it
3. Authorization Request: Client includes the code challenge in the authorization request
4. Code Exchange: Client sends the original code verifier when exchanging the authorization code
5. Verification: Authorization server verifies the code verifier matches the original challenge

Technical Implementation

The PKCE specification (RFC 7636) defines specific requirements for implementation:

Code Verifier Requirements:

• Length: 43-128 characters
• Character set: A-Z, a-z, 0-9, -, ., _, ~ (unreserved characters)
• High entropy for security (recommended: 256 bits)

Code Challenge Methods:

• S256: SHA256 hash of code verifier, base64url-encoded (recommended)
• plain: Code verifier used directly (only for testing, not recommended)

Benefits of Using PKCE

Implementing PKCE provides several security advantages:

• Authorization Code Protection: Even if intercepted, authorization codes are useless without the code verifier
• No Client Secret Required: Perfect for public clients that cannot securely store secrets
• Dynamic Security: Each authorization flow uses a unique code verifier/challenge pair
• Backward Compatibility: Can be implemented alongside existing OAuth flows
• Industry Standard: Recommended by OAuth 2.0 Security Best Practices

Best Practices for PKCE Implementation

• Always use the S256 code challenge method
• Generate code verifiers with sufficient entropy (256 bits minimum)
• Store code verifiers securely in client application memory
• Never log or persist code verifiers
• Implement proper error handling for PKCE validation failures
• Use PKCE even for confidential clients as an additional security layer

Common PKCE Implementation Mistakes

Avoid these common pitfalls when implementing PKCE:

• Using insufficient entropy in code verifier generation
• Storing code verifiers in insecure locations
• Using the 'plain' code challenge method in production
• Not validating code verifier format requirements
• Reusing code verifiers across multiple authorization attempts

PKCE and Modern OAuth 2.0

PKCE has evolved from a mobile-specific security enhancement to a fundamental part of modern OAuth 2.0 implementations. The OAuth 2.1 specification makes PKCE mandatory for all clients, reflecting its importance in the current security landscape.

Whether you're building a mobile app, single-page application, or server-side web application, implementing PKCE is now considered essential for maintaining the security and integrity of your OAuth 2.0 flows.