What is an Authorisation Code with Proof Key for Code Exchange (PKCE)?

Businesses demand robust authentication mechanisms to protect user data and prevent unauthorised access. OAuth 2.0 has emerged as a critical protocol for secure authentication, with the Authorisation Code flow with PKCE representing a significant advancement in protecting users during the authentication process. 

This innovative approach addresses critical security vulnerabilities in traditional authentication methods.

The Traditional Authorisation Code Flow

In the traditional OAuth 2.0 authorisation code flow, authentication involves a series of interactions between the client application, authorisation server, and the user. When a user attempts to access a protected resource, they are redirected to an authorisation server where they authenticate. 

The server then generates an authorisation code, which the client exchanges for an access token. This mechanism was initially designed to provide a secure method for human-to-machine (h2m) and machine-to-machine (m2m) authentication across various application types.

Security Challenges in the Traditional Flow

Token Exposure Vulnerabilities

The original OAuth 2.0 authorisation code flow harboured significant security risks that could compromise user data. These vulnerabilities were particularly pronounced in public clients such as single-page applications and mobile apps. 

Client-Side Authentication Weaknesses

Public clients faced numerous authentication challenges that made them inherently vulnerable. The implicit flow, commonly used for client-side authentication, presented multiple security concerns. Tokens could be exposed in browser history, and there was no reliable method to verify the token's legitimate recipient.

Code Interception Threats

One of the most critical vulnerabilities was the potential for code interception attacks. Malicious applications could register with similar URL schemes, allowing them to capture authorisation codes intended for legitimate applications. 

This meant that attackers could potentially obtain access tokens without the user's knowledge or consent.

What is PKCE?

PKCE, also pronounced "pixy", is an enhanced security extension to the OAuth 2.0 protocol designed to prevent code interception attacks. This OAuth 2.0 grant type introduces a dynamic, cryptographically secure mechanism to ensure that only the original client can exchange an authorisation code for an access token. 

In the PKCE authentication flow, the client generates a unique code verifier—a cryptographically random string—and transforms it into a code challenge before initiating the authorisation request.

The authorisation server securely stores this challenge and later verifies the code verifier during the token exchange. By implementing this additional layer of security, PKCE prevents unauthorised parties from successfully obtaining access tokens, even if they intercept the initial authorisation code. 

This makes the OAuth 2.0 authentication process significantly more resilient against various security threats.

Key Components of PKCE

Code Verifier

The code verifier represents the cornerstone of the PKCE security model. This cryptographically random string is generated uniquely for each authorisation request, serving as a dynamic, one-time secret key. 

With a carefully defined length between 43 and 128 characters, the code verifier ensures unpredictability and prevents potential replay attacks.

Code Challenge

Derived from the original code verifier, the code challenge acts as a secure representation of the initial secret. This transformed value is sent alongside the initial authorisation request, providing a mechanism for secure, one-way verification.

Verification Process

During the token exchange, the authorisation server compares the code verifier against the original code challenge. This cryptographic comparison ensures that only the legitimate client can complete the authentication process, adding an unprecedented level of security to the OAuth 2.0 authorisation flow.

How PKCE Authentication Works: A Step-by-Step Overview

The authorisation code flow with PKCE introduces a robust security mechanism in OAuth 2.0 authentication. Here's how the PKCE authentication process unfolds:

Code Verifier Generation

• The client application creates a unique, random code verifier
• This is a cryptographically secure string between 43-128 characters
• Generated dynamically for each authentication request

Code Challenge Creation

• The code verifier is transformed into a code challenge
• Typically using the SHA-256 hashing method
• Encoded using BASE64URL encoding

Authorisation Request

• Client sends the code challenge with the initial authorisation request
• Includes the code challenge method (usually S256)
• Redirects user to the authorisation server

Token Exchange

• After user authentication, an authorisation code is received
• Client sends the original code verifier to the token endpoint
• The authorisation server validates the verifier against the stored challenge
• If verified, an access token is issued

Why PKCE is Critical for Modern Applications

PKCE authentication has become essential in today's digital ecosystem. Traditional OAuth 2.0 login methods were found to be weak when single-page apps and mobile apps became more popular. Public clients, which cannot securely store client secrets, particularly benefit from PKCE's additional security layer.

The protocol prevents code interception attacks by introducing dynamic, cryptographically secure verification. It ensures that even if an attacker intercepts the authorisation code, they cannot exchange it for an access token without the unique code verifier.

Implementing Authorisation Code Flow with PKCE in Your Application

1. Generate a cryptographically secure code verifier
2. Create a code challenge by hashing the verifier
3. Include the code challenge in the initial authorisation request
4. Store the original code verifier securely
5. Exchange the authorisation code with the stored verifier
6. Validate the token exchange on the server side

Modern OAuth 2.0 libraries often provide built-in support for PKCE, simplifying the implementation process for developers.

Conclusion

PKCE represents a significant advancement in OAuth 2.0 authentication security. By addressing vulnerabilities in the traditional implicit flow, it provides a robust mechanism for protecting user data across web and mobile applications.

As digital platforms continue to evolve, PKCE stands as a critical security enhancement, offering developers a reliable method to implement secure, standards-compliant authentication. Its ability to prevent code interception and provide dynamic verification makes it an indispensable tool in modern application security strategies.

Beyond traditional authentication, we at Instasafe deploy advanced multi-factor verification that dynamically shields your digital identity. Protect against unauthorised access with our intelligent, layered security ecosystem that keeps your data impenetrable.