Authorization Code flow with PKCE

Yes, the Authorization Code flow with PKCE (Proof Key for Code Exchange) is a security extension of the OAuth 2.0 Authorization Code flow. The PKCE extension is designed to prevent code injection attacks that could occur in the Authorization Code flow.

In the standard OAuth 2.0 Authorization Code flow, the client application requests authorization from the user and receives an authorization code from the authorization server. The client application then exchanges the authorization code for an access token and refresh token.

However, in some scenarios, an attacker could intercept the authorization code and use it to obtain an access token and refresh token, compromising the security of the user's data. The PKCE extension was introduced to prevent this type of attack.

PKCE involves an additional step in the Authorization Code flow, where the client generates a random string called a code verifier and calculates a hash value of this string called the code challenge. The client includes the code challenge in the authorization request to the authorization server. The authorization server records the code challenge along with the authorization code it issues to the client.

When the client exchanges the authorization code for an access token, it includes the code verifier along with the request. The authorization server verifies that the code verifier produces the correct code challenge, which demonstrates that the client that's requesting the access token is the same client that initiated the authorization request.

PKCE provides additional security for the Authorization Code flow, making it less susceptible to attacks such as code injection attacks. It is recommended to use PKCE for Authorization Code flows when the client application is public and therefore, not capable of keeping secrets confidential. Sure, I can give you an example of how to generate the code verifier and code challenge in a ReactJS client and how to verify it in a Spring Authorization Server (SAS).

First, let's look at how to generate the code verifier and code challenge in the client.

import { sha256 } from 'crypto-hash';

// Generate a random string to use as the code verifier
const codeVerifier = generateRandomString(64);

// Calculate the code challenge by hashing the code verifier with SHA-256 and base64url encoding the result
const codeChallenge = await sha256(codeVerifier, { encoding: 'buffer' });
const codeChallengeEncoded = base64urlencode(codeChallenge);

In this example, we use the crypto-hash package to generate a SHA-256 hash of the code verifier. We then base64url encode the result to produce the code challenge.

Now, let's look at how to verify the code challenge in the SAS.

import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;

public boolean verifyCodeChallenge(String codeVerifier, String codeChallenge) throws NoSuchAlgorithmException {
    MessageDigest md = MessageDigest.getInstance("SHA-256");
    byte[] codeVerifierBytes = codeVerifier.getBytes(StandardCharsets.US_ASCII);
    md.update(codeVerifierBytes, 0, codeVerifierBytes.length);
    byte[] codeChallengeBytes = md.digest();
    String codeChallengeComputed = Base64.getUrlEncoder().withoutPadding().encodeToString(codeChallengeBytes);
    return codeChallengeComputed.equals(codeChallenge);
}

In this example, we use the MessageDigest class from the java.security package to calculate the SHA-256 hash of the code verifier. We then base64url encode the result to produce the code challenge. Finally, we compare the computed code challenge to the one received from the client to verify that they match.

This is just an example, and you will need to modify the code to fit your specific requirements. However, this should give you a general idea of how to generate the code verifier and code challenge in a ReactJS client and how to verify it in a Spring Authorization Server.