How can I implement PKCE for OAuth 2.0 in a Figma plugin

Steps to Implement PKCE in a Figma Plugin

1. Generate a code verifier: Create a cryptographically random string using a library like `crypto-js` or `pkce-challenge`. This will be the `code_verifier`.

2. Generate a code challenge: Create a SHA-256 hash of the `code_verifier` and then base64url encode it. This will be the `code_challenge`.

3. Start the OAuth flow: In your plugin, open a new browser window (using `window.open()`) and redirect the user to the OAuth authorization URL, passing the following parameters:
- `client_id`: Your application's client ID
- `response_type`: Set this to `code`
- `redirect_uri`: The callback URL on your server
- `scope`: The scopes you're requesting access to
- `code_challenge`: The `code_challenge` you generated in step 2
- `code_challenge_method`: Set this to `S256`

4. Handle the OAuth callback: When the user is redirected back to your callback URL, your server should:
- Verify that the `state` parameter matches the one you sent earlier (to prevent CSRF attacks)
- Exchange the authorization code for an access token using the `code_verifier` you generated in step 1:
- Make a POST request to the token endpoint, passing the following parameters:
- `client_id`: Your application's client ID
- `code`: The authorization code received in the callback
- `code_verifier`: The `code_verifier` you generated in step 1
- `grant_type`: Set this to `authorization_code`
- `redirect_uri`: The same callback URL as before

5. Store the access token: Once you've received the access token, store it securely in your plugin's storage (using `figma.clientStorage`) so you can use it for subsequent API requests.

The key benefits of using PKCE are:

- It eliminates the need to store a client secret, which is important for Figma plugins that run in a sandboxed environment.
- It provides an additional layer of security by binding the authorization code to the client application.

The Figma documentation provides more details on the PKCE flow for Figma plugins: OAuth with Plugins | Plugin API