I am working on the backend of our iOs App for apple sign in. The backend of our app is designed in Ubuntu 14.04 and using framework Codeigniter in PHP. I been following this link from the start. https://developer.okta.com/blog/2019/06/04/what-the-heck-is-sign-in-with-apple but then the JWT token wasn't working for me and apple auth was sending me the following error. {"error":"invalid_client"} I tried a few library and when I used this library [ameen-sarsour] (https://gist.github.com/ameen-sarsour/e14a1d5bae5b61080dfdd5b1430c3e10) for generating the JWT token i.e. client_secret and this library [GriffinLedingham/php-apple-signin] (https://github.com/GriffinLedingham/php-apple-signin) to decrypt the id_token code. The above error turned into {"error":"invalid_grant"} After using the fresh auth_code from the iOs. I started receiving the following response from Apple. I used the TEAM_ID, KEY_ID & CLIENT_ID as I received from Apple. I successfully received the following response from Apple side. { "access_token":"...", "token_type":"Bearer", "expires_in":3600, "refresh_token":"....", "id_token": "..." } I designed the backend of the App for registration and login with Apple sign-in. The next day, the same code is started to throw the following error. {"error":"invalid_grant"} Its been a few days now and I am still stuck on this issue. I also looked into CURL URL provided here [generateandvalidate_tokens] (https://developer.apple.com/documentation/sign_in_with_apple/generate_and_validate_tokens) curl -v POST "https://appleid.apple.com/auth/token" \H 'content-type: application/x-www-form-urlencoded' \ d 'client_id=com.Myapp.app' \ d 'client_secret=<client secret jwt code>' \ d 'code=<auth_code from ios App>' \ d 'grant_type=authorization_code' Here is the response of the above code in the attachment CURL Post auth/token result - https://developer.apple.com/forums/content/attachment/b09f7118-3a4d-429d-aebd-5e7cd193f2e0 It's showing HTTP 400 in response. Please suggest solution for this issue.