OAuth2/PKCE on Android: Authorization Flow & Token Management

OAuth2/PKCE on Android: Authorization Flow & Token Management OAuth2 with PKCE (Proof Key for Code Exchange) is the secure standard for mobile authentication. It protects against authorization code interception attacks. PKCE Code Challenge Generation class OAuth2Manager ( private val clientId : String ) { fun generateCodeChallenge (): Pair < String , String > { val codeVerifier = generateRandomString ( 128 ) val bytes = codeVerifier . toByteArray () val md = MessageDigest . getInstance ( "SHA-256" ) val digest = md . digest ( bytes ) val codeChallenge = Base64 . getUrlEncoder (). withoutPadding (). encodeToString ( digest ) return codeVerifier to codeChallenge } private fun generateRandomString ( length : Int ): String { val chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~" return ( 1 .. length ). map { chars . random () }. joinToString ( "" ) } } Authorization Request fun requestAuthorizationCode ( codeChallenge : String ) { val authUrl = Uri . Builder () .