CryptoKitCodeChallenge.swift

extension Data {
    // Returns a base64 encoded string, replacing reserved characters
    // as per https://tools.ietf.org/html/rfc7636#section-4.2
    func pkce_base64EncodedString() -> String {
        base64EncodedString()
            .replacingOccurrences(of: "+", with: "-")
            .replacingOccurrences(of: "/", with: "_")
            .replacingOccurrences(of: "=", with: "")
            .trimmingCharacters(in: .whitespaces)
    }
}

func generateCodeChallenge(from verifier: String) -> String {
    let data = verifier.data(using: .utf8)!
    let hash = SHA256.hash(data: data)
    return Data(hash).pkce_base64EncodedString()
}