Make --ca and --cipher non-optional in .ovpn

Dodge those annoying scenarios where server cipher is not set
and defaults to BF-CBC, whereas default TunnelKit cipher
is AES-128-CBC. And data channel stalls.

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+- Require explicit `--ca` and `--cipher` in .ovpn configuration file.
+
 ## 2.1.0 (2019-11-03)
 
 ### Added

TunnelKit/Sources/Protocols/OpenVPN/ConfigurationParser.swift

@@ -587,6 +587,13 @@ extension OpenVPN {
                 }
             }
             
+            guard let _ = optCA else {
+                throw ConfigurationError.missingConfiguration(option: "ca")
+            }
+            guard let _ = optCipher else {
+                throw ConfigurationError.missingConfiguration(option: "cipher")
+            }
+            
             //
             
             var sessionBuilder = ConfigurationBuilder()