diff --git a/Sources/IndieWebKit/IndieAuth/AuthenticationRequest.swift b/Sources/IndieWebKit/IndieAuth/AuthenticationRequest.swift
index 219b79b..5231825 100644
--- a/Sources/IndieWebKit/IndieAuth/AuthenticationRequest.swift
+++ b/Sources/IndieWebKit/IndieAuth/AuthenticationRequest.swift
@@ -19,19 +19,19 @@ public class AuthenticationRequest {
     private var codeChallenge: String?
     private let codeChallengeMethod = "S256"
     
-    private var url: URL {
+    private var url: URL? {
         var requestUrl = URLComponents(url: authorizationEndpoint, resolvingAgainstBaseURL: false)
-        requestUrl?.queryItems.push(URLQueryItem(name: "me", value: profile))
-        requestUrl?.queryItems.push(URLQueryItem(name: "client_id", value: clientId))
-        requestUrl?.queryItems.push(URLQueryItem(name: "redirect_uri", value: redirectUri))
-        requestUrl?.queryItems.push(URLQueryItem(name: "state", value: state))
+        requestUrl?.queryItems?.append(URLQueryItem(name: "me", value: profile.absoluteString))
+        requestUrl?.queryItems?.append(URLQueryItem(name: "client_id", value: clientId.absoluteString))
+        requestUrl?.queryItems?.append(URLQueryItem(name: "redirect_uri", value: redirectUri.absoluteString))
+        requestUrl?.queryItems?.append(URLQueryItem(name: "state", value: state))
         
         if codeChallenge != nil {
-            requestUrl?.queryItems.push(URLQueryItem(name: "code_challenge", value: codeChallenge))
-            requestUrl?.queryItems.push(URLQueryItem(name: "code_challenge_method", value: codeChallengeMethod))
+            requestUrl?.queryItems?.append(URLQueryItem(name: "code_challenge", value: codeChallenge))
+            requestUrl?.queryItems?.append(URLQueryItem(name: "code_challenge_method", value: codeChallengeMethod))
         }
         
-        return requestUrl.url
+        return requestUrl?.url
     }
     
     init(for profile: URL, at authorizationEndpoint: URL, clientId: URL, redirectUri: URL, state: String, codeChallenge: String?) {
@@ -47,8 +47,56 @@ public class AuthenticationRequest {
         }
     }
     
-    func start() {
-        ASWebAuthenticationSession(url: url, callbackURLScheme: <#T##String?#>, completionHandler: <#T##ASWebAuthenticationSession.CompletionHandler##ASWebAuthenticationSession.CompletionHandler##(URL?, Error?) -> Void#>)
+    func start(completion: @escaping ((String?) -> ())) {
+        guard url != nil else {
+            // TODO: Throw some type of error
+            return
+        }
+        
+        ASWebAuthenticationSession(url: url!, callbackURLScheme: nil) { [weak self] responseUrl, error in
+            guard error == nil else {
+                // TODO: Throw some type of error
+                return
+            }
+            
+            guard responseUrl != nil else {
+                // TODO: Throw some type of error
+                return
+            }
+            
+            let authorizationCode = self?.parseResponse(responseUrl!)
+            guard authorizationCode != nil else {
+                // TODO: Throw an error because authorization code should not be nil
+                return
+            }
+            
+            verifyAuthenticationCode(authorizationCode!)
+        }.start()
+    }
+    
+    private func parseResponse(_ responseUrl: URL) -> String {
+        let responseComponents = URLComponents(url: responseUrl, resolvingAgainstBaseURL: false)
+        var state = ""
+        var code = ""
+        
+        responseComponents?.queryItems?.forEach { queryItem in
+            if queryItem.name == "code", queryItem.value != nil {
+                code = queryItem.value!
+            } else if queryItem.name == "state", queryItem.value != nil {
+                state = queryItem.value!
+            }
+        }
+    
+        guard state == self.state else {
+            // TODO: Throw some error because state doesn't match
+            return ""
+        }
+        
+        return code
+    }
+    
+    private func verifyAuthenticationCode(_ code: String) {
+        // TODO: Resume #5 here
     }
     
     private func generateDefaultCodeChallenge() -> String? {