diff --git a/examples/gmail_oauth2.rs b/examples/gmail_oauth2.rs
index 50805f1..0fe212c 100644
--- a/examples/gmail_oauth2.rs
+++ b/examples/gmail_oauth2.rs
@@ -13,7 +13,7 @@ struct GmailOAuth2 {
 impl Authenticator for GmailOAuth2 {
     type Response = String;
     #[allow(unused_variables)]
-    fn process(&self, data: String) -> Self::Response {
+    fn process(&self, data: Vec<u8>) -> Self::Response {
         format!(
             "user={}\x01auth=Bearer {}\x01\x01",
             self.user, self.access_token
diff --git a/src/authenticator.rs b/src/authenticator.rs
index 0926924..1e5187f 100644
--- a/src/authenticator.rs
+++ b/src/authenticator.rs
@@ -1,5 +1,5 @@
 /// This will allow plugable authentication mechanisms.
 pub trait Authenticator {
     type Response: AsRef<[u8]>;
-    fn process(&self, String) -> Self::Response;
+    fn process(&self, Vec<u8>) -> Self::Response;
 }
diff --git a/src/client.rs b/src/client.rs
index 734d0e9..017bd1d 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -395,7 +395,14 @@ impl<T: Read + Write> Client<T> {
                     parse_authenticate_response(String::from_utf8(line).unwrap()),
                     self
                 );
-                let raw_response = &authenticator.process(data);
+                let challenge = ok_or_unauth_client_err!(
+                    base64::decode(data.as_str())
+                        .map_err(|_|
+                                 Error::Parse(ParseError::Authentication(data))
+                        ),
+                    self
+                );
+                let raw_response = &authenticator.process(challenge);
                 let auth_response = base64::encode(raw_response);
                 ok_or_unauth_client_err!(
                     self.write_line(auth_response.into_bytes().as_slice()),
@@ -940,7 +947,7 @@ mod tests {
 
     #[test]
     fn authenticate() {
-        let response = b"+\r\n\
+        let response = b"+YmFy\r\n\
                          a1 OK Logged in\r\n".to_vec();
         let command =  "a1 AUTHENTICATE PLAIN\r\n\
                         Zm9v\r\n";
@@ -949,7 +956,11 @@ mod tests {
         enum Authenticate { Auth };
         impl Authenticator for Authenticate {
             type Response = Vec<u8>;
-            fn process(&self, _: String) -> Self::Response {
+            fn process(&self, challenge: Vec<u8>) -> Self::Response {
+                assert!(
+                    challenge == b"bar".to_vec(),
+                    "Invalid authenticate challenge"
+                );
                 b"foo".to_vec()
             }
         }