Address feedback round 1

This commit will be squashed before merge.

csi.proto

@@ -271,15 +271,18 @@ message VolumeMetadata {
 }
 
 // A standard way to encode credential data. The total bytes of the values in
-// the Data field must be less than MaxSecretSize bytes.
+// the Data field must be less than 1 Mebibyte.
 message Credentials {
-  // Data contains the credential data, for example username and password. Each
-  // key must consist of alphanumeric characters, '-', '_' or '.'. The
-  // serialized form of the secret data is a base64 encoded string, representing
-  // the arbitrary (possibly non-string) data value here.
-  // This information is sensitive and should be treated as such (not logged,
-  // etc.)
-  map<string, bytes> Data = 1;
+  // Data contains the credential data, for example username and password.
+  // Each key must consist of alphanumeric characters, '-', '_' or '.'.
+  // Each value MUST contain a valid string. An SP MAY choose to accept binary
+  // (non-string) data by using a binary-to-text encoding scheme, like base64.
+  // An SP SHALL advertise the requirements for credentials in documentation.
+  // COs SHALL permit users to pass through the required credentials.
+  // This information is sensitive and MUST be treated as such (not logged,
+  // etc.) by the CO.
+  // This field is REQUIRED.
+  map<string, string> Data = 1;
 }
 ////////
 ////////
@@ -328,7 +331,8 @@ message ControllerPublishVolumeRequest {
   bool readonly = 5;
 
   // End user credentials used to authenticate/authorize controller publish
-  // request. This field is OPTIONAL.
+  // request.
+  // This field is OPTIONAL.
   Credentials userCredentials = 6;
 }