How can I roll a sphere relative to the camera's Y axis but not X axis?

Hello, I’ve recently started using Unity, and have been using C# as my scripting language of preference. Just as the question says, I’m trying to roll a rigid body sphere in relation to the Y axis of the camera but not the X axis. I have searched both questions and the forums but have been unable to figure this out. The code I’m currently using works well going forward and backward, but unless the camera’s X rotation is exactly 0.0f, it causes the sphere to roll offset by the X rotation of the camera. Here is a listing of my current code:

void FixedUpdate () {
   if (Input.GetKey ("w"))
      rigidbody.AddTorque(Camera.main.transform.right * speed * Time.deltaTime);
   if (Input.GetKey ("a"))
      rigidbody.AddTorque(Camera.main.transform.forward * speed * Time.deltaTime);
   if (Input.GetKey ("s"))
      rigidbody.AddTorque(Camera.main.transform.right * -speed * Time.deltaTime);
   if (Input.GetKey ("d"))
      rigidbody.AddTorque(Camera.main.transform.forward * -speed * Time.deltaTime);

I’m sure there’s a fairly easy fix for this, but I’ve been bashing my head on it for a couple hours so I figured I would ask. Thank you in advance!

You want to do this:

  var directionToMove = Quaternion.AngleAxis(Camera.main.transform.eulerAngles.y, Vector3.up) * Vector3.forward;