Add section about HTEX encryption to docs

docs/userguide/execution.rst

@@ -342,3 +342,50 @@ The following code snippet shows how apps can specify suitable executors in the
      def visualize(inputs=(), outputs=()):
          bash_array = " ".join(inputs)
          return "viz {} -o {}".format(bash_array, outputs[0])
+
+
+Encryption
+----------
+
+Users can enable encryption for the ``HighThroughputExecutor`` by setting its ``encrypted``
+initialization argument to ``True``.
+
+For example,
+
+.. code-block:: python
+
+    from parsl.config import Config
+    from parsl.executors import HighThroughputExecutor
+
+    config = Config(
+        executors=[
+            HighThroughputExecutor(
+                encrypted=True
+            )
+        ]
+    )
+
+Under the hood, we use `CurveZMQ <http://curvezmq.org/>`_ to encrypt all communication channels
+between the executor and related nodes.
+
+Encryption performance
+^^^^^^^^^^^^^^^^^^^^^^
+
+CurveZMQ depends on `libzmq <https://github.com/zeromq/libzmq>`_ and  `libsodium <https://github.com/jedisct1/libsodium>`_,
+which `pyzmq <https://github.com/zeromq/pyzmq>`_ (a Parsl dependency) includes as part of its
+installation via ``pip``. This installation path should work on most systems, but users have
+reported significant performance degradation as a result.
+
+If you experience a significant performance hit after enabling encryption, we recommend installing
+``pyzmq`` with conda:
+
+.. code-block:: bash
+
+    conda install conda-forge::pyzmq
+
+Alternatively, you can `install libsodium <https://doc.libsodium.org/installation>`_, then
+`install libzmq <https://zeromq.org/download/>`_, then build ``pyzmq`` from source:
+
+.. code-block:: bash
+
+    pip3 install parsl --no-binary pyzmq