diff --git a/libagent/device/trezor.py b/libagent/device/trezor.py
index 0dc3d5b..f1a2861 100644
--- a/libagent/device/trezor.py
+++ b/libagent/device/trezor.py
@@ -16,9 +16,7 @@ log = logging.getLogger(__name__)
 
 def _message_box(label, sp=subprocess):
     """Launch an external process for PIN/passphrase entry GUI."""
-    cmd = ('import sys, pymsgbox; '
-           'sys.stdout.write(pymsgbox.password(sys.stdin.read()))')
-    args = [sys.executable, '-c', cmd]
+    args = [sys.executable, '-m', 'libagent.device.ui.simple']
     p = sp.Popen(args=args, stdin=sp.PIPE, stdout=sp.PIPE, stderr=sp.PIPE)
     out, err = p.communicate(label.encode('ascii'))
     exitcode = p.wait()
diff --git a/libagent/device/ui/__init__.py b/libagent/device/ui/__init__.py
new file mode 100644
index 0000000..4313f96
--- /dev/null
+++ b/libagent/device/ui/__init__.py
@@ -0,0 +1 @@
+"""UIs for PIN/passphrase entry."""
diff --git a/libagent/device/ui/simple.py b/libagent/device/ui/simple.py
new file mode 100644
index 0000000..7c59b79
--- /dev/null
+++ b/libagent/device/ui/simple.py
@@ -0,0 +1,6 @@
+"""Simple, cross-platform UI for entering a PIN/passhprase."""
+import sys
+
+import pymsgbox
+
+sys.stdout.write(pymsgbox.password(sys.stdin.read()))