diff --git a/libagent/device/trezor.py b/libagent/device/trezor.py
index 0fbfc2b..19efccb 100644
--- a/libagent/device/trezor.py
+++ b/libagent/device/trezor.py
@@ -7,6 +7,7 @@ import mnemonic
 import semver
 
 from . import interface
+from .. import util
 
 log = logging.getLogger(__name__)
 
@@ -46,7 +47,8 @@ class Trezor(interface.Device):
 
         conn.callback_PinMatrixRequest = new_handler
 
-    cached_passphrase_ack = None
+    # Remembers the passphrase for an hour.
+    cached_passphrase_ack = util.ExpiringCache(seconds=60*60)
     cached_state = None
 
     def _override_passphrase_handler(self, conn):
@@ -57,9 +59,10 @@ class Trezor(interface.Device):
             try:
                 if msg.on_device is True:
                     return self._defs.PassphraseAck()
-                if self.__class__.cached_passphrase_ack:
+                ack = self.__class__.cached_passphrase_ack.get()
+                if ack:
                     log.debug('re-using cached %s passphrase', self)
-                    return self.__class__.cached_passphrase_ack
+                    return ack
 
                 passphrase = self.ui.get_passphrase()
                 passphrase = mnemonic.Mnemonic.normalize_string(passphrase)
@@ -70,7 +73,7 @@ class Trezor(interface.Device):
                     msg = 'Too long passphrase ({} chars)'.format(length)
                     raise ValueError(msg)
 
-                self.__class__.cached_passphrase_ack = ack
+                self.__class__.cached_passphrase_ack.set(ack)
                 return ack
             except:  # noqa
                 conn.init_device()
diff --git a/libagent/tests/test_util.py b/libagent/tests/test_util.py
index 2cef300..e85bc2f 100644
--- a/libagent/tests/test_util.py
+++ b/libagent/tests/test_util.py
@@ -121,3 +121,26 @@ def test_assuan_serialize():
     assert util.assuan_serialize(b'') == b''
     assert util.assuan_serialize(b'123\n456') == b'123%0A456'
     assert util.assuan_serialize(b'\r\n') == b'%0D%0A'
+
+
+def test_cache():
+    timer = mock.Mock(side_effect=range(7))
+    c = util.ExpiringCache(seconds=2, timer=timer)  # t=0
+    assert c.get() is None                          # t=1
+    obj = 'foo'
+    c.set(obj)                                      # t=2
+    assert c.get() is obj                           # t=3
+    assert c.get() is obj                           # t=4
+    assert c.get() is None                          # t=5
+    assert c.get() is None                          # t=6
+
+
+def test_cache_inf():
+    timer = mock.Mock(side_effect=range(6))
+    c = util.ExpiringCache(seconds=float('inf'), timer=timer)
+    obj = 'foo'
+    c.set(obj)
+    assert c.get() is obj
+    assert c.get() is obj
+    assert c.get() is obj
+    assert c.get() is obj
diff --git a/libagent/util.py b/libagent/util.py
index c98891f..7df843b 100644
--- a/libagent/util.py
+++ b/libagent/util.py
@@ -5,6 +5,7 @@ import functools
 import io
 import logging
 import struct
+import time
 
 log = logging.getLogger(__name__)
 
@@ -255,3 +256,25 @@ def assuan_serialize(data):
         escaped = '%{:02X}'.format(ord(c)).encode('ascii')
         data = data.replace(c, escaped)
     return data
+
+
+class ExpiringCache(object):
+    """Simple cache with a deadline."""
+
+    def __init__(self, seconds, timer=time.time):
+        """C-tor."""
+        self.duration = seconds
+        self.timer = timer
+        self.value = None
+        self.set(None)
+
+    def get(self):
+        """Returns existing value, or None if deadline has expired."""
+        if self.timer() > self.deadline:
+            self.value = None
+        return self.value
+
+    def set(self, value):
+        """Set new value and reset the deadline for expiration."""
+        self.deadline = self.timer() + self.duration
+        self.value = value