diff --git a/bin/receive_file.py b/bin/receive_file.py
index 466f66d..b5256fe 100644
--- a/bin/receive_file.py
+++ b/bin/receive_file.py
@@ -1,6 +1,5 @@
 from __future__ import print_function
 import sys, os, json
-from binascii import unhexlify
 from nacl.secret import SecretBox
 from wormhole.blocking.transcribe import Receiver
 from wormhole.blocking.transit import TransitReceiver
@@ -23,7 +22,7 @@ data = json.loads(r.get_data().decode("utf-8"))
 #print("their data: %r" % (data,))
 
 file_data = data["file"]
-xfer_key = unhexlify(file_data["key"].encode("ascii"))
+xfer_key = r.derive_key(APPID+"/xfer-key", SecretBox.KEY_SIZE)
 filename = os.path.basename(file_data["filename"]) # unicode
 filesize = file_data["filesize"]
 encrypted_filesize = filesize + SecretBox.NONCE_SIZE+16
diff --git a/bin/send_file.py b/bin/send_file.py
index a98132d..2e15f52 100644
--- a/bin/send_file.py
+++ b/bin/send_file.py
@@ -10,7 +10,6 @@ APPID = "lothar.com/wormhole/file-xfer"
 # we're sending
 filename = sys.argv[1]
 assert os.path.isfile(filename)
-xfer_key = os.urandom(SecretBox.KEY_SIZE)
 transit_sender = TransitSender()
 transit_key = transit_sender.get_transit_key()
 direct_hints = transit_sender.get_direct_hints()
@@ -19,7 +18,6 @@ relay_hints = transit_sender.get_relay_hints()
 filesize = os.stat(filename).st_size
 data = json.dumps({
     "file": {
-        "key": hexlify(xfer_key),
         "filename": os.path.basename(filename),
         "filesize": filesize,
         },
@@ -38,6 +36,7 @@ print("")
 them_bytes = i.get_data()
 them_d = json.loads(them_bytes.decode("utf-8"))
 #print("them: %r" % (them_d,))
+xfer_key = i.derive_key(APPID+"/xfer-key", SecretBox.KEY_SIZE)
 
 box = SecretBox(xfer_key)
 with open(filename, "rb") as f:
diff --git a/src/wormhole/blocking/transcribe.py b/src/wormhole/blocking/transcribe.py
index aa019a3..20efdba 100644
--- a/src/wormhole/blocking/transcribe.py
+++ b/src/wormhole/blocking/transcribe.py
@@ -82,6 +82,10 @@ class Common:
         r = requests.post(self.url("deallocate"))
         r.raise_for_status()
 
+    def derive_key(self, purpose, length=SecretBox.KEY_SIZE):
+        assert type(purpose) == type(b"")
+        return HKDF(self.key, length, CTXinfo=purpose)
+
 class Initiator(Common):
     def __init__(self, appid, data, relay=RELAY):
         self.appid = appid
@@ -104,13 +108,14 @@ class Initiator(Common):
 
     def get_data(self):
         key = self._poll_pake([])
+        self.key = key
         try:
-            outbound_key = HKDF(key, SecretBox.KEY_SIZE, CTXinfo=b"sender")
+            outbound_key = self.derive_key(b"sender")
             outbound_encrypted = self._encrypt_data(outbound_key, self.data)
             other_msgs = self._post_data(outbound_encrypted)
 
             inbound_encrypted = self._poll_data(other_msgs)
-            inbound_key = HKDF(key, SecretBox.KEY_SIZE, CTXinfo=b"receiver")
+            inbound_key = self.derive_key(b"receiver")
             inbound_data = self._decrypt_data(inbound_key, inbound_encrypted)
         finally:
             self._deallocate()
@@ -155,14 +160,15 @@ class Receiver(Common):
         assert self.channel_id is not None
         other_msgs = self._post_pake()
         key = self._poll_pake(other_msgs)
+        self.key = key
 
         try:
-            outbound_key = HKDF(key, SecretBox.KEY_SIZE, CTXinfo=b"receiver")
+            outbound_key = self.derive_key(b"receiver")
             outbound_encrypted = self._encrypt_data(outbound_key, self.data)
             other_msgs = self._post_data(outbound_encrypted)
 
             inbound_encrypted = self._poll_data(other_msgs)
-            inbound_key = HKDF(key, SecretBox.KEY_SIZE, CTXinfo=b"sender")
+            inbound_key = self.derive_key(b"sender")
             inbound_data = self._decrypt_data(inbound_key, inbound_encrypted)
         finally:
             self._deallocate()