Fix non-transient pairing

Hitherto, a subtle bug existed in the HAP module. During various rounds
of verification, signing was done by a temporally generated key, instead
of the 'accessory' long term (LT) key. Accessory is the ap2-receiver.

We now generate an Ed25519 key at startup and put it into the "pk" TXT
record which makes the LTK available to other devices via the
out-of-band channel, for long term access.

More info: https://developer.apple.com/homekit/specification/

This means that non-transient pairing now works :)

To test, add -ftxor 48 (disable Ft48TransientPairing), e.g.:

python3 ap2-receiver.py -m myap2 -n en0 -ftxor 48

README.md

@@ -3,6 +3,7 @@
 Very quick python implementation of AP2 protocol using **minimal
 multi-room** features. For now it implements:
 - HomeKit transient pairing (SRP/Curve25519/ChaCha20-Poly1305)
+- HomeKit non-transient pairing
 - FairPlay (v3) authentication
 - Receiving of both REALTIME and BUFFERED Airplay2 audio streams
 - Airplay2 Service publication

ap2-receiver.py

@@ -153,6 +153,33 @@ class Feat(IntFlag):
     | Feat.Ft14MFiSoftware | Feat.Ft09AirPlayAudio
 )
 
+
+class LTPK():
+    # Note that this must be Ed25519 (and not Curve25519 i.e. X25519) for
+    # Non Transient Pairing to work. (i.e. disable Ft48TransientPairing)
+    def __init__(self):
+        from cryptography.hazmat.primitives.asymmetric import ed25519
+        from cryptography.hazmat.primitives import serialization
+        self.accessory_curve = ed25519.Ed25519PrivateKey.generate()
+        self.accessory_curve_public = self.accessory_curve.public_key(
+        ).public_bytes(
+            encoding=serialization.Encoding.Raw,
+            format=serialization.PublicFormat.Raw
+        )
+        self.public_int = int.from_bytes(self.accessory_curve_public, byteorder='big')
+        self.public_string = str.lower("{0:0>4X}".format(self.public_int))
+
+    def get_pub_string(self):
+        print('Got pub bytes:', self.public_string)
+        return self.public_string
+
+    def get_pub_bytes(self):
+        return self.accessory_curve_public
+
+    def get_private_bytes(self):
+        return self.accessory_curve
+
+
 DEVICE_ID = None
 IPV4 = None
 IPV6 = None
@@ -163,6 +190,7 @@ class Feat(IntFlag):
 HTTP_CT_PARAM = "text/parameters"
 HTTP_CT_IMAGE = "image/jpeg"
 HTTP_CT_DMAP = "application/x-dmap-tagged"
+LTPK = LTPK()
 
 
 def setup_global_structs(args):
@@ -263,7 +291,7 @@ def setup_global_structs(args):
         "gid": "5dccfd20-b166-49cc-a593-6abd5f724ddb",  # UUID generated casually
         "gcgl": "0",
         # "vn": "65537",
-        "pk": "de352b0df39042e201d31564049023af58a106c6d904b74a68aa65012852997f"
+        "pk": LTPK.get_pub_string()
     }
 
 
@@ -703,7 +731,7 @@ def handle_pair_setup(self):
         hexdump(body)
 
         if not self.server.hap:
-            self.server.hap = Hap()
+            self.server.hap = Hap(LTPK.get_private_bytes(), LTPK.get_pub_bytes())
         res = self.server.hap.pair_setup(body)
 
         self.send_response(200)
@@ -724,7 +752,7 @@ def handle_pair_verify(self):
         body = self.rfile.read(content_len)
 
         if not self.server.hap:
-            self.server.hap = Hap()
+            self.server.hap = Hap(LTPK.get_private_bytes(), LTPK.get_pub_bytes())
         res = self.server.hap.pair_verify(body)
 
         self.send_response(200)

ap2/pairing/hap.py

@@ -7,6 +7,8 @@
 
 import hkdf
 from cryptography.hazmat.primitives.asymmetric import x25519
+# We build the Ed25519 in ap2-receiver, and pass it to HAP()
+# from cryptography.hazmat.primitives.asymmetric import ed25519
 from cryptography.hazmat.primitives import serialization
 import nacl.signing
 from Crypto.Cipher import ChaCha20_Poly1305
@@ -104,14 +106,14 @@ def encode(req):
 
 
 class Hap:
-    def __init__(self):
+    def __init__(self, ltsk, ltpk):
         self.transient = False
         self.encrypted = False
         self.pair_setup_steps_n = 5
 
         self.accessory_id = b"00000000-0000-0000-0000-deadbeef0bad"
-        # self.accessory_ltsk = 0
-        # self.accessory_ltpk = 0
+        self.accessory_ltsk = ltsk
+        self.accessory_ltpk = ltpk
 
     def request(self, req):
         req = Tlv8.decode(req)
@@ -216,14 +218,12 @@ def pair_setup_m5_m6_3(self, session_key):
         prk = hkdf.hkdf_extract(b"Pair-Setup-Accessory-Sign-Salt", self.ctx.session_key)
         accessory_x = hkdf.hkdf_expand(prk, b"Pair-Setup-Accessory-Sign-Info", 32)
 
-        self.accessory_ltsk = nacl.signing.SigningKey.generate()
-        self.accessory_ltpk = bytes(self.accessory_ltsk.verify_key)
-
-        self.accessory_id = b"00000000-0000-0000-0000-f0989d7cbbab"
+        # self.accessory_ltsk = nacl.signing.SigningKey.generate()
+        # self.accessory_ltpk = bytes(self.accessory_ltsk.verify_key)
 
         accessory_info = accessory_x + self.accessory_id + self.accessory_ltpk
         accessory_signed = self.accessory_ltsk.sign(accessory_info)
-        accessory_sig = accessory_signed.signature
+        accessory_sig = accessory_signed  # .signature
 
         dec_tlv = Tlv8.encode([
             Tlv8.Tag.IDENTIFIER, self.accessory_id,
@@ -246,12 +246,12 @@ def pair_verify_m1_m2(self, client_public):
         )
         self.accessory_shared_key = self.accessory_curve.exchange(x25519.X25519PublicKey.from_public_bytes(client_public))
 
-        self.accessory_ltsk = nacl.signing.SigningKey.generate()
-        self.accessory_ltpk = bytes(self.accessory_ltsk.verify_key)
+        # self.accessory_ltsk = nacl.signing.SigningKey.generate()
+        # self.accessory_ltpk = bytes(self.accessory_ltsk.verify_key)
 
         accessory_info = self.accessory_curve_public + self.accessory_id + client_public
         accessory_signed = self.accessory_ltsk.sign(accessory_info)
-        accessory_sig = accessory_signed.signature
+        accessory_sig = accessory_signed  # .signature
 
         sub_tlv = Tlv8.encode([
             Tlv8.Tag.IDENTIFIER, self.accessory_id,