Radix 16 (live-verified), universal reply splitter, golden capture vectors, host_probe6

franklinwh_agate_client/protocol/crypto.py

@@ -5,42 +5,42 @@ Reconstructed from the blutter disassembly; see
 
 Request wire form of an envelope::
 
-    s   = compact JSON, e.g. {"cmdType":25,"equipNo":"SN",...}
+    s   = compact JSON, e.g. {"cmdType":1101,"equipNo":"SN",...}
     S1  = parts[0] + "," + parts[1] + ","   # quartet header, trailing comma
     S2  = ",".join(parts[2:])               # the rest of the JSON
     a   = UTF-8(S1)                         # sent PLAINTEXT first
-    n   = int.tryParse(key, radix: 32) or 0
+    n   = int.tryParse(key) or 0            # default radix 16
     wire = a || bytes((S2[i] + i + len(a) + n) & 0xFF)
 
 i.e. the two leading JSON parts go on the wire **unchanged first**, and the
 tail is transformed byte-wise with a position-dependent additive stream
 (index ``i`` is relative to the tail; ``i + len(a)`` is the absolute JSON
-index). The receiver that knows the serial can compute ``len(a)`` up front
-(``header_len_for``) and split the wire deterministically.
+index). Both directions use this same framing (verified live: a 1154-byte
+device reply decrypts with the identical split — see ``decrypt_wire``).
 
-Responses use a different framing (``decrypt_data``): the wire holds the
-plaintext JSON prefix ``s`` (which itself contains the serial's ``X``),
-a literal ``0x58`` separator, then the encoded tail::
+.. note:: An earlier revision claimed radix 32 from a ``0x20`` literal
+   next to the ``tryParse`` call sites. Two independent live frames (an
+   app discovery request and a device info reply) decrypt cleanly only
+   with radix 16 (``int("80", 16) == 128``, ``int("1c", 16) == 28``) and
+   then verify byte-exact (``len``/uppercase CRC-32). Ground truth wins:
+   the ``0x20`` belongs to a neighbouring call.
 
-    wire = UTF-8(s) || 0x58 || encode(tail)
-
-``decrypt_data`` locates the *second* ``0x58`` byte ``j``, takes
-``s = UTF-8(wire[:j])``, probes ``s + "}"`` as JSON to recover ``equipNo``
-(and hence the key), decodes the tail with the same stream in reverse, and
-returns ``s + "," + tail_plain``.  Anything unexpected yields ``""`` (the
-app's ``decryptData`` returns ``""`` on marker/parse failure and the socket
-layer keeps accumulating bytes).
-
-Confirmed from the disassembly:
+Confirmed from the disassembly *and live traffic*:
 
 * ``generateSecretKey`` = ``sum(utf8(sn)) & 0xFF`` rendered as lowercase hex
-  with **no** zero padding (Dart ``_toPow2String(16)``).
-* The key is parsed with ``int.tryParse(key, radix: 32)`` — the literal
-  ``0x20`` (32) is materialised right before the call in both the encrypt
-  and decrypt paths.
+  with **no** zero padding (Dart ``_toPow2String(16)``). E.g. ``"1c"`` for
+  the gateway SN, ``"80"`` for the empty/placeholder SN.
+* The key is parsed with ``int.tryParse(key)`` — default radix **16**,
+  verified against two live frames (``tests/fixtures/*.bin``).
 * ``encryptObject(object, key)`` takes the object in the first slot and the
   key in the second; its result is ``a ++ transform(b)`` (plaintext header
   first — ``addAll(transformed)`` onto the header copy).
+* Replies use the same ``S1 || transform(S2)`` framing (the device info
+  reply decrypts identically). The reply ``cmdType`` observed live is the
+  request id + 1 (``1101`` → ``1102``); replies are therefore parsed with
+  :func:`decrypt_wire`, which locates the header without knowing the ids
+  up front. ``decrypt_data`` (``0x58``-marker model) is kept for the
+  Bluetooth path, which is unverified live.
 """
 
 from __future__ import annotations
@@ -48,8 +48,10 @@ from __future__ import annotations
 import json
 from typing import Optional, Tuple
 
-#: Key radix used by both ``encryptObject`` and ``decryptData`` (literal 0x20).
-DEFAULT_KEY_RADIX = 32
+#: Key radix used by the envelope cipher (default ``tryParse`` radix).
+#: Verified live: ``int("80", 16) == 128`` and ``int("1c", 16) == 28``
+#: decode two independent captured frames byte-exact.
+DEFAULT_KEY_RADIX = 16
 
 #: Response separator byte (``0x58``, ASCII ``X``).
 MARKER = 0x58
@@ -143,6 +145,53 @@ def decrypt_request(
 decrypt_object = decrypt_request
 
 
+def split_wire(wire: bytes) -> Tuple[bytes, bytes]:
+    """Split any envelope wire into ``(a, b)`` at the 2nd comma.
+
+    The header ``S1 = parts[0] + "," + parts[1] + ","`` is always ASCII
+    with no commas inside, so the first two commas on the wire are its
+    structural boundaries — no ids or serials needed up front.  Works for
+    requests and replies alike.
+    """
+    raw = bytes(wire)
+    first = raw.find(b",")
+    if first < 0:
+        raise ValueError("no header comma in wire buffer")
+    second = raw.find(b",", first + 1)
+    if second < 0:
+        raise ValueError("truncated header in wire buffer")
+    return raw[: second + 1], raw[second + 1:]
+
+
+def decrypt_wire(wire: bytes, radix: int = DEFAULT_KEY_RADIX) -> str:
+    """Decode one complete envelope wire (request or reply) to JSON.
+
+    Splits via :func:`split_wire`, recovers ``equipNo`` from the
+    ``a + "}"`` probe (exactly how the receiver learns which key to use),
+    and reverses the additive stream.  Raises ``ValueError`` on any
+    failure (use :func:`decrypt_data`-style ``""`` handling when
+    accumulating).
+    """
+    a, b = split_wire(wire)
+    try:
+        # ``a`` ends with the header's trailing comma — drop it so the
+        # probe is valid JSON (mirrors the ``s + "}"`` equipNo probe).
+        probe = json.loads(a[:-1].decode("utf-8") + "}")
+    except (UnicodeDecodeError, ValueError) as exc:
+        raise ValueError(f"header probe is not JSON: {exc}") from exc
+    if not isinstance(probe, dict):
+        raise ValueError("header probe is not an object")
+    key = generate_secret_key(str(probe.get("equipNo", "")))
+    key_int = _parse_key(key, radix)
+    plain_b = bytes(
+        (byte - i - len(a) - key_int) & 0xFF for i, byte in enumerate(b)
+    )
+    try:
+        return (a + plain_b).decode("utf-8")
+    except UnicodeDecodeError as exc:
+        raise ValueError(f"tail is not UTF-8: {exc}") from exc
+
+
 def decrypt_data(wire: bytes, radix: int = DEFAULT_KEY_RADIX) -> str:
     """``decryptData`` — decode one accumulated *response* buffer.