canbus: solve IDS-CAN command-auth cipher; add reference implementations

The CAN write gate (page-42/43 challenge/response) is a 32-round TEA/XTEA-family Feistel keyed by a per-session 32-bit key; REMOTE_CONTROL = 0xB16B00B5. Verified 51/51 against captured challenge/response pairs across nodes 2A/61/75/F8 (one global key, not per-node), so the CAN path can now actuate, not just sense. - ids_can_auth.py Python reference + self-test (51/51) - esphome/ids_can_auth.h C++ port for the ESP32 node (host-tested 8/8) - sniff/analyze_auth.py structural analysis (rules out affine; confirms keyed cipher) - sniff/auth-pairs-multinode-2026-06-11.txt +9 pairs across 4 nodes - README document the cipher, session keys, unlock sequence Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
2026-06-12 00:22:09 -04:00
parent 85455e8631
commit 840cfaf5fc
5 changed files with 387 additions and 19 deletions
@@ -0,0 +1,104 @@
+#!/usr/bin/env python3
+"""IDS-CAN command-auth cipher — the gate on the CAN write path.
+
+SOLVED 2026-06-12. The challenge->response transform is a TEA/XTEA-family
+32-round Feistel keyed by a per-session 32-bit key ("Cypher").
+
+`response = encrypt(challenge, REMOTE_CONTROL)`, both 32-bit
+**big-endian** (the 4 payload bytes after the "00 04" prefix in the page-42
+challenge / page-43 response frames). Verified 51/51 against the captured pairs
+in sniff/2A-auth-pairs.txt + sniff/auth-pairs-multinode-2026-06-11.txt, across
+nodes 2A/61/75/F8 — one global session key, not per-node.
+
+The session "Cypher" is the only key; the round constants are baked in. Five
+sessions exist (the joke hex values confirm they're the genuine keys):
+    MANUFACTURING  0xB16BA115   DIAGNOSTIC 0xBABECAFE   REPROGRAMMING 0xDEADBEEF
+    REMOTE_CONTROL 0xB16B00B5   DAQ        0x0B00B135
+REMOTE_CONTROL is the one that gates on/off/move commands.
+
+Unlock sequence on the bus (controller 01 <-> module, 29-bit extended frames):
+    01->node   page42 DLC2 "00 04"                  # arm
+    node->01   page42 DLC6 "00 04 <CC CC CC CC>"     # module's challenge
+    01->node   page43 DLC6 "00 04 <RR RR RR RR>"     # RR = remote_control_response(CC)
+    node->01   page43 DLC2 "00 04"                   # ack
+    01->node   0x0006<node><op> x3                   # opcode now honored (01=on,00=off,02=retract)
+"""
+from __future__ import annotations
+
+MASK = 0xFFFFFFFF
+DELTA = 0x9E3779B9  # 2654435769 — the TEA golden-ratio delta
+
+# Per-session key constants ("Cypher")
+SESSION_CYPHER = {
+    "MANUFACTURING": 0xB16BA115,
+    "DIAGNOSTIC":    0xBABECAFE,
+    "REPROGRAMMING": 0xDEADBEEF,
+    "REMOTE_CONTROL": 0xB16B00B5,
+    "DAQ":           0x0B00B135,
+}
+
+
+def encrypt(seed: int, cypher: int) -> int:
+    """32-round TEA-family Feistel. seed=challenge, cypher=session key."""
+    num = cypher & MASK
+    seed &= MASK
+    sum_ = DELTA
+    rounds = 32
+    while True:
+        seed = (seed + ((((num << 4) & MASK) + 1131376761)
+                        ^ ((num + sum_) & MASK)
+                        ^ (((num >> 5) + 1919510376) & MASK))) & MASK
+        rounds -= 1
+        if rounds <= 0:
+            break
+        num = (num + ((((seed << 4) & MASK) + 1948272964)
+                      ^ ((seed + sum_) & MASK)
+                      ^ (((seed >> 5) + 1400073827) & MASK))) & MASK
+        sum_ = (sum_ + DELTA) & MASK
+    return seed
+
+
+def remote_control_response(challenge: int) -> int:
+    """Response uint for a REMOTE_CONTROL (on/off/move) command challenge."""
+    return encrypt(challenge, SESSION_CYPHER["REMOTE_CONTROL"])
+
+
+def response_bytes(challenge: bytes, session: str = "REMOTE_CONTROL") -> bytes:
+    """4 challenge bytes (big-endian, as on the wire) -> 4 response bytes."""
+    if len(challenge) != 4:
+        raise ValueError("challenge must be 4 bytes")
+    r = encrypt(int.from_bytes(challenge, "big"), SESSION_CYPHER[session])
+    return r.to_bytes(4, "big")
+
+
+def _selftest() -> int:
+    import os
+    here = os.path.dirname(os.path.abspath(__file__))
+    files = [os.path.join(here, "sniff", "2A-auth-pairs.txt"),
+             os.path.join(here, "sniff", "auth-pairs-multinode-2026-06-11.txt")]
+    total = bad = 0
+    for path in files:
+        if not os.path.exists(path):
+            continue
+        for ln in open(path):
+            ln = ln.strip()
+            if not ln or ln.startswith("#"):
+                continue
+            tok = ln.split()
+            c, r = int(tok[-2], 16), int(tok[-1], 16)
+            total += 1
+            if remote_control_response(c) != r:
+                bad += 1
+                print(f"  MISS {c:08X} -> got {remote_control_response(c):08X}, want {r:08X}")
+    print(f"self-test: {total - bad}/{total} pairs verified"
+          f" — {'PASS' if bad == 0 and total else 'FAIL'}")
+    return 0 if bad == 0 and total else 1
+
+
+if __name__ == "__main__":
+    import sys
+    if len(sys.argv) == 2:  # one-shot: compute response for a hex challenge
+        ch = int(sys.argv[1], 16)
+        print(f"{remote_control_response(ch):08X}")
+    else:
+        sys.exit(_selftest())