refactor(crypto): extract AES-CTR backend adapter and keep cryptography as desktop backend

proxy/crypto_backend.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+import os
+from typing import Protocol
+
+
+class AesCtrTransform(Protocol):
+    def update(self, data: bytes) -> bytes:
+        ...
+
+    def finalize(self) -> bytes:
+        ...
+
+
+def _create_cryptography_transform(key: bytes,
+                                   iv: bytes) -> AesCtrTransform:
+    from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+
+    cipher = Cipher(algorithms.AES(key), modes.CTR(iv))
+    return cipher.encryptor()
+
+
+def create_aes_ctr_transform(key: bytes, iv: bytes,
+                             backend: str | None = None) -> AesCtrTransform:
+    """
+    Create a stateful AES-CTR transform.
+
+    The backend name is configurable so Android can supply an alternative
+    implementation later without touching proxy logic.
+    """
+    selected = backend or os.environ.get(
+        'TG_WS_PROXY_CRYPTO_BACKEND', 'cryptography')
+
+    if selected == 'cryptography':
+        return _create_cryptography_transform(key, iv)
+
+    raise ValueError(f"Unsupported AES-CTR backend: {selected}")

proxy/tg_ws_proxy.py

@@ -11,7 +11,8 @@ import struct
 import sys
 import time
 from typing import Dict, List, Optional, Set, Tuple
-from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+
+from proxy.crypto_backend import create_aes_ctr_transform
 
 
 DEFAULT_PORT = 1080
@@ -365,8 +366,7 @@ def _dc_from_init(data: bytes) -> Tuple[Optional[int], bool]:
     try:
         key = bytes(data[8:40])
         iv = bytes(data[40:56])
-        cipher = Cipher(algorithms.AES(key), modes.CTR(iv))
-        encryptor = cipher.encryptor()
+        encryptor = create_aes_ctr_transform(key, iv)
         keystream = encryptor.update(b'\x00' * 64) + encryptor.finalize()
         plain = bytes(a ^ b for a, b in zip(data[56:64], keystream[56:64]))
         proto = struct.unpack('<I', plain[0:4])[0]
@@ -396,8 +396,7 @@ def _patch_init_dc(data: bytes, dc: int) -> bytes:
     try:
         key_raw = bytes(data[8:40])
         iv = bytes(data[40:56])
-        cipher = Cipher(algorithms.AES(key_raw), modes.CTR(iv))
-        enc = cipher.encryptor()
+        enc = create_aes_ctr_transform(key_raw, iv)
         ks = enc.update(b'\x00' * 64) + enc.finalize()
         patched = bytearray(data[:64])
         patched[60] = ks[60] ^ new_dc[0]
@@ -424,8 +423,7 @@ class _MsgSplitter:
     def __init__(self, init_data: bytes):
         key_raw = bytes(init_data[8:40])
         iv = bytes(init_data[40:56])
-        cipher = Cipher(algorithms.AES(key_raw), modes.CTR(iv))
-        self._dec = cipher.encryptor()
+        self._dec = create_aes_ctr_transform(key_raw, iv)
         self._dec.update(b'\x00' * 64)  # skip init packet
 
     def split(self, chunk: bytes) -> List[bytes]: