feat(crypto): add android-compatible pure-python AES-CTR backend

proxy/crypto_backend.py

@@ -4,6 +4,33 @@ import os
 from typing import Protocol
 
 
+_SBOX = (
+    0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B,
+    0xFE, 0xD7, 0xAB, 0x76, 0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0,
+    0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0, 0xB7, 0xFD, 0x93, 0x26,
+    0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15,
+    0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2,
+    0xEB, 0x27, 0xB2, 0x75, 0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0,
+    0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84, 0x53, 0xD1, 0x00, 0xED,
+    0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF,
+    0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F,
+    0x50, 0x3C, 0x9F, 0xA8, 0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5,
+    0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2, 0xCD, 0x0C, 0x13, 0xEC,
+    0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73,
+    0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14,
+    0xDE, 0x5E, 0x0B, 0xDB, 0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C,
+    0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79, 0xE7, 0xC8, 0x37, 0x6D,
+    0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08,
+    0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F,
+    0x4B, 0xBD, 0x8B, 0x8A, 0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E,
+    0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E, 0xE1, 0xF8, 0x98, 0x11,
+    0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF,
+    0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F,
+    0xB0, 0x54, 0xBB, 0x16,
+)
+_RCON = (0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1B, 0x36)
+
+
 class AesCtrTransform(Protocol):
     def update(self, data: bytes) -> bytes:
         ...
@@ -12,6 +39,147 @@ class AesCtrTransform(Protocol):
         ...
 
 
+def _xtime(value: int) -> int:
+    value <<= 1
+    if value & 0x100:
+        value ^= 0x11B
+    return value & 0xFF
+
+
+def _mul2(value: int) -> int:
+    return _xtime(value)
+
+
+def _mul3(value: int) -> int:
+    return _xtime(value) ^ value
+
+
+def _add_round_key(state: list[int], round_key: bytes):
+    for idx in range(16):
+        state[idx] ^= round_key[idx]
+
+
+def _sub_bytes(state: list[int]):
+    for idx in range(16):
+        state[idx] = _SBOX[state[idx]]
+
+
+def _shift_rows(state: list[int]):
+    state[1], state[5], state[9], state[13] = (
+        state[5], state[9], state[13], state[1]
+    )
+    state[2], state[6], state[10], state[14] = (
+        state[10], state[14], state[2], state[6]
+    )
+    state[3], state[7], state[11], state[15] = (
+        state[15], state[3], state[7], state[11]
+    )
+
+
+def _mix_columns(state: list[int]):
+    for offset in range(0, 16, 4):
+        s0, s1, s2, s3 = state[offset:offset + 4]
+        state[offset + 0] = _mul2(s0) ^ _mul3(s1) ^ s2 ^ s3
+        state[offset + 1] = s0 ^ _mul2(s1) ^ _mul3(s2) ^ s3
+        state[offset + 2] = s0 ^ s1 ^ _mul2(s2) ^ _mul3(s3)
+        state[offset + 3] = _mul3(s0) ^ s1 ^ s2 ^ _mul2(s3)
+
+
+def _rot_word(word: list[int]) -> list[int]:
+    return word[1:] + word[:1]
+
+
+def _sub_word(word: list[int]) -> list[int]:
+    return [_SBOX[value] for value in word]
+
+
+def _expand_round_keys(key: bytes) -> tuple[list[bytes], int]:
+    if len(key) not in (16, 24, 32):
+        raise ValueError("AES key must be 16, 24, or 32 bytes long")
+
+    nk = len(key) // 4
+    nr = {4: 10, 6: 12, 8: 14}[nk]
+    words = [list(key[idx:idx + 4]) for idx in range(0, len(key), 4)]
+    total_words = 4 * (nr + 1)
+
+    for idx in range(nk, total_words):
+        temp = words[idx - 1][:]
+        if idx % nk == 0:
+            temp = _sub_word(_rot_word(temp))
+            temp[0] ^= _RCON[idx // nk - 1]
+        elif nk > 6 and idx % nk == 4:
+            temp = _sub_word(temp)
+        words.append([
+            words[idx - nk][byte_idx] ^ temp[byte_idx]
+            for byte_idx in range(4)
+        ])
+
+    round_keys = []
+    for round_idx in range(nr + 1):
+        start = round_idx * 4
+        round_keys.append(bytes(sum(words[start:start + 4], [])))
+    return round_keys, nr
+
+
+class _PurePythonAesCtrTransform:
+    def __init__(self, key: bytes, iv: bytes):
+        if len(iv) != 16:
+            raise ValueError("AES-CTR IV must be 16 bytes long")
+        self._round_keys, self._rounds = _expand_round_keys(key)
+        self._counter = bytearray(iv)
+        self._buffer = b""
+        self._buffer_offset = 0
+
+    def update(self, data: bytes) -> bytes:
+        if not data:
+            return b""
+
+        out = bytearray(len(data))
+        data_offset = 0
+
+        while data_offset < len(data):
+            if self._buffer_offset >= len(self._buffer):
+                self._buffer = self._encrypt_block(bytes(self._counter))
+                self._buffer_offset = 0
+                self._increment_counter()
+
+            available = len(self._buffer) - self._buffer_offset
+            chunk_size = min(len(data) - data_offset, available)
+            for chunk_idx in range(chunk_size):
+                out[data_offset + chunk_idx] = (
+                    data[data_offset + chunk_idx]
+                    ^ self._buffer[self._buffer_offset + chunk_idx]
+                )
+            data_offset += chunk_size
+            self._buffer_offset += chunk_size
+
+        return bytes(out)
+
+    def finalize(self) -> bytes:
+        return b""
+
+    def _encrypt_block(self, block: bytes) -> bytes:
+        state = list(block)
+        _add_round_key(state, self._round_keys[0])
+
+        for round_idx in range(1, self._rounds):
+            _sub_bytes(state)
+            _shift_rows(state)
+            _mix_columns(state)
+            _add_round_key(state, self._round_keys[round_idx])
+
+        _sub_bytes(state)
+        _shift_rows(state)
+        _add_round_key(state, self._round_keys[self._rounds])
+        return bytes(state)
+
+    def _increment_counter(self):
+        for idx in range(15, -1, -1):
+            self._counter[idx] = (self._counter[idx] + 1) & 0xFF
+            if self._counter[idx] != 0:
+                break
+
+
 def _create_cryptography_transform(key: bytes,
                                    iv: bytes) -> AesCtrTransform:
     from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
@@ -25,8 +193,8 @@ def create_aes_ctr_transform(key: bytes, iv: bytes,
     """
     Create a stateful AES-CTR transform.
 
-    The backend name is configurable so Android can supply an alternative
+    Windows keeps using `cryptography` by default. Android can select the
-    implementation later without touching proxy logic.
+    pure-Python backend to avoid native build dependencies.
     """
     selected = backend or os.environ.get(
         'TG_WS_PROXY_CRYPTO_BACKEND', 'cryptography')
@@ -34,4 +202,7 @@ def create_aes_ctr_transform(key: bytes, iv: bytes,
     if selected == 'cryptography':
         return _create_cryptography_transform(key, iv)
 
+    if selected == 'python':
+        return _PurePythonAesCtrTransform(key, iv)
+
     raise ValueError(f"Unsupported AES-CTR backend: {selected}")

tests/test_crypto_mtproto.py

@@ -36,6 +36,28 @@ def _encrypt_after_init(init_packet: bytes, plaintext: bytes) -> bytes:
 
 
 class CryptoBackendTests(unittest.TestCase):
+    def test_python_backend_matches_cryptography_stream(self):
+        cryptography_transform = create_aes_ctr_transform(
+            KEY, IV, backend="cryptography")
+        python_transform = create_aes_ctr_transform(KEY, IV, backend="python")
+
+        chunks = [
+            b"",
+            b"\x00" * 16,
+            bytes(range(31)),
+            b"telegram-proxy",
+            b"\xff" * 64,
+        ]
+
+        cryptography_out = b"".join(
+            cryptography_transform.update(chunk) for chunk in chunks
+        ) + cryptography_transform.finalize()
+        python_out = b"".join(
+            python_transform.update(chunk) for chunk in chunks
+        ) + python_transform.finalize()
+
+        self.assertEqual(python_out, cryptography_out)
+
     def test_unknown_backend_raises_error(self):
         with self.assertRaises(ValueError):
             create_aes_ctr_transform(KEY, IV, backend="missing")