Improve handling of deprecated ciphers

This commit eliminates deprecation warnings from cryptography 43.0.0,
and automates the handling of "decrepit" ciphers going forward to
avoid needing to update AsyncSSH each time a new cipher is retired.

asyncssh/crypto/cipher.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2014-2021 by Ron Frederick <ronf@timeheart.net> and others.
+# Copyright (c) 2014-2024 by Ron Frederick <ronf@timeheart.net> and others.
 #
 # This program and the accompanying materials are made available under
 # the terms of the Eclipse Public License v2.0 which accompanies this
@@ -20,22 +20,23 @@
 
 """A shim around PyCA for accessing symmetric ciphers needed by AsyncSSH"""
 
+from types import ModuleType
 from typing import Any, MutableMapping, Optional, Tuple
 import warnings
 
 from cryptography.exceptions import InvalidTag
 from cryptography.hazmat.primitives.ciphers import Cipher, CipherContext
 from cryptography.hazmat.primitives.ciphers.aead import AESGCM
-from cryptography.hazmat.primitives.ciphers.algorithms import AES, ARC4
-from cryptography.hazmat.primitives.ciphers.algorithms import TripleDES
 from cryptography.hazmat.primitives.ciphers.modes import CBC, CTR
 
-with warnings.catch_warnings():
-    warnings.simplefilter('ignore')
+import cryptography.hazmat.primitives.ciphers.algorithms as _algs
+
+_decrepit_algs: Optional[ModuleType]
 
-    from cryptography.hazmat.primitives.ciphers.algorithms import Blowfish
-    from cryptography.hazmat.primitives.ciphers.algorithms import CAST5
-    from cryptography.hazmat.primitives.ciphers.algorithms import SEED
+try:
+    import cryptography.hazmat.decrepit.ciphers.algorithms as _decrepit_algs
+except ImportError:
+    _decrepit_algs = None
 
 
 _CipherAlgs = Tuple[Any, Any, int]
@@ -140,27 +141,44 @@ def get_cipher_params(cipher_name: str) -> _CipherParams:
 
 
 _cipher_alg_list = (
-    ('aes128-cbc',   AES,       CBC,     0, 16, 16, 16),
-    ('aes192-cbc',   AES,       CBC,     0, 24, 16, 16),
-    ('aes256-cbc',   AES,       CBC,     0, 32, 16, 16),
-    ('aes128-ctr',   AES,       CTR,     0, 16, 16, 16),
-    ('aes192-ctr',   AES,       CTR,     0, 24, 16, 16),
-    ('aes256-ctr',   AES,       CTR,     0, 32, 16, 16),
-    ('aes128-gcm',   None,      None,    0, 16, 12, 16),
-    ('aes256-gcm',   None,      None,    0, 32, 12, 16),
-    ('arcfour',      ARC4,      None,    0, 16,  1,  1),
-    ('arcfour40',    ARC4,      None,    0,  5,  1,  1),
-    ('arcfour128',   ARC4,      None, 1536, 16,  1,  1),
-    ('arcfour256',   ARC4,      None, 1536, 32,  1,  1),
-    ('blowfish-cbc', Blowfish,  CBC,     0, 16,  8,  8),
-    ('cast128-cbc',  CAST5,     CBC,     0, 16,  8,  8),
-    ('des-cbc',      TripleDES, CBC,     0,  8,  8,  8),
-    ('des2-cbc',     TripleDES, CBC,     0, 16,  8,  8),
-    ('des3-cbc',     TripleDES, CBC,     0, 24,  8,  8),
-    ('seed-cbc',     SEED,      CBC,     0, 16, 16, 16)
+    ('aes128-cbc',   'AES',       CBC,     0, 16, 16, 16),
+    ('aes192-cbc',   'AES',       CBC,     0, 24, 16, 16),
+    ('aes256-cbc',   'AES',       CBC,     0, 32, 16, 16),
+    ('aes128-ctr',   'AES',       CTR,     0, 16, 16, 16),
+    ('aes192-ctr',   'AES',       CTR,     0, 24, 16, 16),
+    ('aes256-ctr',   'AES',       CTR,     0, 32, 16, 16),
+    ('aes128-gcm',   None,        None,    0, 16, 12, 16),
+    ('aes256-gcm',   None,        None,    0, 32, 12, 16),
+    ('arcfour',      'ARC4',      None,    0, 16,  1,  1),
+    ('arcfour40',    'ARC4',      None,    0,  5,  1,  1),
+    ('arcfour128',   'ARC4',      None, 1536, 16,  1,  1),
+    ('arcfour256',   'ARC4',      None, 1536, 32,  1,  1),
+    ('blowfish-cbc', 'Blowfish',  CBC,     0, 16,  8,  8),
+    ('cast128-cbc',  'CAST5',     CBC,     0, 16,  8,  8),
+    ('des-cbc',      'TripleDES', CBC,     0,  8,  8,  8),
+    ('des2-cbc',     'TripleDES', CBC,     0, 16,  8,  8),
+    ('des3-cbc',     'TripleDES', CBC,     0, 24,  8,  8),
+    ('seed-cbc',     'SEED',      CBC,     0, 16, 16, 16)
 )
 
-for _cipher_name, _cipher, _mode, _initial_bytes, \
-        _key_size, _iv_size, _block_size in _cipher_alg_list:
-    _cipher_algs[_cipher_name] = (_cipher, _mode, _initial_bytes)
-    register_cipher(_cipher_name, _key_size, _iv_size, _block_size)
+with warnings.catch_warnings():
+    warnings.simplefilter('ignore')
+
+    for _cipher_name, _alg, _mode, _initial_bytes, \
+            _key_size, _iv_size, _block_size in _cipher_alg_list:
+        if _alg:
+            try:
+                _cipher = getattr(_algs, _alg)
+            except AttributeError as exc: # pragma: no cover
+                if _decrepit_algs:
+                    try:
+                        _cipher = getattr(_decrepit_algs, _alg)
+                    except AttributeError:
+                        raise exc from None
+                else:
+                    raise
+        else:
+            _cipher = None
+
+        _cipher_algs[_cipher_name] = (_cipher, _mode, _initial_bytes)
+        register_cipher(_cipher_name, _key_size, _iv_size, _block_size)