AEAD: handle EAX, using Cryptodome if it is available

pgpy/symenc.py

@@ -2,6 +2,7 @@
 """
 
 from typing import Optional, Union
+from types import ModuleType
 
 from cryptography.exceptions import UnsupportedAlgorithm
 
@@ -15,6 +16,12 @@
 from .errors import PGPEncryptionError
 from .errors import PGPInsecureCipherError
 
+AES_Cryptodome: Optional[ModuleType]
+try:
+    from Cryptodome.Cipher import AES as AES_Cryptodome
+except ModuleNotFoundError:
+    AES_Cryptodome = None
+
 __all__ = ['_cfb_encrypt',
            '_cfb_decrypt',
            'AEAD']
@@ -62,16 +69,49 @@ def _cfb_decrypt(ct: bytes, key: bytes, alg: SymmetricKeyAlgorithm, iv: Optional
 
 
 class AEAD:
+    class AESEAX:
+        '''This class supports the same interface as AESOCB3 and AESGCM from python's cryptography module
+
+        We don't use that module because it doesn't support EAX
+        (see https://github.com/pyca/cryptography/issues/6903)
+        '''
+
+        def __init__(self, key: bytes) -> None:
+            self._key: bytes = key
+
+        def decrypt(self, nonce: bytes, data: bytes, associated_data: Optional[bytes] = None) -> bytes:
+            if AES_Cryptodome is None:
+                raise NotImplementedError("AEAD Mode EAX needs the python cryptodome module installed")
+            if len(nonce) != AEADMode.EAX.iv_len:
+                raise ValueError(f"EAX nonce should be {AEADMode.EAX.iv_len} octets, got {len(nonce)}")
+            a = AES_Cryptodome.new(self._key, AES_Cryptodome.MODE_EAX, nonce, mac_len=AEADMode.EAX.tag_len)
+            if associated_data is not None:
+                a.update(associated_data)
+            return a.decrypt_and_verify(data[:-AEADMode.EAX.tag_len], data[-AEADMode.EAX.tag_len:])
+
+        def encrypt(self, nonce: bytes, data: bytes, associated_data: Optional[bytes] = None) -> bytes:
+            if AES_Cryptodome is None:
+                raise NotImplementedError("AEAD Mode EAX needs the python cryptodome module installed")
+            if len(nonce) != AEADMode.EAX.iv_len:
+                raise ValueError(f"EAX nonce should be {AEADMode.EAX.iv_len} octets, got {len(nonce)}")
+            a = AES_Cryptodome.new(self._key, AES_Cryptodome.MODE_EAX, nonce, mac_len=AEADMode.EAX.tag_len)
+            if associated_data is not None:
+                a.update(associated_data)
+            ciphertext, tag = a.encrypt_and_digest(data)
+            return ciphertext + tag
+
     def __init__(self, cipher: SymmetricKeyAlgorithm, mode: AEADMode, key: bytes) -> None:
-        self._aead: Union[AESOCB3, AESGCM]
+        self._aead: Union[AESOCB3, AESGCM, AEAD.AESEAX]
         if cipher not in [SymmetricKeyAlgorithm.AES128, SymmetricKeyAlgorithm.AES192, SymmetricKeyAlgorithm.AES256]:
             raise NotImplementedError(f"Cannot do AEAD with non-AES cipher (requested cipher: {cipher!r})")
         if mode == AEADMode.OCB:
             self._aead = AESOCB3(key)
         elif mode == AEADMode.GCM:
             self._aead = AESGCM(key)
+        elif mode == AEADMode.EAX:
+            self._aead = AEAD.AESEAX(key)
         else:
-            raise NotImplementedError(f"Cannot do AEAD mode other than OCB, and GCM (requested mode: {mode!r})")
+            raise NotImplementedError(f"Cannot do AEAD mode other than OCB, GCM, and EAX (requested mode: {mode!r})")
 
     def encrypt(self, nonce: bytes, data: bytes, associated_data: Optional[bytes] = None) -> bytes:
         return self._aead.encrypt(nonce, data, associated_data)

requirements.txt

@@ -1,2 +1,3 @@
 argon2_cffi
 cryptography>=3.3.2
+pycryptodomex[eax]

setup.cfg

@@ -47,6 +47,8 @@ install_requires =
     cryptography>=3.3.2
     sop>=0.5.1
 python_requires = >=3.6
+extras_require =
+    eax = pycryptodomex
 
 # doc_requires =
 #    sphinx

tests/test_12_crypto_refresh.py

@@ -3,6 +3,7 @@
 """
 
 from typing import Dict, Optional, Tuple
+from types import ModuleType
 
 import pytest
 
@@ -11,6 +12,12 @@
 from pgpy import PGPKey, PGPSignature, PGPMessage
 from pgpy.errors import PGPDecryptionError
 
+Cryptodome:Optional[ModuleType]
+try:
+    import Cryptodome
+except ModuleNotFoundError:
+    Cryptodome = None
+
 class TestPGP_CryptoRefresh(object):
     def test_v4_sigs(self) -> None:
         (k, _) = PGPKey.from_file('tests/testdata/crypto-refresh/v4-ed25519-pubkey-packet.key')
@@ -30,8 +37,8 @@ def test_v4_skesk_argon2(self, cipher: str) -> None:
     def test_v6_skesk(self, aead: str) -> None:
         msg = PGPMessage.from_file(f'tests/testdata/crypto-refresh/v6skesk-aes128-{aead}.pgp')
         assert msg.is_encrypted
-        if aead == 'eax':
-            pytest.xfail('AEAD Mode EAX not supported')
+        if aead == 'eax' and Cryptodome is None:
+            pytest.xfail('AEAD Mode EAX is not supported unless the Cryptodome module is available')
         unlocked = msg.decrypt('password')
         assert not unlocked.is_encrypted
         assert unlocked.message == b'Hello, world!'

tox.ini

@@ -30,6 +30,8 @@ deps =
     pytest
     pytest-cov
     pytest-order
+extras =
+    eax
 
 install_command = pip install {opts} --no-cache-dir {packages}
 commands =