Move GCM tag size/value validation farther forward -- this makes it easier by not requiring future backends to implement the same checks

3 files changed, 13 insertions, 7 deletions

diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 6231aadb..0e824165 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -354,18 +354,11 @@ class _CipherContext(object):
             )
             assert res != 0
             if operation == self._DECRYPT:
-                if not mode.tag or len(mode.tag) < 4:
-                    raise ValueError("Authentication tag must be provided and "
-                                     "be 4 bytes or longer when decrypting")
                 res = self._backend.lib.EVP_CIPHER_CTX_ctrl(
                     ctx, self._backend.lib.EVP_CTRL_GCM_SET_TAG,
                     len(mode.tag), mode.tag
                 )
                 assert res != 0
-            else:
-                if mode.tag:
-                    raise ValueError("Authentication tag must be None when "
-                                     "encrypting")
 
         # pass key/iv
         res = self._backend.lib.EVP_CipherInit_ex(ctx, self._backend.ffi.NULL,
diff --git a/cryptography/hazmat/primitives/ciphers/base.py b/cryptography/hazmat/primitives/ciphers/base.py
index b8615cb9..d1ca6d2a 100644
--- a/cryptography/hazmat/primitives/ciphers/base.py
+++ b/cryptography/hazmat/primitives/ciphers/base.py
@@ -44,8 +44,16 @@ class Cipher(object):
     def _wrap_ctx(self, ctx, encrypt):
         if isinstance(self.mode, interfaces.ModeWithAuthenticationTag):
             if encrypt:
+                if self.mode.tag is not None:
+                    raise ValueError(
+                        "Authentication tag must be None when encrypting"
+                    )
                 return _AEADEncryptionContext(ctx)
             else:
+                if self.mode.tag is None:
+                    raise ValueError(
+                        "Authentication tag must be provided when decrypting"
+                    )
                 return _AEADCipherContext(ctx)
         else:
             return _CipherContext(ctx)
diff --git a/cryptography/hazmat/primitives/ciphers/modes.py b/cryptography/hazmat/primitives/ciphers/modes.py
index e1c70185..ab8501c6 100644
--- a/cryptography/hazmat/primitives/ciphers/modes.py
+++ b/cryptography/hazmat/primitives/ciphers/modes.py
@@ -65,5 +65,10 @@ class GCM(object):
     name = "GCM"
 
     def __init__(self, initialization_vector, tag=None):
+        if tag is not None and len(tag) < 4:
+            raise ValueError(
+                "Authentication tag must be 4 bytes or longer"
+            )
+
         self.initialization_vector = initialization_vector
         self.tag = tag