update docs to explain tag requirements and valueerror

2 files changed, 6 insertions, 2 deletions

diff --git a/docs/hazmat/bindings/interfaces.rst b/docs/hazmat/bindings/interfaces.rst
index c55d86dc..711c82c2 100644
--- a/docs/hazmat/bindings/interfaces.rst
+++ b/docs/hazmat/bindings/interfaces.rst
@@ -69,6 +69,8 @@ A specific ``backend`` may provide one or more of these interfaces.
         :returns:
             :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
 
+        :raises ValueError: When tag is not None in an AEAD mode
+
 
     .. method:: create_symmetric_decryption_ctx(cipher, mode)
 
@@ -86,6 +88,8 @@ A specific ``backend`` may provide one or more of these interfaces.
         :returns:
             :class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
 
+        :raises ValueError: When tag is None in an AEAD mode
+
 
 .. class:: HashBackend
 
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 8d8d558b..46148689 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -349,8 +349,8 @@ Modes
                                         Do not reuse an ``initialization_vector``
                                         with a given ``key``.
 
-    :param bytes tag: The tag bytes to verify during decryption. Must be provided
-                      for decryption, but is ignored when encrypting.
+    :param bytes tag: The tag bytes to verify during decryption. When encrypting
+                      this must be None.
 
     .. doctest::