Fixes #1200 -- disallow GCM truncation by default

1 files changed, 12 insertions, 9 deletions

diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index abc2b076..fffad6e8 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -317,14 +317,22 @@ Modes
         Cryptography will generate a 128-bit tag when finalizing encryption.
         You can shorten a tag by truncating it to the desired length but this
         is **not recommended** as it lowers the security margins of the
-        authentication (`NIST SP-800-38D`_ recommends 96-bits or greater).
-        If you must shorten the tag the minimum allowed length is 4 bytes
-        (32-bits). Applications **must** verify the tag is the expected length
-        to guarantee the expected security margin.
+        authentication (`NIST SP-800-38D`_ recommends 96-bits or greater). If
+        you must shorten the tag the minimum allowed length is 4 bytes
+        (32-bits). Applications wishing to allow truncation must pass the
+        ``min_tag_length`` parameter.
+
+        .. versionchanged:: 0.5
+
+            The ``min_tag_length`` parameter was added in ``0.5``, previously
+            truncation up to ``4`` bytes was always allowed.
 
     :param bytes tag: The tag bytes to verify during decryption. When
         encrypting this must be ``None``.
 
+    :param bytes min_tag_length: The minimum length ``tag`` must be. By default
+        this is ``16``, meaning tag truncation is not allowed.
+
     .. testcode::
 
         import os
@@ -356,11 +364,6 @@ Modes
             return (iv, ciphertext, encryptor.tag)
 
         def decrypt(key, associated_data, iv, ciphertext, tag):
-            if len(tag) != 16:
-                raise ValueError(
-                    "tag must be 16 bytes -- truncation not supported"
-                )
-
             # Construct a Cipher object, with the key, iv, and additionally the
             # GCM tag used for authenticating the message.
             decryptor = Cipher(