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Diffstat (limited to 'docs/hazmat/primitives/symmetric-encryption.rst')
| -rw-r--r-- | docs/hazmat/primitives/symmetric-encryption.rst | 23 |
1 files changed, 14 insertions, 9 deletions
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst index abc2b076..586285b7 100644 --- a/docs/hazmat/primitives/symmetric-encryption.rst +++ b/docs/hazmat/primitives/symmetric-encryption.rst @@ -288,7 +288,7 @@ Modes Must be the same number of bytes as the ``block_size`` of the cipher. Do not reuse an ``initialization_vector`` with a given ``key``. -.. class:: GCM(initialization_vector, tag=None) +.. class:: GCM(initialization_vector, tag=None, min_tag_length=16) .. danger:: @@ -318,13 +318,23 @@ Modes 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. + 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 down 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. Allowing tag + truncation is strongly discouraged for most applications. + + :raises ValueError: This is raised if ``len(tag) < min_tag_length``. + .. testcode:: import os @@ -356,11 +366,6 @@ 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( |