From 688e0f673bfbf43fa898994326c6877f00ab19ef Mon Sep 17 00:00:00 2001
From: Paul Kehrer <paul.l.kehrer@gmail.com>
Date: Tue, 17 Jul 2018 10:47:57 +0800
Subject: [PATCH] disallow implicit tag truncation with finalize_with_tag

---
 docs/hazmat/primitives/symmetric-encryption.rst     |  1 +
 src/cryptography/hazmat/backends/openssl/ciphers.py |  5 +++++
 src/cryptography/hazmat/primitives/ciphers/modes.py |  1 +
 tests/hazmat/primitives/test_aes.py                 | 16 ++++++++++++++++
 5 files changed, 28 insertions(+)

diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 5ebcca754..5b6000902 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -670,6 +670,7 @@ Interfaces
         :raises ValueError: This is raised when the data provided isn't
             a multiple of the algorithm's block size, if ``min_tag_length`` is
             less than 4, or if ``len(tag) < min_tag_length``.
+            ``min_tag_length`` is an argument to the ``GCM`` constructor.
         :raises NotImplementedError: This is raised if the version of the
             OpenSSL backend used is 1.0.1 or earlier.
 
diff --git a/src/cryptography/hazmat/backends/openssl/ciphers.py b/src/cryptography/hazmat/backends/openssl/ciphers.py
index 462ffea25..e0ee06ee2 100644
--- a/src/cryptography/hazmat/backends/openssl/ciphers.py
+++ b/src/cryptography/hazmat/backends/openssl/ciphers.py
@@ -199,6 +199,11 @@ def finalize_with_tag(self, tag):
                 "finalize_with_tag requires OpenSSL >= 1.0.2. To use this "
                 "method please update OpenSSL"
             )
+        if len(tag) < self._mode._min_tag_length:
+            raise ValueError(
+                "Authentication tag must be {0} bytes or longer.".format(
+                    self._mode._min_tag_length)
+            )
         res = self._backend._lib.EVP_CIPHER_CTX_ctrl(
             self._ctx, self._backend._lib.EVP_CTRL_AEAD_SET_TAG,
             len(tag), tag
diff --git a/src/cryptography/hazmat/primitives/ciphers/modes.py b/src/cryptography/hazmat/primitives/ciphers/modes.py
index 598dfaa4a..543015fef 100644
--- a/src/cryptography/hazmat/primitives/ciphers/modes.py
+++ b/src/cryptography/hazmat/primitives/ciphers/modes.py
@@ -220,6 +220,7 @@ def __init__(self, initialization_vector, tag=None, min_tag_length=16):
                         min_tag_length)
                 )
         self._tag = tag
+        self._min_tag_length = min_tag_length
 
     tag = utils.read_only_property("_tag")
     initialization_vector = utils.read_only_property("_initialization_vector")
diff --git a/tests/hazmat/primitives/test_aes.py b/tests/hazmat/primitives/test_aes.py
index d6f83ebc2..4ceccf155 100644
--- a/tests/hazmat/primitives/test_aes.py
+++ b/tests/hazmat/primitives/test_aes.py
@@ -439,3 +439,19 @@ def test_gcm_tag_decrypt_finalize(self, backend):
             decryptor.finalize()
         else:
             decryptor.finalize_with_tag(tag)
+
+    @pytest.mark.supported(
+        only_if=lambda backend: (
+            not backend._lib.CRYPTOGRAPHY_OPENSSL_LESS_THAN_102 or
+            backend._lib.CRYPTOGRAPHY_IS_LIBRESSL
+        ),
+        skip_message="Not supported on OpenSSL 1.0.1",
+    )
+    def test_gcm_tag_decrypt_finalize_tag_length(self, backend):
+        decryptor = base.Cipher(
+            algorithms.AES(b"0" * 16),
+            modes.GCM(b"0" * 12),
+            backend=backend
+        ).decryptor()
+        with pytest.raises(ValueError):
+            decryptor.finalize_with_tag(b"tagtooshort")