Reduce the minimum length of the EAX MAC tag to 2 bytes

AUTHORS.rst

@@ -48,3 +48,4 @@ Eric Young
 Hannes van Niekerk
 Stefan Seering
 Koki Takahashi
+Lauro de Lima

Changelog.rst

@@ -4,6 +4,10 @@ Changelog
 3.17.0 (under development)
 ++++++++++++++++++++++++++
 
+New features
+---------------
+* Reduce the minimum tag length for the EAX cipher to 2 bytes.
+
 Resolved issues
 ---------------
 * GH#526: improved typing for ``RSA.contruct``.

Doc/src/cipher/modern.rst

@@ -194,8 +194,9 @@ a new EAX cipher object for the relevant base algorithm.
   :param bytes nonce: the value of the fixed nonce.
     It must be unique for the combination message/key.
     If not present, the library creates a random nonce (16 bytes long for AES).
-  :param integer mac_len: the desired length of the 
+  :param integer mac_len: the length of the MAC tag, in bytes.
-    MAC tag (default if not present: the cipher's block size, 16 bytes for AES).
+    At least 2, and not larger than the cipher's block size (default),
+    which is 16 bytes for AES.
   :return: an EAX cipher object
 
 The cipher object has a read-only attribute :attr:`nonce`.

lib/Crypto/Cipher/_mode_eax.py

@@ -94,8 +94,8 @@ class EaxMode(object):
                       "digest", "verify"]
 
         # MAC tag length
-        if not (4 <= self._mac_len <= self.block_size):
+        if not (2 <= self._mac_len <= self.block_size):
-            raise ValueError("Parameter 'mac_len' must not be larger than %d"
+            raise ValueError("'mac_len' must be at least 2 and not larger than %d"
                              % self.block_size)
 
         # Nonce cannot be empty and must be a byte string

lib/Crypto/SelfTest/Cipher/test_EAX.py

@@ -151,12 +151,12 @@ class EaxTests(unittest.TestCase):
     def test_mac_len(self):
         # Invalid MAC length
         self.assertRaises(ValueError, AES.new, self.key_128, AES.MODE_EAX,
-                          nonce=self.nonce_96, mac_len=3)
+                          nonce=self.nonce_96, mac_len=2-1)
         self.assertRaises(ValueError, AES.new, self.key_128, AES.MODE_EAX,
                           nonce=self.nonce_96, mac_len=16+1)
 
         # Valid MAC length
-        for mac_len in range(5, 16 + 1):
+        for mac_len in range(2, 16 + 1):
             cipher = AES.new(self.key_128, AES.MODE_EAX, nonce=self.nonce_96,
                              mac_len=mac_len)
             _, mac = cipher.encrypt_and_digest(self.data_128)