dirnode: don't check MAC on entries in dirnodes

In an ancient version of directories, we needed a MAC on each entry.  In modern times, the entire dirnode comes with a digital signature, so the MAC on each entry is redundant.
With this patch, we no longer check those MACs when reading directories, but we still produce them so that older readers will accept directories that we write.

src/allmydata/dirnode.py

@@ -152,14 +152,12 @@ class NewDirectoryNode:
         mac = hashutil.hmac(key, IV + crypttext)
         assert len(mac) == 32
         return IV + crypttext + mac
+        # The MAC is not checked by readers in Tahoe >= 1.3.0, but we still produce it for the sake of older readers.
 
     def _decrypt_rwcapdata(self, encwrcap):
         IV = encwrcap[:16]
         crypttext = encwrcap[16:-32]
-        mac = encwrcap[-32:]
         key = hashutil.mutable_rwcap_key_hash(IV, self._node.get_writekey())
-        if mac != hashutil.hmac(key, IV+crypttext):
-            raise hashutil.IntegrityCheckError("HMAC does not match, crypttext is corrupted")
         cryptor = AES(key)
         plaintext = cryptor.process(crypttext)
         return plaintext

src/allmydata/test/test_dirnode.py

@@ -119,17 +119,17 @@ class Dirnode(unittest.TestCase,
                 filenode = dn._node
                 si = IURI(filenode.get_uri()).storage_index
                 old_contents = filenode.all_contents[si]
-                # we happen to know that the writecap is encrypted near the
-                # end of the string. Flip one of its bits and make sure we
-                # detect the corruption.
+                # We happen to know that the writecap MAC is near the end of the string. Flip
+                # one of its bits and make sure we ignore the corruption.
                 new_contents = testutil.flip_bit(old_contents, -10)
                 # TODO: also test flipping bits in the other portions
                 filenode.all_contents[si] = new_contents
             d.addCallback(_corrupt)
             def _check2(res):
-                self.shouldFail(hashutil.IntegrityCheckError, "corrupt",
-                                "HMAC does not match, crypttext is corrupted",
-                                dn.list)
+                d = dn.list()
+                def _c3(res):
+                    self.failUnless(res.has_key('child'))
+                d.addCallback(_c3)
             d.addCallback(_check2)
             return d
         d.addCallback(_created)

src/allmydata/util/hashutil.py

@@ -13,9 +13,6 @@ from allmydata.util.netstring import netstring
 # kinds.
 CRYPTO_VAL_SIZE=32
 
-class IntegrityCheckError(Exception):
-    pass
-
 class _SHA256d_Hasher:
     # use SHA-256d, as defined by Ferguson and Schneier: hash the output
     # again to prevent length-extension attacks