vdrive: protect dirnode contents with an HMAC

src/allmydata/test/test_vdrive.py

@@ -255,3 +255,35 @@ class Test(unittest.TestCase):
                              sorted(expected_keys))
         return res
 
+def flip_bit(data, offset):
+    if offset < 0:
+        offset = len(data) + offset
+    return data[:offset] + chr(ord(data[offset]) ^ 0x01) + data[offset+1:]
+
+class Encryption(unittest.TestCase):
+    def test_loopback(self):
+        key = "k" * 16
+        data = "This is some plaintext data."
+        crypttext = vdrive.encrypt(key, data)
+        plaintext = vdrive.decrypt(key, crypttext)
+        self.failUnlessEqual(data, plaintext)
+
+    def test_hmac(self):
+        key = "j" * 16
+        data = "This is some more plaintext data."
+        crypttext = vdrive.encrypt(key, data)
+        # flip a bit in the IV
+        self.failUnlessRaises(vdrive.IntegrityCheckError,
+                              vdrive.decrypt,
+                              key, flip_bit(crypttext, 0))
+        # flip a bit in the crypttext
+        self.failUnlessRaises(vdrive.IntegrityCheckError,
+                              vdrive.decrypt,
+                              key, flip_bit(crypttext, 16))
+        # flip a bit in the HMAC
+        self.failUnlessRaises(vdrive.IntegrityCheckError,
+                              vdrive.decrypt,
+                              key, flip_bit(crypttext, -1))
+        plaintext = vdrive.decrypt(key, crypttext)
+        self.failUnlessEqual(data, plaintext)
+

src/allmydata/util/hashutil.py

@@ -79,3 +79,13 @@ def generate_dirnode_keys_from_writekey(write_key):
 def generate_dirnode_keys_from_readkey(read_key):
     index = dir_index_hash(read_key)
     return None, None, read_key, index
+
+def _xor(a, b):
+    return "".join([chr(ord(c) ^ ord(b)) for c in a])
+
+def hmac(tag, data):
+    ikey = _xor(tag, "\x36")
+    okey = _xor(tag, "\x5c")
+    h1 = SHA256.new(ikey + data).digest()
+    h2 = SHA256.new(okey + h1).digest()
+    return h2

src/allmydata/vdrive.py

@@ -27,23 +27,29 @@ def create_directory_node(client, diruri):
     d.addCallback(_got)
     return d
 
+IV_LENGTH = 14
 def encrypt(key, data):
-    # TODO: add the hmac
-    IV = os.urandom(14)
-    counterstart = IV + "\x00"*2
+    IV = os.urandom(IV_LENGTH)
+    counterstart = IV + "\x00"*(16-IV_LENGTH)
     assert len(counterstart) == 16, len(counterstart)
     cryptor = AES.new(key=key, mode=AES.MODE_CTR, counterstart=counterstart)
     crypttext = cryptor.encrypt(data)
-    return IV + crypttext
+    mac = hashutil.hmac(key, IV + crypttext)
+    assert len(mac) == 32
+    return IV + crypttext + mac
+
+class IntegrityCheckError(Exception):
+    pass
 
 def decrypt(key, data):
-    # TODO: validate the hmac
-    assert len(data) >= 14, len(data)
-    IV = data[:14]
-    counterstart = IV + "\x00"*2
+    assert len(data) >= (32+IV_LENGTH), len(data)
+    IV, crypttext, mac = data[:IV_LENGTH], data[IV_LENGTH:-32], data[-32:]
+    if mac != hashutil.hmac(key, IV+crypttext):
+        raise IntegrityCheckError("HMAC does not match, crypttext is corrupted")
+    counterstart = IV + "\x00"*(16-IV_LENGTH)
     assert len(counterstart) == 16, len(counterstart)
     cryptor = AES.new(key=key, mode=AES.MODE_CTR, counterstart=counterstart)
-    plaintext = cryptor.decrypt(data[14:])
+    plaintext = cryptor.decrypt(crypttext)
     return plaintext