ensure server nonce .startswith() our nonce

ircrobots/scram.py

@@ -31,12 +31,12 @@ def _scram_xor(s1: bytes, s2: bytes) -> bytes:
     return bytes(a ^ b for a, b in zip(s1, s2))
 
 class SCRAMState(enum.Enum):
-    Uninitialised = 0
-    ClientFirst = 1
-    ClientFinal = 2
-    Success = 3
-    Failed = 4
-    VerifyFailed = 5
+    NONE           = 0
+    CLIENT_FIRST   = 1
+    CLIENT_FINAL   = 2
+    SUCCESS        = 3
+    FAILURE        = 4
+    VERIFY_FAILURE = 5
 
 class SCRAMError(Exception):
     pass
@@ -50,11 +50,13 @@ class SCRAMContext(object):
         self._username = username.encode("utf8")
         self._password = password.encode("utf8")
 
-        self.state = SCRAMState.Uninitialised
+        self.state = SCRAMState.NONE
         self.error = ""
         self.raw_error = ""
 
         self._client_first    = b""
+        self._client_nonce    = b""
+
         self._salted_password = b""
         self._auth_message    = b""
 
@@ -70,10 +72,19 @@ class SCRAMContext(object):
     def _constant_time_compare(self, b1: bytes, b2: bytes):
         return hmac.compare_digest(b1, b2)
 
+    def _fail(self, error: str):
+        self.raw_error = error
+        if error in SCRAM_ERRORS:
+            self.error = error
+        else:
+            self.error = "other-error"
+        self.state = SCRAMState.FAILURE
+
     def client_first(self) -> bytes:
-        self.state = SCRAMState.ClientFirst
+        self.state = SCRAMState.CLIENT_FIRST
+        self._client_nonce = _scram_nonce()
         self._client_first = b"n=%s,r=%s" % (
-            _scram_escape(self._username), _scram_nonce())
+            _scram_escape(self._username), self._client_nonce)
 
         # n,,n=<username>,r=<nonce>
         return b"n,,%s" % self._client_first
@@ -81,25 +92,24 @@ class SCRAMContext(object):
     def _assert_error(self, pieces: Dict[bytes, bytes]) -> bool:
         if b"e" in pieces:
             error = pieces[b"e"].decode("utf8")
-            self.raw_error = error
-            if error in SCRAM_ERRORS:
-                self.error = error
-            else:
-                self.error = "other-error"
-
-            self.state = SCRAMState.Failed
+            self._fail(error)
             return True
         else:
             return False
 
     def server_first(self, data: bytes) -> bytes:
-        self.state = SCRAMState.ClientFinal
+        self.state = SCRAMState.CLIENT_FINAL
 
         pieces = self._get_pieces(data)
         if self._assert_error(pieces):
             return b""
 
         nonce = pieces[b"r"] # server combines your nonce with it's own
+        if (not nonce.startswith(self._client_nonce) or
+                nonce == self._client_nonce):
+            self._fail("nonce-unacceptable")
+            return b""
+
         salt = base64.b64decode(pieces[b"s"]) # salt is b64encoded
         iterations = int(pieces[b"i"])
 
@@ -133,8 +143,8 @@ class SCRAMContext(object):
         server_signature = self._hmac(server_key, self._auth_message)
 
         if server_signature == verifier:
-            self.state = SCRAMState.Success
+            self.state = SCRAMState.SUCCESS
             return True
         else:
-            self.state = SCRAMState.VerifyFailed
+            self.state = SCRAMState.VERIFY_FAILURE
             return False