support for limiting acceptable curve encodings

as some standards, like PKIX in X.509 certificates, don't allow
for explicit curve paramters, provide an API that limits the
supported formats

Author: tomato42

src/ecdsa/curves.py

@@ -140,19 +140,36 @@ def to_pem(self, encoding=None, point_encoding="uncompressed"):
         )
 
     @staticmethod
-    def from_der(data):
+    def from_der(data, valid_encodings=None):
         """Decode the curve parameters from DER file.
 
         :param data: the binary string to decode the parameters from
-        :type data: bytes-like object
+        :type data: :term:`bytes-like object`
+        :param valid_encodings: set of names of allowed encodings, by default
+            all (set by passing ``None``), supported ones are ``named_curve``
+            and ``explicit``
+        :type valid_encodings: :term:`set-like object`
         """
+        if not valid_encodings:
+            valid_encodings = set(("named_curve", "explicit"))
+        if not all(i in ["named_curve", "explicit"] for i in valid_encodings):
+            raise ValueError(
+                "Only named_curve and explicit encodings supported"
+            )
         data = normalise_bytes(data)
         if not der.is_sequence(data):
+            if "named_curve" not in valid_encodings:
+                raise der.UnexpectedDER(
+                    "named_curve curve parameters not allowed"
+                )
             oid, empty = der.remove_object(data)
             if empty:
                 raise der.UnexpectedDER("Unexpected data after OID")
             return find_curve(oid)
 
+        if "explicit" not in valid_encodings:
+            raise der.UnexpectedDER("explicit curve parameters not allowed")
+
         seq, empty = der.remove_sequence(data)
         if empty:
             raise der.UnexpectedDER(
@@ -168,7 +185,9 @@ def from_der(data):
         order, rest = der.remove_integer(rest)
         cofactor = None
         if rest:
-            cofactor, rest = der.remove_integer(rest)
+            # the ASN.1 specification of ECParameters allows for future
+            # extensions of the sequence, so ignore the remaining bytes
+            cofactor, _ = der.remove_integer(rest)
 
         # decode the ECParameters.fieldID sequence
         field_type, rest = der.remove_object(field_id)
@@ -213,10 +232,14 @@ def from_der(data):
         return tmp_curve
 
     @classmethod
-    def from_pem(cls, string):
+    def from_pem(cls, string, valid_encodings=None):
         """Decode the curve parameters from PEM file.
 
         :param str string: the text string to decode the parameters from
+        :param valid_encodings: set of names of allowed encodings, by default
+            all (set by passing ``None``), supported ones are ``named_curve``
+            and ``explicit``
+        :type valid_encodings: :term:`set-like object`
         """
         if not PY2 and isinstance(string, str):  # pragma: no branch
             string = string.encode()
@@ -225,7 +248,9 @@ def from_pem(cls, string):
         if ec_param_index == -1:
             raise der.UnexpectedDER("EC PARAMETERS PEM header not found")
 
-        return cls.from_der(der.unpem(string[ec_param_index:]))
+        return cls.from_der(
+            der.unpem(string[ec_param_index:]), valid_encodings
+        )
 
 
 # the SEC curves

src/ecdsa/test_curves.py

@@ -38,6 +38,32 @@ def test_from_pem(self):
 
         self.assertIs(curve, NIST256p)
 
+    def test_from_pem_with_explicit_when_explicit_disabled(self):
+        pem_params = (
+            "-----BEGIN EC PARAMETERS-----\n"
+            "MIHgAgEBMCwGByqGSM49AQECIQD/////AAAAAQAAAAAAAAAAAAAAAP/////////\n"
+            "//////zBEBCD/////AAAAAQAAAAAAAAAAAAAAAP///////////////AQgWsY12K\n"
+            "o6k+ez671VdpiGvGUdBrDMU7D2O848PifSYEsEQQRrF9Hy4SxCR/i85uVjpEDyd\n"
+            "wN9gS3rM6D0oTlF2JjClk/jQuL+Gn+bjufrSnwPnhYrzjNXazFezsu2QGg3v1H1\n"
+            "AiEA/////wAAAAD//////////7zm+q2nF56E87nKwvxjJVECAQE=\n"
+            "-----END EC PARAMETERS-----\n"
+        )
+        with self.assertRaises(der.UnexpectedDER) as e:
+            Curve.from_pem(pem_params, ["named_curve"])
+
+        self.assertIn("explicit curve parameters not", str(e.exception))
+
+    def test_from_pem_with_named_curve_with_named_curve_disabled(self):
+        pem_params = (
+            "-----BEGIN EC PARAMETERS-----\n"
+            "BggqhkjOPQMBBw==\n"
+            "-----END EC PARAMETERS-----\n"
+        )
+        with self.assertRaises(der.UnexpectedDER) as e:
+            Curve.from_pem(pem_params, ["explicit"])
+
+        self.assertIn("named_curve curve parameters not", str(e.exception))
+
     def test_from_pem_with_wrong_header(self):
         pem_params = (
             "-----BEGIN PARAMETERS-----\n"
@@ -119,6 +145,12 @@ def test_decoding_well_known_from_explicit_params(self):
 
         self.assertIs(curve, NIST256p)
 
+    def test_decoding_with_incorrect_valid_encodings(self):
+        with self.assertRaises(ValueError) as e:
+            Curve.from_der(b"", ["explicitCA"])
+
+        self.assertIn("Only named_curve", str(e.exception))
+
     def test_compare_curves_with_different_generators(self):
         curve_fp = CurveFp(23, 1, 7)
         base_a = PointJacobi(curve_fp, 13, 3, 1, 9, generator=True)