don't allow for creation of malformed DER files

when ancoding in ASN.1 the public point has to use uncompressed,
compressed or hybrid representation, raw is not allowed
so disallow it in API

src/ecdsa/keys.py

@@ -226,6 +226,8 @@ def to_pem(self):
         return der.topem(self.to_der(), "PUBLIC KEY")
 
     def to_der(self, point_encoding="uncompressed"):
+        if point_encoding == "raw":
+            raise ValueError("raw point_encoding not allowed in DER")
         point_str = b("\x00") + self.to_string(point_encoding)
         return der.encode_sequence(der.encode_sequence(encoded_oid_ecPublicKey,
                                                        self.curve.encoded_oid),
@@ -356,6 +358,8 @@ def to_pem(self):
     def to_der(self, point_encoding="uncompressed"):
         # SEQ([int(1), octetstring(privkey),cont[0], oid(secp224r1),
         #      cont[1],bitstring])
+        if point_encoding == "raw":
+            raise ValueError("raw encoding not allowed in DER")
         encoded_vk = b("\x00") + \
                 self.get_verifying_key().to_string(point_encoding)
         return der.encode_sequence(der.encode_integer(1),

src/ecdsa/test_pyecdsa.py

@@ -277,6 +277,19 @@ def order(self):
         pub2 = VerifyingKey.from_pem(pem)
         self.assertTruePubkeysEqual(pub1, pub2)
 
+    def test_vk_to_der_with_invalid_point_encoding(self):
+        sk = SigningKey.generate()
+        vk = sk.verifying_key
+
+        with self.assertRaises(ValueError):
+            vk.to_der("raw")
+
+    def test_sk_to_der_with_invalid_point_encoding(self):
+        sk = SigningKey.generate()
+
+        with self.assertRaises(ValueError):
+            sk.to_der("raw")
+
     def test_vk_from_der_garbage_after_curve_oid(self):
         type_oid_der = encoded_oid_ecPublicKey
         curve_oid_der = der.encode_oid(*(1, 2, 840, 10045, 3, 1, 1)) + \