Support explicit curve params

build-requirements-3.4.txt

@@ -4,3 +4,4 @@ hypothesis
 pytest>=4.6.0
 PyYAML<5.3
 coverage
+attrs<21

src/ecdsa/curves.py

@@ -1,7 +1,9 @@
 from __future__ import division
 
-from . import der, ecdsa
-from .util import orderlen
+from six import PY2
+from . import der, ecdsa, ellipticcurve
+from .util import orderlen, number_to_string, string_to_number
+from ._compat import normalise_bytes
 
 
 # orderlen was defined in this module previously, so keep it in __all__,
@@ -29,9 +31,15 @@
     "BRAINPOOLP320r1",
     "BRAINPOOLP384r1",
     "BRAINPOOLP512r1",
+    "PRIME_FIELD_OID",
+    "CHARACTERISTIC_TWO_FIELD_OID",
 ]
 
 
+PRIME_FIELD_OID = (1, 2, 840, 10045, 1, 1)
+CHARACTERISTIC_TWO_FIELD_OID = (1, 2, 840, 10045, 1, 2)
+
+
 class UnknownCurveError(Exception):
     pass
 
@@ -47,11 +55,203 @@ def __init__(self, name, curve, generator, oid, openssl_name=None):
         self.verifying_key_length = 2 * orderlen(curve.p())
         self.signature_length = 2 * self.baselen
         self.oid = oid
-        self.encoded_oid = der.encode_oid(*oid)
+        if oid:
+            self.encoded_oid = der.encode_oid(*oid)
+
+    def __eq__(self, other):
+        if isinstance(other, Curve):
+            return (
+                self.curve == other.curve and self.generator == other.generator
+            )
+        return NotImplemented
+
+    def __ne__(self, other):
+        return not self == other
 
     def __repr__(self):
         return self.name
 
+    def to_der(self, encoding=None, point_encoding="uncompressed"):
+        """Serialise the curve parameters to binary string.
+
+        :param str encoding: the format to save the curve parameters in.
+            Default is ``named_curve``, with fallback being the ``explicit``
+            if the OID is not set for the curve.
+        :param str point_encoding: the point encoding of the generator when
+            explicit curve encoding is used. Ignored for ``named_curve``
+            format.
+
+        :return: DER encoded ECParameters structure
+        :rtype: bytes
+        """
+        if encoding is None:
+            if self.oid:
+                encoding = "named_curve"
+            else:
+                encoding = "explicit"
+
+        if encoding == "named_curve":
+            if not self.oid:
+                raise UnknownCurveError(
+                    "Can't encode curve using named_curve encoding without "
+                    "associated curve OID"
+                )
+            return der.encode_oid(*self.oid)
+
+        # encode the ECParameters sequence
+        curve_p = self.curve.p()
+        version = der.encode_integer(1)
+        field_id = der.encode_sequence(
+            der.encode_oid(*PRIME_FIELD_OID), der.encode_integer(curve_p)
+        )
+        curve = der.encode_sequence(
+            der.encode_octet_string(
+                number_to_string(self.curve.a() % curve_p, curve_p)
+            ),
+            der.encode_octet_string(
+                number_to_string(self.curve.b() % curve_p, curve_p)
+            ),
+        )
+        base = der.encode_octet_string(self.generator.to_bytes(point_encoding))
+        order = der.encode_integer(self.generator.order())
+        seq_elements = [version, field_id, curve, base, order]
+        if self.curve.cofactor():
+            cofactor = der.encode_integer(self.curve.cofactor())
+            seq_elements.append(cofactor)
+
+        return der.encode_sequence(*seq_elements)
+
+    def to_pem(self, encoding=None, point_encoding="uncompressed"):
+        """
+        Serialise the curve parameters to the :term:`PEM` format.
+
+        :param str encoding: the format to save the curve parameters in.
+            Default is ``named_curve``, with fallback being the ``explicit``
+            if the OID is not set for the curve.
+        :param str point_encoding: the point encoding of the generator when
+            explicit curve encoding is used. Ignored for ``named_curve``
+            format.
+
+        :return: PEM encoded ECParameters structure
+        :rtype: str
+        """
+        return der.topem(
+            self.to_der(encoding, point_encoding), "EC PARAMETERS"
+        )
+
+    @staticmethod
+    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: :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(
+                "Unexpected data after ECParameters structure"
+            )
+        # decode the ECParameters sequence
+        version, rest = der.remove_integer(seq)
+        if version != 1:
+            raise der.UnexpectedDER("Unknown parameter encoding format")
+        field_id, rest = der.remove_sequence(rest)
+        curve, rest = der.remove_sequence(rest)
+        base_bytes, rest = der.remove_octet_string(rest)
+        order, rest = der.remove_integer(rest)
+        cofactor = None
+        if 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)
+        if field_type == CHARACTERISTIC_TWO_FIELD_OID:
+            raise UnknownCurveError("Characteristic 2 curves unsupported")
+        if field_type != PRIME_FIELD_OID:
+            raise UnknownCurveError(
+                "Unknown field type: {0}".format(field_type)
+            )
+        prime, empty = der.remove_integer(rest)
+        if empty:
+            raise der.UnexpectedDER(
+                "Unexpected data after ECParameters.fieldID.Prime-p element"
+            )
+
+        # decode the ECParameters.curve sequence
+        curve_a_bytes, rest = der.remove_octet_string(curve)
+        curve_b_bytes, rest = der.remove_octet_string(rest)
+        # seed can be defined here, but we don't parse it, so ignore `rest`
+
+        curve_a = string_to_number(curve_a_bytes)
+        curve_b = string_to_number(curve_b_bytes)
+
+        curve_fp = ellipticcurve.CurveFp(prime, curve_a, curve_b, cofactor)
+
+        # decode the ECParameters.base point
+
+        base = ellipticcurve.PointJacobi.from_bytes(
+            curve_fp,
+            base_bytes,
+            valid_encodings=("uncompressed", "compressed", "hybrid"),
+            order=order,
+            generator=True,
+        )
+        tmp_curve = Curve("unknown", curve_fp, base, None)
+
+        # if the curve matches one of the well-known ones, use the well-known
+        # one in preference, as it will have the OID and name associated
+        for i in curves:
+            if tmp_curve == i:
+                return i
+        return tmp_curve
+
+    @classmethod
+    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()
+
+        ec_param_index = string.find(b"-----BEGIN EC PARAMETERS-----")
+        if ec_param_index == -1:
+            raise der.UnexpectedDER("EC PARAMETERS PEM header not found")
+
+        return cls.from_der(
+            der.unpem(string[ec_param_index:]), valid_encodings
+        )
+
 
 # the SEC curves
 SECP112r1 = Curve(