Add BigNum class method gen_rand

umbral/bignum.py

@@ -0,0 +1,48 @@
+import os
+from cryptography.hazmat.backends.openssl import backend
+
+
+class BigNum(object):
+    """
+    Represents an OpenSSL BIGNUM except more Pythonic
+    """
+
+    def __init__(self, bignum, curve_nid, curve_group, curve_order):
+        self.bignum = bignum
+        self.curve_nid = curve_nid
+        self.curve_group = curve_group
+        self.curve_order = curve_order
+
+    @classmethod
+    def gen_rand(cls, curve):
+        """
+        Returns a BigNum object with a cryptographically secure BigNum based
+        on the given curve.
+        """
+        curve_nid = backend._elliptic_curve_to_nid(curve)
+
+        group = backend._lib.EC_GROUP_new_by_curve_name(curve_nid)
+        backend.openssl_assert(group != backend._ffi.NULL)
+
+        order = backend._lib.BN_new()
+        backend.openssl_assert(order != backend._ffi.NULL)
+        order = backend._ffi.gc(order, backend._lib.BN_free)
+
+        with backend._tmp_bn_ctx() as bn_ctx:
+            res = backend._lib.EC_GROUP_get_order(group, order, bn_ctx)
+            backend.openssl_assert(res == 1)
+
+        order_int = backend._bn_to_int(order)
+
+        # Generate random number on curve
+        rand_num = int.from_bytes(os.urandom(curve.key_size // 8), 'big')
+        while rand_num >= order_int or rand_num <= 0:
+            rand_num = int.from_bytes(os.urandom(curve.key_size // 8), 'big')
+
+        new_rand_bn = backend._int_to_bn(rand_num)
+        new_rand_bn = backend._ffi.gc(new_rand_bn, backend._lib.BN_free)
+
+        return BigNum(new_rand_bn, curve_nid, group, order)
+
+    def __int__(self):
+        return backend._bn_to_int(self.bignum)