py-bindings: add SecretKeyFactory to docs and .pyi

umbral-pre-python/docs/index.rst

@@ -26,6 +26,18 @@ API reference
 
         Generates a new secret key.
 
+.. py:class:: SecretKeyFactory
+
+    A deterministic generator of :py:class:`SecretKey` objects.
+
+    .. py:staticmethod:: random() -> SecretKeyFactory
+
+        Generates a new random factory.
+
+    .. py:method:: secret_key_by_label(label: bytes) -> SecretKey
+
+        Generates a new :py:class:`SecretKey` using ``label`` as a seed.
+
 .. py:class:: PublicKey
 
     An ``umbral-pre`` public key object.
@@ -38,7 +50,6 @@ API reference
 
     An encapsulated symmetric key.
 
-
 .. py:function:: encrypt(pk: PublicKey, plaintext: bytes) -> Tuple[Capsule, bytes]
 
     Creates a symmetric key, encrypts ``plaintext`` with it, and returns the encapsulated symmetric key along with the ciphertext. ``pk`` is the public key of the recipient.

umbral-pre-python/umbral_pre/__init__.pyi

@@ -6,6 +6,17 @@ class SecretKey:
     def random() -> SecretKey:
         ...
 
+
+class SecretKeyFactory:
+
+    @staticmethod
+    def random() -> SecretKeyFactory:
+        ...
+
+    def secret_key_by_label(self, label: bytes) -> SecretKey:
+        ...
+
+
 class PublicKey:
     @staticmethod
     def from_secret_key(sk: SecretKey) -> PublicKey: