Add source for some secure bytes, e.g. to provide a nonce

libp2p/security/base_transport.py

@@ -1,14 +1,30 @@
+import secrets
+from typing import Callable
+
 from libp2p.crypto.keys import KeyPair
 from libp2p.peer.id import ID
 from libp2p.security.secure_transport_interface import ISecureTransport
 
 
+def default_secure_bytes_provider(n: int) -> bytes:
+    return secrets.token_bytes(n)
+
+
 class BaseSecureTransport(ISecureTransport):
     """
     ``BaseSecureTransport`` is not fully instantiated from its abstract classes as it
     is only meant to be used in clases that derive from it.
+
+    Clients can provide a strategy to get cryptographically secure bytes of a given length.
+    A default implementation is provided using the ``secrets`` module from the
+    standard library.
     """
 
-    def __init__(self, local_key_pair: KeyPair) -> None:
+    def __init__(
+        self,
+        local_key_pair: KeyPair,
+        secure_bytes_provider: Callable[[int], bytes] = default_secure_bytes_provider,
+    ) -> None:
         self.local_private_key = local_key_pair.private_key
         self.local_peer = ID.from_pubkey(local_key_pair.public_key)
+        self.secure_bytes_provider = secure_bytes_provider