Fix: pad numbers to requested size before XOR-ing them (#22)

Problem: rebuilding the random number from the random bytes in the
executor responses does not work when the generated random number is
smaller than the requested number of bytes. This occurs because the
random number is not padded before being XOR-ed with the others.

Solution: pad the random number of each executor to guarantee that it is
N bytes wide.

src/aleph_vrf/coordinator/vrf.py

@@ -241,7 +241,7 @@ def generate_final_vrf(
             )
 
         random_numbers_list.append(
-            int_to_bytes(int(vrf_publish_response.random_number))
+            int_to_bytes(int(vrf_publish_response.random_number), n=vrf_request.nb_bytes)
         )
 
         node_response = CRNVRFResponse(

src/aleph_vrf/utils.py

@@ -15,9 +15,12 @@ def xor_all(x: List[bytes]) -> bytes:
     return result
 
 
-def int_to_bytes(x: int) -> bytes:
-    """Converts an integer to bytes."""
-    return x.to_bytes((x.bit_length() + 7) // 8, "big")
+def int_to_bytes(x: int, n: int = 0) -> bytes:
+    """
+    Converts an integer to bytes.
+    If `n` is specified, pads the number to reach n bytes.
+    """
+    return x.to_bytes(max((x.bit_length() + 7) // 8, n), "big")
 
 
 def bytes_to_int(x: bytes) -> int:
@@ -30,9 +33,9 @@ def bytes_to_binary(x: bytes) -> str:
     return "".join(format(b, "08b") for b in x)
 
 
-def binary_to_bytes(s: str):
+def binary_to_bytes(s: str, n: int = 0):
     """Converts binary string to bytes."""
-    return int(s, 2).to_bytes((len(s) + 7) // 8, byteorder="big")
+    return int(s, 2).to_bytes(max((len(s) + 7) // 8, n), byteorder="big")
 
 
 def generate_nonce() -> Nonce: