extend and refactor nut11 sig_all message aggregation (#804)

cashu/core/nuts/nut11.py

@@ -0,0 +1,18 @@
+from typing import List
+
+from ..base import BlindedMessage, Proof
+
+
+def sigall_message_to_sign(proofs: List[Proof], outputs: List[BlindedMessage]) -> str:
+    """
+    Creates the message to sign for sigall spending conditions.
+    The message is a concatenation of all proof secrets and signatures + all output attributes (amount, id, B_).
+    """
+
+    # Concatenate all proof secrets
+    message = "".join([p.secret + p.C for p in proofs])
+
+    # Concatenate all output attributes
+    message += "".join([str(o.amount) + o.id + o.B_ for o in outputs])
+
+    return message