Fix #11: Attack by using a nonce multiple times

verifier.py

@@ -55,6 +55,15 @@ while True:
     with open(input('entries: ')) as f:
         entries = ast.literal_eval(f.read())
 
+    nonces = set()
+    for entry in entries:
+        if type(entry) == str:
+            break
+        if entry in nonces:
+            print('A given nonce was used several times!')
+            exit(1)
+        nonces.add(entry)
+
     merkleTreeLevels = [{} for _ in range(common.LEVELS)]
 
     for index in indexesRequest: