Expose which keys signed the Transaction in the SDK

programs/bpf/c/src/move_funds.c

@@ -20,6 +20,11 @@ extern bool entrypoint(const uint8_t *input) {
     return false;
   }
 
+  if (!ka[0].is_signer) {
+    sol_log("Transaction not signed by key 0");
+    return false;
+  }
+
   int64_t tokens = *(int64_t *)data;
   if (*ka[0].tokens >= tokens) {
     *ka[0].tokens -= tokens;