diff --git a/src/core/lib/BitwiseOp.mjs b/src/core/lib/BitwiseOp.mjs
index fe33c812..516a816b 100644
--- a/src/core/lib/BitwiseOp.mjs
+++ b/src/core/lib/BitwiseOp.mjs
@@ -34,10 +34,10 @@ export function bitOp (input, key, func, nullPreserving, scheme) {
             !(nullPreserving && (o === 0 || o === k))) {
             switch (scheme) {
                 case "Input differential":
-                    key[i % key.length] = x;
+                    key[i % key.length] = o;
                     break;
                 case "Output differential":
-                    key[i % key.length] = o;
+                    key[i % key.length] = x;
                     break;
             }
         }