From b3c10424a49309285a54dea14644b3fba79e59ae Mon Sep 17 00:00:00 2001
From: Manuel Thalmann <m@nuth.ch>
Date: Wed, 8 Nov 2023 22:27:12 +0100
Subject: [PATCH] Add an `xtime` implementation

---
 aes/aes.cpp | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/aes/aes.cpp b/aes/aes.cpp
index 82ba954..669b72e 100644
--- a/aes/aes.cpp
+++ b/aes/aes.cpp
@@ -100,7 +100,16 @@ void shiftRows(t_state s) {
 }
 
 uint8_t xtime(uint8_t a) {
-    return 0; /* ??? */
+    uint8_t mask;
+
+    if (a & 0b10000000) {
+        mask = 0x1B;
+    }
+    else {
+        mask = 0x00;
+    }
+
+    return ((a << 1) ^ mask) & 0xFF;
 }
 
 // not mandatory - mix a single column