diff --git a/aes/aes.cpp b/aes/aes.cpp
index 942f55b..95418b9 100644
--- a/aes/aes.cpp
+++ b/aes/aes.cpp
@@ -116,12 +116,22 @@ uint8_t xtime(uint8_t a) {
 
 // not mandatory - mix a single column
 uint32_t mixColumn(uint32_t c) {
-    return 0; /* ??? */
+    uint32_t result = c;
+    uint8_t *source = (uint8_t*)(&c);
+    uint8_t *target = (uint8_t*)(&result);
+    uint8_t base = *source ^ *(source + 1) ^ *(source + 2) ^ *(source + 3);
+    *target ^= base ^ xtime(*source ^ *(source + 1));
+    *(target + 1) ^= base ^ xtime(*(source + 1) ^ *(source + 2));
+    *(target + 2) ^= base ^ xtime(*(source  + 2) ^ *(source + 3));
+    *(target + 3) ^= base ^ xtime(*(source + 3) ^ *source);
+    return result;
 }
 
 
 void mixColumns(t_state s) {
-    /* ??? */
+    for (uint8_t i = 0; i < 4; i++) {
+        s[i] = mixColumn(s[i]);
+    }
 }
 
 /*