diff --git a/aes/aes.cpp b/aes/aes.cpp
index e1379bb..ff69abf 100644
--- a/aes/aes.cpp
+++ b/aes/aes.cpp
@@ -162,7 +162,10 @@ void expandKey(uint8_t k[16], uint32_t ek[44]) {
 
 /* Adding expanded round key (prepared before) */
 void addRoundKey(t_state s, uint32_t ek[], short round) {
-    /* ??? */
+    s[0] ^= ek[round];
+	s[1] ^= ek[round + 1];
+	s[2] ^= ek[round + 2];
+	s[3] ^= ek[round + 3];
 }
 
 void aes(uint8_t *in, uint8_t *out, uint8_t *skey)