diff --git a/substitution.c b/substitution.c
index ebda538..4732453 100644
--- a/substitution.c
+++ b/substitution.c
@@ -32,15 +32,19 @@ int validate_key(string key)
     {
         int sum = 0;
 
-        for (int i = 0; i < strlen(key); i++)
+        for (char c = 'A'; c <= 'Z'; c++)
         {
-            if (isalpha(key[i]))
+            for (int i = 0; i < 26; i++)
             {
-                sum += toupper(key[i]);
+                if (toupper(key[i]) == c)
+                {
+                    sum++;
+                    break;
+                }
             }
         }
 
-        if (sum == 2015) // Sum of 26 unique uppercase characters
+        if (sum == 26) // Sum of 26 unique uppercase characters, but also MMCcEFGHIJKLMNOPQRqTUVWXeZ
         {
             return 0;
         }