EDIT 2:

You have an array of pointers, I would use

 

    char * dictionary = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    char key[12];    //to copy the characters

To use a single character we can do:
  

    key [1] = '\0';
         //for loop
             key [0] = dictionary [i];

keep in mind that string key[52] is an **array of pointers**