The ABZ Affair by Joan H. Young
(Dubois Files book #3) George Harris liked writing messages in secret code. In August of 1953, he learned one that was tough to crack, and taught it to his sister Ruby, and then to his friends Jimmie Mosher and Cora Dubois. But someone listened in on their conversation and stole a coder. The children were playing games, but what were the adults up to?