A plain substitution falls to counting: E is the commonest letter in English, so the commonest symbol is probably E. Homophonic substitution kills that. Give E eight different numbers, T six, and rare letters one each, then pick a different one every time.
Now every symbol appears about as often as every other, and the counting attack has nothing to grip. This is what the Zodiac killer used, and one of his messages held out for fifty-one years.
The message that held out for fifty-one years
The Zodiac killer sent newspapers a cipher of strange symbols where common letters had several symbols each, so counting them gave nothing. It was solved in 2020, fifty-one years later, by three amateurs working together: an American software developer, an Australian mathematician and a Belgian programmer. None of them had ever met.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.