How it works
There are four lists of words, one for each part of a sentence: who, does what, how and where. Every list has one entry for each letter of the alphabet.
The first letter of your message picks a word from the first list, the second letter from the second list, and so on. After four letters you have a whole sentence, and you start again.
To read it back, your friend takes each piece of the sentence, finds it in the right list, and writes down its letter.
Now read one
Paste a message made with the lists below, and the letters come back out.
The four lists
Copy these out by hand and give a copy to a friend. Anyone without the lists sees a strange little prayer and nothing more.
Where it comes from
Johannes Trithemius was a German abbot who lived from 1462 to 1516. His book Polygraphia came out in 1518, two years after he died, and it is often called the first printed book about cryptography.
Its first part is page after page of Latin words in columns. Each letter of the alphabet has a word in each column, and the columns are built so that, read one after another, the words make sense together. The first column is all names for God, such as Deus and Creator. Spell out a secret with it and it comes out as a Latin prayer. That is why people call it the Ave Maria cipher.
This is steganography as well as a cipher: the message is scrambled, and it also hides the fact that there is a message at all. The same book holds the Trithemius shifting alphabet, which led to the Vigenère cipher.
How to break it
On its own it is weak. Each list always uses the same word for the same letter, so it is a simple substitution in disguise, and counting the words will crack it. Its real strength was never the maths. It was that nobody thought to look.
The Trithemius cipher More ways to hide a message Try the challengesRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.