Which letters count?
A null is a letter or a word that means nothing. In a null cipher, almost everything is a null: only a few letters carry the message, and the rule tells you which.
Press every third word and watch the message come out. Press every word and you get gibberish, from exactly the same page. The text has not changed. The rule has.
That is the whole danger of it. Without the rule there is nothing to attack. With the rule it takes four seconds.
The most famous one
A null cipher can hide in anything: first letters, second letters, the word after every comma, the last letter of every line. Censors in wartime rewrote innocent-looking telegrams for exactly this reason, changing words for their synonyms so any hidden pattern collapsed. "Father is dead" would come back as "father is deceased", which is how one famous story goes, and the reply, "is father dead or deceased", told them the trick had worked.
This is the sister of the acrostic on the front page of this section, and the ancestor of every message hidden in the first letters of a poem.
Back to HIPS Invisible inkRead more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.