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 inkWhere it comes from
Censors reading civilian mail were trained to rewrite innocent-sounding messages, swapping words for their synonyms so that any hidden pattern collapsed. The story every code-breaker tells is of the telegram saying FATHER IS DEAD coming back as FATHER IS DECEASED, and the reply that followed: IS FATHER DEAD OR DECEASED. Whether or not it happened, it is exactly why they did it.
Try it away from the screen
Write a short note where the first letter of every line spells something, then read it aloud to somebody and see whether they notice. The hard part is making the lines sound ordinary.
Read more elsewhere
Longer, harder and written for grown-ups. Worth a look when this page runs out.