Null Cipher — Bench tool
Published 29 Sep 2026 ·
A null cipher does not disguise a letter — it hides which letters are the message. Every character in the transmitted text is a perfectly ordinary character. Only the positions are secret, and an attacker who does not know the rule has no ciphertext to attack at all, only a letter about the weather.
- Era
- Medieval
- Type
- Substitution
- Key space
- carrier text + position rule
- Level
- Field
IWhat this tool does
Here is a cover sentence: THE WEATHER HOLDS FAIR AND THE LOOM KEEPS ITS OWN COUNSEL ALL OF THE LONG DAY.
Read every fifth letter from the start and you get W H D A H K T N A H G A — which is not a message, because this cover was not written to carry one. But suppose the rule is read every fifth letter backwards from the end, or read the last letter of each word, or the acrostic rule read the first letter of each line.
The bench writes nulls in the simplest mechanical form: the plaintext letters are dropped into position N of each block of N characters, and the gaps are filled from a fixed carrier sentence.
| Rule | Where the message hides | Detectability |
|---|---|---|
| every Nth character | mechanical, easy to automate | low, if the cover reads naturally |
| first letter of each sentence | acrostic | very low |
| last letter of each word | word-final | low |
| Nth word of each sentence | word-level | lowest |
The word-level variants are the strongest, because normal prose has highly variable word lengths and sentence lengths, and variation is camouflage. A mechanical every-fifth-character rule makes the cover text stilted — and stilted prose is itself a signal.
IIThe key
The key is a pair: the carrier text plus the position rule. Neither alone is sufficient, and both are chosen freely, which means the theoretical space is unbounded.
That is an unusual property in this collection. Almost every other cipher here has a keyspace you can write as a number. The null cipher's keyspace is the number of sentences a person could plausibly write — which is not a number anyone can search, and also not a number anyone can administer.
Unbounded keyspace is not the same as strong cryptography. If your rule is memorable, it is guessable; if it is not memorable, you will not remember it under pressure.
The historical compromise, used by prisoners of war and by agents in occupied territory, was to use a rule that is not a secret at all: the text that happens to be in front of you. A passage from a book both parties own is enough.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →