Null Cipher Concealment by selective reading · ancient → modern
Spell the secret with every Nth letter of every Mth word. Trivial when noticed; surprisingly hard to notice.
Weak (concealment, not encryption)
Why This Matters
A null cipher hides selected letters inside a longer carrier text. Acrostics and related wordplay have a long history, but the evidence here does not establish this as the oldest named cipher or one universal ancient practice.
A carrier may select word initials, second letters or another agreed position. The often-repeated “PERSHING” example uses a different position rule from word initials, and its attribution needs independent evidence. Jeremiah Denton’s televised blinking in captivity was Morse signalling, not a word-position null cipher.
Pick a position rule. The simplest is first letter of every word:
Carrier: Help Every Little Pup Hidden : H E L P
The demo on this page builds carrier sentences from a dictionary of common English words and selects ones whose chosen letter matches the next character of your secret. Position keys it accepts:
first— first letter of each word (default)last— last letter of each word2,3, … — Nth letter of each word
When no real word fits a needed letter, the demo fabricates a plausible-looking filler so the round-trip always closes. In real use a writer would simply rewrite the carrier sentence around the constraint.
Once concealment is suspected, an analyst can try simple selection rules. That can expose a naive construction, but neither success nor a fixed recovery time is guaranteed for every carrier and rule.
Because the carrier is constrained to spell something, its letter and word-length distributions drift away from natural English. Bletchley-era censors maintained statistical baselines for ordinary correspondence and flagged outliers.
Null ciphers illustrate the broader idea of a covert channel: information is carried through a feature of an ordinary-looking message. Digital techniques use different mechanisms; resemblance does not establish a single historical lineage.
The defensive lesson is also unchanged. Defenders cannot watch every field of every packet, so attackers have rich choices for where to hide. Best modern practice is to assume covert channels exist and to design protocols that minimise the bandwidth available for them — not to chase every clever new carrier.
| Origin | Roman & medieval correspondents |
| Notable uses | Bacon's Advancement of Learning; WWII POW letters; spy classifieds |
| Key Type | Position rule — first/last letter, Nth letter, Nth word, … |
| Family | Steganography (concealment cipher) |
| Modern Lesson | A hidden message that nobody looks for is, briefly, secure |