Editing Null cipher (section)

== Dot concealment cipher ==
A dot or pinprick concealment cipher is a common classical encryption method in which dot or pinprick is placed above or below certain letters in a piece of writing.<ref name=":1" /> An early reference to this appears in [[Aeneas Tacticus]]'s  book ''On the Defense of Fortifications.''<ref name=":3">{{Cite web |title=Aeneas Tacticus • Siege Defense, XXI‑XXXI |url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Aeneas_Tacticus/E*.html |access-date=2022-05-28 |website=penelope.uchicago.edu}}</ref> The Germans improved upon this, using a dot of [[invisible ink]] during [[World War I]] and [[World War II]].

In 19th-century England, pinpricks in newspapers were once a popular way to send letters with little or no cost.{{clarify|reason=How did this work? Could you send a newspaper to someone else for free?|date=November 2024}}

If dots were placed far apart, this cipher could be used effectively. The dots should be small and the null text must make sense in the context of the senders and their relationship. Both also must have agreed on the page, chapter, article, or section to be used, typically several.<ref name=":1" /> Another option is to have an indicator, such as the date in a newspaper, which shows which page the message is on.<ref name=":4">{{Cite book |last1=Travis |first1=Falcon |title=The Spy's Guidebook |last2=Hindley |first2=Judy |last3=Thomson |first3=Ruth |last4=Amery |first4=Heather |last5=Rawson |first5=Christopher |last6=Harper |first6=Anita |publisher=Usborne Pocketbooks |year=1978}}</ref> This version is less secure.