From the Proceedings of the IEEE:
A large number of techniques were invented or reported by Æneas the Tactician … He also proposed hiding text by making very small holes above or below letters or by changing the heights of letter-strokes in a cover text. These dots were masked by the contrast between the black letters and the white paper. This technique was still in use during the 17th century, but was improved by Wilkins who used invisible ink to print very small dots instead of making holes and was reused by German spies during both World Wars. A modern adaptation of this technique is still in use for document security1.
The classic purpose of steganography was the prevention of sensitive information falling into the wrong hands:
The word steganography is of Greek origin and means “covered, or hidden writing”. Its ancient origins can be traced back to 440 BC. Herodotus mentions two examples of steganography in The Histories of Herodotus. Demaratus sent a warning about a forthcoming attack to Greece by writing it on a wooden panel and covering it in wax. Wax tablets were in common use then as re-usable writing surfaces, sometimes used for shorthand. Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden. The purpose was to instigate a revolt against the Persians. Later, Johannes Trithemius published the book Steganographia, a treatise on cryptography and steganography disguised as a grimoire.2
There is an interesting theory, however, that the Amora Rav utilized a steganographic technique quite similar to the ones described in the IEEE article, for a novel purpose – to minimize an infraction of the prohibition against writing תורה שבעל פה. The Gemara relates:
תלה ליה רב לרבי ביני חיטי “האחין ששיעבדו מאי?”3
Rashi explains:
שלח לו אגרת שלומים ותלה לו שאלה זו בין השיטים:4
Rashi does not explain what exactly he means by writing the question “between the lines”, or why Rav did so. Hafla’ah elaborates:
עיין פירש”י נראה דרצונו דלשון תלה משמע שתולה בין השיטין כמו שאמרו בתולה את השם אם שכחו והיינו משום דרב לא רצה לכתוב שאלתו משום דקיימא לן דברים שבעל פה אי אתה רשאי לכתוב אלא דהתירו משום עת לעשות וגו’ ועדיפא ליה לרב לעשות כן דהיינו שכתב אגרת שלומים ובתוך הכתב עשה עצמו כאלו שכח האות וכתב למעלה כגון שבתחלה תלה אות ה’ ובתיבה אחרת או בשיטה אחרת תלה אות א’ וכן עשה עד שבצירוף כל האותיות התלויות בין השיטין בא מצורף אלו התיבות האחים ששיעבדו מאי וק”ל5
Incidentally, it is noteworthy that Hafla’ah seems to assume that Rav’s query was subject to the injunction against writing תורה שבעל פה, even though it was a pure interrogatory, with no declarative content.