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systems available today just embed the ‘hidden’ data in the least significant bits of an audio or video file – which is trivial for a capable opponent to detect and remove.

B. Camouflage

Fig. 4. Hiding information into music scores: Gaspar Schott simply maps the letters of the alphabet to the notes. Clearly, one should not try to play the music [29, p. 322].

hid messages in letters home using the dots and dashes on i, j, t and f to spell out a hidden text in Morse code. These ‘semagrams’ concealed messages but have an inher- ent problem, that the cover-text tends to be laborious to construct and often sounds odd enough to alert the censor. During both World Wars, censors intercepted many such messages. A famous one, from World War I, was a cable- gram saying ‘Father is dead’ which the censor modified into ‘Father is deceased’. The reply was a giveaway: ‘Is Father dead or deceased?’ [32, pp. 515–516].

Although steganography is different from cryptography, we can borrow many of the techniques and much practical wisdom from the latter, more thoroughly researched dis- cipline. In 1883, Auguste Kerckhoffs enunciated the first principles of cryptographic engineering, in which he ad- vises that we assume the method used to encipher data is known to the opponent, so security must lie only in the choice of key2 [33]. The history of cryptology since then has repeatedly shown the folly of ‘security-by-obscurity’ – the assumption that the enemy will remain ignorant of the system in use.

Applying this wisdom, we obtain a tentative definition of a secure stego-system: one where an opponent who un- derstands the system, but does not know the key, can ob- tain no evidence (or even grounds for suspicion) that a communication has taken place. In other words, no in- formation about the embedded text can be obtained from knowledge of the stego (and perhaps also cover) texts. We will revisit this definition later, to take account of robust- ness and other issues; but it will remain a central prin- ciple that steganographic processes intended for wide use should be published, just like commercial cryptographic al- gorithms and protocols. This teaching of Kerckhoffs holds with particular force for marking techniques intended for use in evidence, which implies their disclosure in court [34].

That any of the above ‘security-by-obscurity’ systems ever worked was a matter of luck. Yet many steganographic

2Il faut qu’il n’exige pas le secret, et qu’il puisse sans inconve´nient tomber entre les mains de l’ennemi. [33, p. 12]

The situation may be improved by intelligent use of ca- mouflage. Even if the method is known in principle, mak- ing the hidden data expensive to look for can be beneficial, especially where there is a large amount of cover traffic.

Since the early days of architecture, artists have under- stood that works of sculpture or painting appear differ- ent from certain angles, and established rules for perspec- tive and anamorphosis [35]. Through the 16th and 17th centuries anamorphic images supplied an ideal means of camouflaging dangerous political statements and heretical ideas [36]. A masterpiece of hidden anamorphic imagery

  • the Vexierbild – was created in the 1530s by Sho¨, a

N¨urnberg engraver, pupil of Albrecht D¨urer (1471–1528): when one looks at it normally one sees a strange landscape, but looking from the side reveals portraits of famous kings.

In his Histories [37], Herodotus (c.486–425 B.C.) tells how around 440 B.C. Histiæus shaved the head of his most trusted slave and tattooed it with a message which disap- peared after the hair had regrown. The purpose was to instigate a revolt against the Persians. Astonishingly the method was still used by some German spies at the be- ginning of the 20th century [38]. Herodotus also tells how Demeratus, a Greek at the Persian court, warned Sparta of an imminent invasion by Xerxes: he removed the wax from a writing tablet, wrote his message on the wood underneath and then covered the message with wax. The tablet looked exactly like a blank one (it almost fooled the recipient as well as the customs men).

A large number of techniques were invented or reported by Æneas the Tactician [1], including letters hidden in mes- sengers’ soles or women’s ear-rings, text written on wood tablets and then whitewashed, and notes carried by pi- geons. The centerpiece is a scheme for winding thread through 24 holes bored in an astragal: each hole represents a letter and a word is represented by passing the thread through the corresponding letters. He also proposed hid- ing 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 [2] and was reused by German spies during both World Wars [32, p. 83]. A modern adaptation of this technique is still in use for document security [39].

Invisible inks were used extensively. They were originally made of available organic substances (such as milk or urine) or ‘salt armoniack dissolved in water’ [2, V, pp. 37–47] and developed with heat; progress in chemistry helped to create more sophisticated combinations of ink and developer by the first World War, but the technology fell into disuse with the invention of ‘universal developers’ which could deter- mine which parts of a piece of paper had been wetted from

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