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The story is often told that Histiaeus tattooed a secret message on his slave's head, waited for his hair to grow back, then sent him off to Miletus. Why would he have done this?

The story is usually cited as an early historical example of steganography. But a message in someone's memory is less obvious than a message on one's skull. A message in someone's memory doesn't seem more resistant to torture (a strong-willed slave might not disclose the message, while a strong-willed torturer would almost certainly discover the tattoo). It also doesn't seem likely that the message could be kept secret from the messenger (since the messenger could just ask a friend to read it off his skull).

In short, is there some information-theoretic advantage to this anecdotal method?

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    $\begingroup$ It also doesn't seem likely that the message could be kept secret from the messenger (since the messenger could just ask a friend to read it off his skull) This assumes that slaves know how to read, which easily may not have been true (or that the slave had a friend who was not a slave, which seems improbable). $\endgroup$ – Ella Rose Mar 26 '18 at 18:55
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    $\begingroup$ Also, there is no reason that the tattooed slave should even know that there is a secret message being tattooed on his scalp. I could have sworn that when I first heard of this tale (and, to be clear, this vignette is generally not considered to be one of Herodotus' more factual stories), that the slave was told that the tattooing was supposed to be some kind of medical procedure. $\endgroup$ – mhum Mar 26 '18 at 22:21
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    $\begingroup$ You assume that a) the slave would remember a longer message exactly over weeks (good luck with that) and b) that if he could remember the message perfectly, the slave would want to tell it the recipient (because nothing bad ever happens to the messenger of bad news). The unreliability of messengers used to be one of the biggest problems of diplomacy. There's a fun extra history episode that talks about this for a bit, maybe I'll look for it later. $\endgroup$ – Voo Mar 27 '18 at 8:33
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Exact motives will remain unknown, since we only have Herodotus's word for the whole story, and he doesn't say. However, we can imagine a plausible reason: the whole ordeal provides some level of authentication.

Indeed, Histiaeus's slave could hardly have made himself the tattoo, and certainly not without Histiaeus noticing. Since hair growing is slow (and cannot be accelerated, at least not in Ancient Greek times, where hair grafting was not known), the presence of the message as a tattoo shows at least a purposeful dedication in time to the process. In modern terminology, it is a "proof of work". A contrario, a purely verbal message could be made up on the spot by the slave himself. Aristagoras could believe such an oral transmission to be merely the invention of a disgruntled slave.

Alternatively, this could be Herodotus having absorbed too much Samos wine. He tries to be careful, but since most of what he writes is hearsay, he at times swallows wonderfully extravagant stories, such as some about giant ants that can devour adult camels (one hypothesis is that he confused "giant ants" with marmots, but these peaceful rodents don't attack camels either!).

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    $\begingroup$ I suppose one might confuse a grizzly bear for a giant marmot. Sorry, what did you say? Grizzlies don't eat camels? Are you sure? $\endgroup$ – David Richerby Mar 27 '18 at 14:47
  • $\begingroup$ Up to about 11000 years ago, North America's fauna included camels (camelops genus) and also a species of a very large, carnivorous bear (arctodus simus), and palaeontological clues point to the definite possibility that the latter at least occasionally preyed on the former. $\endgroup$ – Thomas Pornin Mar 27 '18 at 17:09
  • $\begingroup$ OK, that must have been what Herodotus was talking about. $\endgroup$ – David Richerby Mar 27 '18 at 17:21
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This sounds like weak steganography ... with a stronger, albeit algorithm dependent :) , signing/anti tampering system.

Altering a tattoo would be significant work, which the messenger would have a hard time doing to himself, while him/her requesting someone else to do so would have raised attention and probably put the slave at risk. Furthermore, an alteration to a tattoo would be visible as an alteration for at least several days, since the altered parts would be in a different stage of skin healing - and that is for tattoos done in a sanitary, modern environment with modern inks. Also, if the ink or method used for altering it was different, that could be obvious for a long time. Also, even with hair, a head tattoo could be exposed to plenty of sunlight in southern europe, even with hair growing over it - giving fading effects that would be very hard to falsify evenly.

The tattooist combined with his tools and materials and the time of tattooing would form the signing key to the message.

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