Sometimes it's coded information placed inconspicuously inside a newspaper article and other times it's a note tattooed on the back of a slave's head, but the hiding of a secret message in plain sight (or steganography) is a tactic that goes back millennia.
Last week, reports in The New York Times and The Intercept revealed the National Security Agency (NSA) has been communicating with Russian spies using their public Twitter account, giving the old practice a 21st-century spin.
This was part of a joint effort from members of the CIA and NSA to recover classified government documents stolen by Russian hackers. According to the reports, the spy in question was also willing to offer government officials the dirt on President Trump.
In the end, the deal fell through because of US concerns over the legitimacy of the intel – but that was not before the NSA sent a bunch of very public messages to the spy. The tweets, almost a dozen in total, were designed not to spark suspicion.
One read, "Samuel Morse patented the telegraph 177 years ago. Did you know you can still send telegrams? Faster than post & pay only if it's delivered."
The spy was told what the message would say and when it would be posted. The purpose of these messages: to show officials wanted "to communicate with the Russians or reassure them that the US was still supporting the channel", reports The Intercept.
So, what is steganography? Essentially, it's the opposite of cryptography. Cryptography is a coded message that can only be deciphered by a person possessing a key. Anyone who comes across that message will know it's a coded message, even if they can't understand the content. In contrast, steganography is very public but, when successful, the message can only be interpreted by the desired recipient, while the rest of the world remains oblivious.
The practice goes back years. According to the ancient Greek historian Herodotus, Histiaeus of Miletus tattooed a message on the bald head of a slave in 513 BCE. He waited for the hair to grow before sending his slave to the Greeks, who, again, shaved the head of the poor, tattooed slave to read Histiaeus' message.
More recently, spies have turned to the realm of the digital. In one case, the FBI discovered secret messages from Russian spies in New Jersey hidden in the pixels of photos posted on a public website.