Balearic techno, a style of dance music born on the tropical shores of Ibiza, is optimized for going out of your mind on the dance floor, oblivious to everything but the rhythm. 
 
 That could be why Polish cybersecurity researcher Krzysztof Szczypiorski saw club music as the perfect genre to serve as a vehicle for Morse code-like messages, hidden in nearly imperceptible changes in tempo. Basically, he turned tunes that were designed to put people in a druggy trance into a method of sending secret messages across an entire open-air beach rave, without most people noticing what they’re really listening to. 
 
 The basic system is this: just like Morse code, letters are ascribed a dot-and-line series, but the “dot” is a slight increase in tempo, and the “line” is a slight decrease. By modulating the tempo ever so slightly, an entire sentence can be communicated over time. To decode the messages, you could use an algorithm that analyzes the beats per minute or simply listen very closely, if your ear is finely tuned enough. 
 
 "For the live mixing it is just usage of tempo slide on the DJ controller—limited only by manual skills of a DJ" 
 
 “It is very easy—it is just tuning the tempo which is visible as a curve in [Digital Audio Workstations like Logic],” Szczypiorski wrote me in an email. “The shape of this curve could be changed by a mouse and follow adopted Morse mode or any coding scheme. For the live mixing it is just usage of tempo slide on the DJ controller—limited only by manual skills of a DJ.” 
 
 In a paper published on the arXiv preprint server, which hasn’t been peer reviewed, Szczypiorski describes two experiments with the system, which he called StegIbiza, after the technical term for hiding messages in other media, steganography. In the first experiment, twenty people—half of which had musical backgrounds—listened to dance music that Szczypiorski had manipulated to contain messages. When the tempo was changed by less than one percent, “none of them could identify a difference in the audio,” he wrote. 
 
 The second experiment is sure to be a bit more familiar to people who actually enjoy Balearic dance music: Szczypiorski DJ’d an open-air summer party with about 70 people in attendance. He reported similar results to the more controlled first experiment, but wrote that after he manipulated the tempo too much, the experiment had to stop “because the rest of the party did not care about the music.” 
 
 For all of his expertise, it looks like Szczypiorski didn’t know that there’s nothing worse than a DJ who just won’t stop screwing with the tempo. 
 
 It’s worth noting that music steganography is a very old practice, and Szczypiorski points out in his paper that hiding messages in musical compositions and scores goes back at least 500 years—but not with club music, which is way more fun than whatever the kids were bumping when people were still drilling into skulls to cure bad humors. 
 
 Next time you’re at a rave, keep a keen ear out. The DJ might just be trying to send you a hidden message. ||||| Plenty of research has looked at adults’ emotional responses to music. But research with babies is more piecemeal and eclectic, perhaps reflecting the difficulty of asking them what they like. Researchers know that babies can hear and remember music even while they are still in the womb. And one curious study found that newborn babies prefer Bach to Aerosmith. 
 
 Most systematic work has found young babies have clear preferences for consonance over dissonance and can remember the tempo and timbre of music they’ve heard before. Babies prefer the female voice but like it even more when it takes on the qualities of “motherese” (the high-energy singsong tone we all naturally adopt when talking to babies). But their emotional responses to music is a bit more of a mystery. What kind of music makes them calm and content? And what makes them happy? 
 
 I am an expert on baby laughter and was intrigued when the C&G baby club approached me and music psychologist Lauren Stewart to create “a song scientifically proven to make babies happy” that they could give away to parents. We thought this was an interesting challenge. However, our first proviso was that they shouldn’t use the word “prove”. Our second was that they had let us do real science. They readily agreed. 
 
 The first step was to discover what was already known about the sounds and music that might make babies happy. We had some experience. My previous work on the Baby Laughter project had asked parents about the nursery rhymes and silly sounds that appealed to babies. Lauren’s previous research has looked at “earworms”, songs that get stuck in your head. But we discovered surprisingly little research on babies’ musical preferences. This was encouraging as it meant this was a worthwhile project from a scientific point of view. 
 
 The next step was to find the right composer: Grammy-award winner Imogen Heap. Imogen is a highly tech-savvy musician who just happened to have an 18-month-old daughter of her own. She was also intrigued by the challenges of the project. Few musicians had taken on the task of writing real music to excite babies while still appealing to parents. Musician Michael Janisch recorded a whole album of Jazz for Babies, but that was very slow and designed to soothe babies. Most music written specifically for babies sounds frankly deranged. 
 
 We met with Heap and gave her a set of recommendations based on what we had discovered from the past research. The song ought to be in an major key with a simple and repetitive main melody with musical devices like drum rolls, key changes and rising pitch glides to provide opportunities for anticipation and surprise. Because babies’ heart rates are much faster than ours so the music ought to be more uptempo than we would expect. And finally, it should have an energetic female vocal, ideally recorded in the presence of an actual baby. 
 
 Setting up the experiment 
 
 Fortunately Heap had her daughter, Scout, to help her with the composition. Heap created four melodies for us to test in the lab, two fast and two slow ones. For each of these she created a version with and without simple sung lyrics. Some 26 babies between six and 12 months then came to our lab with their mums and a few dads to give us their opinion. Amazingly most of the parents and 20 out of 26 babies seemed to share a clear preference for one particular melody. In line with our predictions this was a faster melody. Even more amazingly, this was the tune that had started out as a little ditty made up by Scout. 
 
 We knew which song the mums liked because we could ask them. We also asked the parents to tell us what their babies preferred best, because they are the experts on their own babies. But we also filmed the babies’ responses and coded the videos for laughs, smiles and dancing. 
 
 Now that we had a winning melody, Heap needed to turn it into a full-length song and it needed to be funny (to a baby). The secret was to make it silly and make it social. Around 2,500 parents from the C&G baby club and Heapäs fan club voted on silly sounds that made their babies happy. The top ten sounds included “boo!” (66%), raspberries (57%), sneezing (51%), animal sounds (23%) and baby laughter (28%). We also know babies respond better to “plosive” vocal sounds like “pa” and “ba” compared to “sonorant” sounds like “la”. Heap very cleverly worked many of these elements into the song. 
 
 Next it needed to be something that parents could enjoy themselves and share with their children. Happiness is a shared emotion and the success of nursery rhymes is that they are interactive. Heap carefully crafted the lyrics to tell a joyous tale of how we love our little babies wherever we are – from the sky to the ocean, on a bike or on a rocket. The transport theme permitted lots of plosives “beep, beep” and bouncing actions. 
 
 Our baby music consultants came back to the lab and listened to two slightly different sketches of the full song. This time we found that slightly slower seemed to work better (163 vs 168 beats per minute). Perhaps because it gave parents and babies a little more time to respond to the lyrics. We also found that the chorus was the most effective part of the song and determined which lyrics and sound effects worked better or worse. 
 
 After one final round of tweaks from Heap, we went for a different kind of test. We assembled about 20 of the babies in one room and played them the song all together. If you ever met an excited toddler or young baby, you will know that two and a half minutes is a long time to hold the attention of even one child, let alone two dozen. When The Happy Song played we were met by a sea of entranced little faces. This final bit wasn’t the most scientific as tests go but it definitely convinced me that we had a hit on our hands. 
 
 Now that we have a song that is both new and highly baby friendly, Lauren and I have a range of follow-up studies planned. We are planning to use the song in a range of experiments looking at how parents introduce their babies to music and hope to look more in depth at babies’ physiological responses to happy music. ||||| This story is featured in BBC Future’s “Best of 2017” collection. Discover more of our picks. 
 
 On 3 June, FBI agents arrived at the house of government contractor Reality Leigh Winner in Augusta, Georgia. They had spent the last two days investigating a top secret classified document that had allegedly been leaked to the press. In order to track down Winner, agents claim they had carefully studied copies of the document provided by online news site The Intercept and noticed creases suggesting that the pages had been printed and “hand-carried out of a secured space”. 
 
 In an affidavit, the FBI alleges that Winner admitted printing the National Security Agency (NSA) report and sending it to The Intercept. Shortly after a story about the leak was published, charges against Winner were made public. 
 
 Many colour printers add the dots to documents without people ever knowing they’re there 
 
 At that point, experts began taking a closer look at the document, now publicly available on the web. They discovered something else of interest: yellow dots in a roughly rectangular pattern repeated throughout the page. They were barely visible to the naked eye, but formed a coded design. After some quick analysis, they seemed to reveal the exact date and time that the pages in question were printed: 06:20 on 9 May, 2017 – at least, this is likely to be the time on the printer’s internal clock at that moment. The dots also encode a serial number for the printer. 
 
 These “microdots” are well known to security researchers and civil liberties campaigners. Many colour printers add them to documents without people ever knowing they’re there. 
 
 In this case, the FBI has not said publicly that these microdots were used to help identify their suspect, and the bureau declined to comment for this article. The US Department of Justice, which published news of the charges against Winner, also declined to provide further clarification. 
 
 In a statement, The Intercept said, “Winner faces allegations that have not been proven. The same is true of the FBI’s claims about how it came to arrest Winner.” 
 
 But the presence of microdots on what is now a high-profile document (against the NSA’s wishes) has sparked great interest. 
 
 Based on their positions when plotted against a grid, they denote specific hours, minutes, dates and numbers 
 
 “Zooming in on the document, they were pretty obvious,” says Ted Han at cataloguing platform Document Cloud, who was one of the first to notice them. “It is interesting and notable that this stuff is out there.” 
 
 Another observer was security researcher Rob Graham, who published a blog post explaining how to identify and decode the dots. Based on their positions when plotted against a grid, they denote specific hours, minutes, dates and numbers. Several security experts who decoded the dots came up with the same print time and date. 
 
 Microdots have existed for many years. The Electronic Frontier Foundation (EFF) maintains a list of colour printers known to use them. The images below, captured by the EFF, demonstrate how to decode them: 
 
 As well as perhaps being of interest to spies, microdots have other potential uses, says Tim Bennett, a data analyst at software consultancy Vector 5 who also examined the allegedly leaked NSA document. 
 
 The Electronic Frontier Foundation has an online tool that should reveal what information the pattern encodes 
 
 “People could use this to check for forgeries,” he explains. “If they get a document and someone says it’s from 2005, [the microdots might reveal] it’s from the last several months.” 
 
 If you do encounter microdots on a document at some point, the EFF has an online tool that should reveal what information the pattern encodes. 
 
 Hidden messages 
 
 Similar kinds of steganography – secret messages hidden in plain sight – have been around for much longer. 
 
 Slightly more famously, many banknotes around the world feature a peculiar five-point pattern called the Eurion constellation. In an effort to avoid counterfeiting, many photocopiers and scanners are programmed not to produce copies of the banknotes when this pattern is recognised. 
 
 READ MORE: The secret codes on banknotes 
 
 The NSA itself points to a fascinating historical example of tiny dots forming messages – from World War Two. German spies in Mexico were found to have taped tiny dots inside the envelope concealing a memo for contacts in Lisbon. 
 
 At the time, these spies were operating undercover and were trying to get materials from Germany, such as radio equipment and secret ink. The Allies intercepted these messages, however, and disrupted the mission. The tiny dots used by the Germans were often simply bits of unencrypted text miniaturised to the size of a full-stop. 
 
 This sort of communication was widely used during WWII and afterwards, notably during the Cold War. There are reports of agents operating for the Soviet Union, but based undercover in West Germany and using letter drops to transmit these messages. 
 
 And today, anyone can try using microtext to protect their property – some companies, such as Alpha Dot in the UK, sell little vials of permanent adhesive full of pin-head sized dots, which are covered in microscopic text containing a unique serial number. If the police recover a stolen item, the number can in theory be used to match it with its owner. 
 
 Many examples of these miniature messages do not involve a coded pattern as with the output of many colour printers, but they remain good examples of how miniscule dispatches physically applied to documents or objects can leave an identifying trail. 
 
 One project has tracked more than 45,000 complaints to printer companies about the technology 
 
 Some forms of text-based steganography don’t even use alphanumeric characters or symbols at all. Alan Woodward, a security expert at the University of Surrey, notes the example of ‘Snow’ – Steganographic Nature Of Whitespace – which places spaces and tabs at the end of lines in a piece of text. The particular number and order of these white spaces can be used to encode an invisible message. 
 
 “Locating trailing whitespace in text is like finding a polar bear in a snowstorm,” the Snow website explains. 
 
 Woodward points out, though, that there are usually multiple ways of tracing documents back to whoever printed or accessed them. 
 
 “Organisations such as the NSA have logs of every time something is printed, not just methods of tracking paper once printed,” he says. “They know that people know about the yellow dots and so they don’t rely upon it for traceability.” 
 
 There is a long-running debate over whether it is ethical for printers to be attaching this information to documents without users knowing. In fact, there has even been a suggestion that it is a violation of human rights and one MIT project has tracked more than 45,000 complaints to printer companies about the technology. 
 
 Still, many believe that the use of covert measures to ensure the secrecy of classified documents remains necessary in some cases. 
 
 “There are things that governments should be able to keep secret,” says Ted Han. 
 
 Is your printer sharing your history? According to a freedom of information request to the US Secret Service made by journalist Theo Karantsalis in 2012, these printer manufacturers agreed to fulfil "document identification requests": Canon 
 
 Brother 
 
 Casio 
 
 Hewlett-Packard 
 
 Konica 
 
 Minolta 
 
 Mita 
 
 Ricoh 
 
 Sharp 
 
 Xerox 
 
 However, he adds, “I hope that folks think about their operational security and also about how journalists can protect themselves – and their sources as well.” 
 
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– There is a long and melodious history of hiding secret messages in song—so-called musical steganography—that dates back at least 500 years. But now a cybersecurity expert from Poland has revealed a new way to send messages that are otherwise undetectable through song: tiny adjustments in tempo. To test this, Krzysztof Szczypiorski turned to club music for its trance-like repetition and happily packed his bags for Ibiza off the coast of Spain, arguably the dance-club capital of the world, reports Vice. (Hey, somebody's gotta do it.) Szczypiorski reports in the open journal arXiv (it has not yet been peer reviewed) that in experiments DJing for dancers, he was able to adjust a song's tempo up or down by as much as 2% without anyone noticing any change. By having a faster tempo represent a dash and a slower one a dot, he used Apple's Logic Pro X digital audio workstation to manipulate the speeds of songs like "Rhythm Is a Dancer" by Snap (normally 130 beats per minute) and randomly encode the message "Steganography is a dancer" in Morse code, reports MIT Technology Review. So long as the change in tempo was subtle enough, nobody was the wiser. (Listen here.) His conclusion: It would be pretty simple to develop software that automatically encodes or decodes messages sent this way. An algorithm could be used to decode the message, though it's possible that some very careful listeners could do so on their own. (Music might also help your beer taste better.)