They re almost invisible but contain a hidden code and now their presence on
a leaked document has sparked speculation about their usefulness to FBI
investigators.
By Chris Baraniuk
7 June 2017
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.