FRANKFURT/ISTANBUL, Aug 3 (Reuters) - Turkish authorities were able to trace thousands of people they accuse of participating in an underground network linked to last month’s failed military coup by cracking the weak security features of a little-known smartphone messaging app.

Security experts who looked at the app, known as ByLock, at the request of Reuters said it appeared to be the work of amateur software developers and had left important information about its users unencrypted.

A senior Turkish official said Turkish intelligence cracked the app earlier this year and was able to use it to trace tens of thousands of members of a religious movement the government blames for last month’s failed coup.

Members of the group stopped using the app several months ago after realising it had been compromised, but it still made it easier to swiftly purge tens of thousands of teachers, police, soldiers and justice officials in the wake of the coup.

Turkey blames followers of U.S.-based exiled Muslim cleric Fethullah Gulen for the July 15-16 attempted coup. Gulen denies any connection to the plot.

“The ByLock data made it possible for us to map their network -- at least a large part of it,” a senior Turkish official said. “What I can say is that a large number of people identified via ByLock were directly involved in the coup attempt.”

The Turkish official said ByLock may have been created by the Gulenists themselves so they could communicate. However, experts consulted by Reuters were not able to verify this.

“ByLock is an insecure messaging application that is not widely used today,” Tim Strazzere, director of mobile research at U.S.-Israeli security firm SentinelOne told Reuters. “Anyone who wanted to reverse engineer the app could do so in minutes.”

More than a dozen security and messaging experts contacted by Reuters had never heard of ByLock until it was mentioned in recent days by the Turkish authorities.

According to Matthew Green, a cryptologist and assistant professor of computer science at Johns Hopkins University in the United States who examined the app’s code after being contacted by Reuters, the ByLock network generates a private security key for each device, intended to keep users anonymous.

But these keys are sent to a central server along with user passwords in plain, unencrypted text, meaning that anyone who can break into the server can decrypt the message traffic, he said.

“From what I can tell it was either an amateur app (most likely) or something that someone wrote for the purpose,” he said in an email.

MINISCULE

The ByLock messaging app appears to have been launched in 2014 on both Apple and the Google Play app stores, only to be removed by the developers later the same year. New versions subsequently appeared on less secure app downloading websites targeting Android, Windows Phone and Blackberry users.

An anonymous blog post in November 2014 purporting to be from the developer claims ByLock had attracted around 1 million users, making it difficult to maintain, in part because the app had come under attack from unnamed “Middle East countries”.

Even if it had reached 1 million users, that would still make it miniscule compared to mainstream smartphone messaging apps like Facebook Messenger or WhatsApp, which each have around a billion users worldwide, or iMessage, the messaging app available on all Apple iPhones.

According to some websites that allowed users to download ByLock, and to the security certificate inside the software itself, the author of the app was listed as David Keynes of Beaverton, Oregon. Reuters was unable to locate anyone matching that name or verify whether this identity is genuine.

Starting in May 2015, Turkey’s intelligence agency was able to identify close to 40,000 undercover Gulenist operatives, including 600 ranking military personnel, by mapping connections between ByLock users, the Turkish official said.

However, the Turkish official said that while ByLock helped the intelligence agency identify Gulen’s wider network, it was not used for planning the coup itself. Once Gulen network members realised ByLock had been compromised they stopped using it, the official said.

Instead, the coup plotters seem to have switched to the far more secure WhatsApp by the time they launched their putsch.

While WhatsApp encryption is harder to crack from the outside than ByLock, the authorities have been able to access messages sent that night by getting their hands on the phones of detained plotters.

Transcripts published by Turkish media show officers coordinating troops movements in WhatsApp chat groups.

“With thousands of people in a single WhatsApp chat, it only takes one person to get captured while their phone is unlocked to discover every planned detail,” said Dan Guido, head of New York-based information security firm Trail of Bits. (Additional reporting by Jim Finkle in Las Vegas and Joseph Menn in San Francisco; Editing by Nick Tattersall and Peter Graff)