On the evening of March 5, 2012 in Cairo, Egyptian revolutionaries stormed the headquarters of the secret police known as the State Security Investigations (SSI), a building known as the "City of Hell" because it was where the country's ruthless officials tortured prisoners. Inside, protesters found intact and shredded documents, torture instruments, hard drives, CDs and DVDs, all documenting nightmarish torture and widespread surveillance.

Among the documents, protesters found a memo written in Arabic by a National Security Intelligence Service official about a mysterious piece of software called FinFisher, which was made by the British and German company Gamma International.

The officials reported that FinFisher was an "advanced hacking system" with a variety of capabilities, including accessing email inboxes, uploading "spy files" on target devices, tracking their communications, "complete control" of hacked targets' devices, and - crucially - recording their "successful hacking" of people's accounts on the Skype network, which is known as "the most secure form of communication" because it is encrypted.

In the early 2010s, Skype was the most popular Internet calling application in the world, and not just in Egypt.

Skype, launched in 2003, promised users unprecedented privacy protection. Calls are "end-to-end encrypted and highly secure." In theory, cyber hackers or spies cannot read the chat content, nor can they monitor the call content transmitted over the Internet. That’s why Egyptian spies used FinFisher to hack directly into other people’s computers to eavesdrop on their targets’ Skype calls.

"Skype calls have excellent sound quality and use end-to-end encryption technology for extremely high security," the Skype homepage wrote in 2004.

Skype's encryption feature was revolutionary and groundbreaking at the time. In the mid-1990s, legendary cryptographer Phil Zimmermann created PrettyGoodPrivacy (PGP) software, which allowed people to protect the privacy of files or emails through end-to-end encryption, which meant that only the sender and receiver could read the message content. But PGP is very clunky and not included in easy-to-use chat and calling applications.

Today, more than 20 years later, end-to-end encryption is built into applications used by billions of people, but most of them may not realize that their messages and calls are protected by this data encryption technology. Apple's iMessage and FaceTime, Facebook Messenger, Signal and WhatsApp all use end-to-end encryption technology by default.

But in 2003, Skype was the first to offer this level of encryption and privacy protection.

After Skype was launched, it sparked outrage from law enforcement agencies around the world. In Italy, the Polizia Postale (postal and communications police), which investigates internet crime, asked HackingTeam, a small cybersecurity consulting startup, to develop mobile phone spyware capable of bypassing snooping features such as Skype encryption.

Around the world, other governments have also found different ways to spy on Skype users. In 2008, CitizenLab, a digital rights research group at the University of Toronto, discovered that Skype had been modified to allow Chinese security services to collect information exchanged through the service. In China, Skype was operated by Tom-Skype, a joint venture between a Chinese wireless carrier and eBay, which owned Skype at the time.

Years later, secret documents leaked by former U.S. government contractor Edward Snowden revealed that Microsoft, which now owns Skype, had modified the app to allow the NSA and other agencies to collect calls and information, effectively undermining the app's touted encryption capabilities.

This week, Microsoft announced that it will shut down Skype on May 5. Currently, Skype has become a marginal application. Microsoft said that by 2023, Skype will still have 36 million users, a far cry from the 300 million users at its peak.

Although Skype is a thing of the past and will soon cease operations, Skype's technological legacy lives on, providing communications security for all of the world's most popular chat applications. Thanks to the pioneering ideas about privacy from the original developers of Skype, the world is a safer and freer place.