Keeping information confidential is difficult. Everyone from children to celebrities understands this. National security experts are no exception.

The problem is about to become even more challenging.

There's always someone who wants to get their hands on sensitive information. Yet, vast amounts of private data travel constantly across the internet through cables and fiber optics. This information's privacy depends on encryption, a method that mathematically scrambles data to prevent anyone, even with powerful computers, from deciphering it.

However, the mathematical basis of these techniques is under threat from a recently hypothetical foe: quantum computers.

In the 1990s, scientists discovered that these computers could exploit the strange physics of the minuscule world of atoms and electrons to perform calculations beyond the reach of standard computers. This means that once quantum machines are powerful enough, they could crack the mathematical locks protecting encrypted data, exposing the world's secrets.

Today's quantum computers are far too weak to defeat current security measures. But with more powerful machines being built by companies like IBM and Google, scientists, governments, and others are starting to take action. Experts are urging preparation for a significant event some call Y2Q. This refers to the year when quantum computers will be able to break the encryption schemes that safeguard electronic communication.

"If that encryption is ever broken," says mathematician Michele Mosca, "it would be a catastrophe for our systems."

## Y2Q is coming. What does it mean?

Encryption is essential in our digital lives, protecting emails, financial and medical data, online shopping transactions, and more. Encryption is also woven into many physical devices that transmit information, from cars to robot vacuums to baby monitors. Even critical infrastructure like power grids relies on encryption. The tools Y2Q threatens are everywhere.

"The stakes are incredibly high," says Mosca, of the University of Waterloo in Canada, who is also the CEO of the cybersecurity company evolutionQ.

The name Y2Q references the infamous Y2K bug, which threatened to cause computer havoc in the year 2000 because software typically used only two digits to mark the year. Y2Q is a similarly widespread issue, but in many ways, it's not a fair comparison. Fixing Y2Q is far more complex than changing how dates are represented, and computers are now even more integrated into society than two decades ago. Additionally, no one knows exactly when Y2Q will arrive.

Faced with the Y2Q threat, cryptography, the study and practice of techniques used to encode information, is undergoing a significant revision. Scientists and mathematicians are urgently working to prepare for this unknown date by developing new methods of encrypting data that will be resistant to quantum decryption. The National Institute of Standards and Technology (NIST) is leading an effort to release new standards for these post-quantum cryptography algorithms next year.

In the meantime, a longer-term effort is taking a "can't beat 'em, join 'em" approach: using quantum technology to build a more secure, quantum internet. Scientists worldwide are constructing networks that transmit quantum information between cities, pursuing the dream of communication that could theoretically be unhackable.

## How public-key cryptography works

Imagine you want to share a secret message with someone. You can encrypt it, scrambling the information so that it can be decoded later.

Schoolchildren might do this with a simple cipher: For instance, replace the letter A with the number 1, B with 2, and so on. Anyone who knows this secret key used to encrypt the message can later decode the message and read it, whether it's the intended recipient or a curious classmate.

This is a simplified example of what's called symmetric-key cryptography: the same key is used to encode and decode a message. In more serious communication, the key would be much more complex, essentially impossible for anyone to guess. But in both cases, the same secret key is used for encoding and decoding.

This strategy has been used in cryptography for thousands of years, says computer scientist Peter Schwabe of the Max Planck Institute for Security and Privacy in Bochum, Germany. "It was either used in a military context or between lovers that weren't supposed to love each other."

However, in today's globally connected world, symmetric-key cryptography has a problem. How do you securely transmit the secret key to someone on the other side of the planet, someone you've never met?

To solve this dilemma, cryptographers developed public-key cryptography in the 1970s. It uses special mathematical tricks to solve the symmetric-key conundrum. It utilizes two different, mathematically related keys. A public key is used to encrypt messages, and a mathematically related private key decodes them.

## How to swap secrets with strangers

For instance, say Alice wants to send a secret message to Bob. She finds his public key, which is kind of like a digital lockbox with an open door. Anyone can put a message in, but only someone with the right key can open it and read what's inside.

Alice uses Bob's public key to scramble her message. This scrambling process makes the message unreadable to anyone who intercepts it. Once the message is encrypted, Alice sends it off to Bob.

Here's the twist: Bob has another, different key called a private key. This private key is like the actual key to the lockbox. Only Bob has access to his private key, and it's what he uses to decrypt the message Alice sent him.

The beauty of this system is that Alice never needs to share her private key with Bob. She only uses his public key, which is designed to be shared freely. Even if someone eavesdropped and stole the encrypted message, they wouldn't be able to decrypt it without Bob's private key.

This is a simplified explanation of public-key cryptography, but it hopefully gives you a basic understanding of how it works.

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