https://www.scientificamerican.com/article/new-encryption-system-protects-data-from-quantum-computers/

Once quantum computers become functional, experts warn, they could perform calculations exponentially faster than classical computers—potentially enabling them to destroy the encryption that currently protects our data, from online banking records to personal documents on hard drives. That’s why the National Institute of Standards and Technology is already pushing researchers to look ahead to this “postquantum” era. Most recently, IBM successfully demonstrated a quantum-proof encryption method it developed.

To send secure messages online or encrypt the files on a computer, most modern systems employ asymmetric, or public-key, cryptography. With this technique, data are encoded with a so-called public key, which is accessible to all; decoding that information requires a private key that only one party knows. Although both parts of this system are called “keys,” the public key is more like a slotted lockbox: anyone can drop something in, or encode a secret message, but only the private key’s holder can unlock the box, or decrypt the message. This arrangement makes such asymmetric cryptography more secure than a symmetric system—one that is more like an unlocked lockbox (security depends on keeping the box hidden, because a person who can get to it to drop in a message can also access its contents). Think of symmetric cryptography as a more complex version of a substitution cipher: if the message is encoded by shifting each letter of the alphabet ahead by three places, one can crack the code by simply shifting each letter back by three. That ability means anyone who knows how to put the code in place can also reverse engineer it. In contrast, public-key cryptography uses a mathematical algorithm to generate much more complex keys so the code cannot be run backward in this way. Different public-key systems can utilize different algorithms, as long as they are based on mathematical problems that are easy to put into place but hard to reverse engineer. For instance, any computer can multiply two extremely large prime numbers together, yet factoring the result is nearly impossible—at least, it would be for a classical machine.