Some operations can be easily carried out in one direction, but they are very difficult to reverse. For example, an image can be quickly transformed into a puzzle, but to arrange all the pieces in order to turn the puzzle back into the picture – that’s much longer, both for humans and for computers. Another example - to multiply two large numbers -a trivial task for a computer - but we don’t know how to “unmultiply” this big number and single out multipliers.
Modern public key cryptography is based on this principle. To draw an analogy: I know two numbers, I have multiplied them and told everyone the result. Now everyone can encrypt a message for me using the result of multiplication, but only I can decipher this using multipliers. Cryptography is widely used in almost all spheres of human activity. It is no exaggeration to say that the world will go through a huge shock if suddenly all that is encrypted suddenly becomes decrypted.
The surprise is that there are already quantum computers that allow you to quickly solve problems that seem to be back-breaking for conventional computers, for example, to quickly decompose a composite number. The only obstacle to the digital end of the world (let's call it Cryptoapocalypse) is that for the time being quantum computers can work with small numbers only.
But the day will soon come when security services or rich villains will access a universal decryption and do terrible things. For example, if such a villain steals your credit card, then he will immediately recognize its pin. In addition, the ability to decipher brings along the ability to forge. For example, an ATM informs the bank that thousand dollars was withdrawn, but the villain falsified the message so that it says that only one dollar has been withdrawn.
Another thing that encryption gives us is identifiability. Encryption allows you to be sure that you have entered the actual bank's website, not a fake one.
And that's half the trouble. Even when the first supervillains appear, the world will continue to use the same algorithms for a long time. Quick shift to new safety standards is not the strongest point of humanity.