Public Key Cryptography

Public key cryptography is a cryptographic method that separates the key used for encryption (the public key) from the key used for decryption (the private key). The public key can be distributed to anyone, and only its owner holds the private key. Data encrypted with the public key can only be decrypted with the corresponding private key, and this solved the key distribution problem.

HTTPS communication on websites is based on this technology. When a URL stored in a QR code starts with https://, communication after the scan is protected by public key cryptography. The icon shown in the address bar (its design differs by browser and version) indicates that the connection is encrypted with TLS, and it is not a sign meaning "public key cryptography is active". Public key cryptography is used at the entrance of the connection, that is, the part that verifies the signature of the server certificate and safely hands over the shared key used in the communication that follows. The encryption of the communication itself is handled by the shared key handed over there.

Representative algorithms include RSA and elliptic curve cryptography (ECC). It has been pointed out that advances in quantum computers may break current public key cryptography in the future, and the standardization of post-quantum cryptography is being advanced, led by NIST.