What distinguishes a public key from a private key?

A public key and a private key play crucial roles in asymmetric encryption, which is a cornerstone of modern cryptography. The correct answer highlights that public keys can be used to encrypt data, while private keys are used for decryption. This distinction is fundamental because, in a secure communication system, the public key is openly shared and can be used by anyone to encrypt messages intended for the owner of the corresponding private key.

When someone wants to send a confidential message, they use the recipient's public key to encrypt that message. Only the recipient, who possesses the corresponding private key, can decrypt the message, ensuring confidentiality. This method not only protects the content of the messages but also facilitates secure communications over an unsecured channel.

The other answers do not accurately represent the roles of public and private keys. For example, stating that public keys are kept secret and private keys are shared is misleading, as the strength of the public-private key system relies on the public key being openly available. Additionally, the role of each key in encryption and decryption is defined by the nature of asymmetric encryption, making interchangeable use of keys incorrect. Thus, understanding the distinct functions of these keys is essential for grasping the principles of encryption and secure communication.