Solution

In public-private key encryption , also known as asymmetric encryption, the system relies on two keys: a public key and a private key . The public key is used to encrypt data, and it can be freely shared with anyone. However, the private key , which is used for decryption , must remain confidential and private. Only the holder of the private key can decrypt the data encrypted with the corresponding public key. This is the basis of secure communications, where the sender encrypts the message with the recipient's public key, and the recipient decrypts it with their private key. Option 1 (Used for encryption and shared with others) is incorrect because the private key is used for decryption, not encryption. Option 3 (Used for encryption and publicly available) describes the public key, not the private key. Option 4 (Used for decryption but shared with everyone) is incorrect because sharing the private key would compromise the security of the encryption. Option 5 (Used for authentication in place of passwords) is incorrect as private keys are used for decryption, not for authenticating in place of passwords.