Olivia is employed as a security professional at InfoGuard Ltd. She has been assigned the responsibility of safeguarding the authentication and integrity of messages transmitted across the corporate network. To achieve message encryption, she introduced a security model wherein each user within the network possesses a set of public keys. In this model, sending users encrypt messages using the recipient's public key, and only the designated recipient can decrypt the message using their private key. What is the security model implemented by Olivia to enhance the security of corporate messages?