Which feature must be enabled on iPhones to utilize passkeys?

To utilize passkeys on iPhones, it is essential to have iCloud Keychain enabled. This feature allows users to store and synchronize their passkeys across multiple Apple devices securely. iCloud Keychain provides a seamless way to manage passwords and passkeys, ensuring they are readily accessible while maintaining a level of security through end-to-end encryption. By leveraging iCloud Keychain, users can take full advantage of the passkey feature, which simplifies the login process by eliminating the need to remember complex passwords and providing a more secure authentication method. Other features like Face ID or two-factor authentication may enhance security, but they are not strictly required for the functionality of passkeys.