A Secure Sockets Layer (SSL)/Transport Layer Security (TLS) certificate is required to establish a secure connection between a web server and a client using the HTTPS protocol. This certificate ensures that the data transferred is encrypted and the server's authenticity is validated by a Certificate Authority (CA). A self-signed certificate provides encryption but does not offer third-party validation of the server's identity and can cause trust issues with clients' browsers. A digital signature is used to verify that data has not been altered during transfer but does not by itself enable encrypted communications. A wildcard certificate is used to secure multiple subdomains but still requires the SSL/TLS protocol to function for secure communications.