SSL, HTTPS, S-HTTP, S/MIME, and SSH

Services that are used to connect to systems such as Telnet, FTP, and Sendmail are considered insecure services because data is transported in an unencrypted format. Data can be captured as it crosses the wire. Much like the message portion of a postcard, which anybody can read, these cleartext protocols are vulnerable to anyone who decides to monitor that traffic. Several of these services can be replaced with secure, encrypted versions, such as SSL, S-HTTP, HTTPS, S/MIME, and SSH:

• Secure Sockets Layer (SSL) and HTTPS— SSL is an encryption method that supports several different encryption protocols for client and server authentication. SSL operates at the transport layer. Data transferred over SSL is encrypted and is used in many online shopping applications. If you are going to run an e-commerce site on your Linux server, you will have to use SSL to encrypt information. HTTPS is the protocol that uses SSL.

• S-HTTP— S-HTTP is a security protocol utilized by Internet applications. It was designed to provide confidentiality, authentication, integrity, and nonrepudiation and to support multiple key-management implementations. It encrypts information. It is not as widely used as SSL.

• Secure Multipurpose Internet Mail Extension (S/MIME)— S/MIME is used to encrypt electronic mail and other types of messages. This open standard is being implemented more often as Internet commerce expands.

• SSH— SSH (secure shell) is the de facto communication program used for login to remote systems. It provides encryption of the entire process and can replace such services as Telnet, rsh, and rlogin. It uses public-key cryptography to encrypt communications between two hosts and for authentication.