The AH header is 24 bytes and includes a security parameter index (SPI) field, a sequence number field and an authentication data with variable length. The SPI iden- tifies a security association (SA) while the sequence number field is counter for anti- replay service. AH can be used in the transport mode and the tunnel mode. 6.13 CONFIDENTIALITY AND INTEGRITY FOR MAIL BY S/MIME E-mail has been used within the Internet community for a very long time. From the beginning, all e-mails were text-based, and text was the only transferred information. Later on, the mail standards were extended to embrace enhanced text, graphics and audio in the body part of the mail. The new standard was multipurpose Internet mail extensions (MIME). The Internet standard secure MIME or S/MIME was developed to add security by providing authentication, integrity, confidentiality and non-repudiation for Internet e-mail. S/MIME is also attractive for HTTP information retrieval and for corporate mails, now that more and more mobile users need to communicate securely with colleagues on corporate intranets. See Figure 6.26. S/MIME relies on the existence of a PKI under the control of a trusted CA. S/MIME is fundamentally used for point-to-point security. Security Security mail gateway Internal mail Intranet user server Corporate PKI server Corporate network Mail server Firewall (IMAP/POP) Firewall IMAP/POP Internet Mobile user Extranet or mail service provider Email in plain text Email protected with S/MIME PKI traffic cert processing Figure 6.26 Corporate e-mail architecture example - The Internet and corporate sections fire- wall-security mail gateway protected by S/MIME 6.14 APPLICATIONS AND SOLUTIONS 6.14.1 VPN As mentioned before, the main VPN idea is to create a secure path through the insecure Internet environment by using tunnelling, encryption, keys and other security enablers. VPN technology allows an organization to securely extend its networking services over the Internet to remote users, branch offices and partner companies, allow