confidentiality. Encrypting the communications between mail servers to protect the confidentiality of both message body and message header. The first two methods, message signing and message body encryption, are often used together; however, encrypting the transmissions between mail servers is typically used only when two organizations want to protect emails regularly sent between them. For example, the organizations could establish a virtual private network (VPN) to encrypt communications between their mail servers. Unlike methods that only encrypt a message body, a VPN can encrypt all communication over the connection, including email header information such as senders, recipients, and subjects. However, a VPN does not provide a message signing mechanism, nor can it provide protection for email messages along the entire route from sender to recipient. Message Authentication Code A Message authentication code (MAC) is a cryptography method that uses a secret key to digitally sign a message. This method outputs a MAC value that can be decrypted by the receiver, using the same secret key used by the sender. The Message Authentication Code protects both a message's data integrity as well as its authenticity. Firewalls A computer firewall controls access to a single computer. A network firewall controls access to an entire network. A firewall is a security device — computer hardware or software — that filters traffic and blocks outsiders. It generally consists of gateways and filters. Firewalls can also screen network traffic and block traffic deemed unauthorized. Web security Firewalls restrict incoming and outgoing network packets. Only authorized traffic is allowed to pass through it. Firewalls create checkpoints between networks and computers. Firewalls can block traffic based on IP source and TCP port number. They can also serve as the platform for IPsec. Using tunnel mode, firewalls can implement VPNs. Firewalls can also limit network exposure by hiding the internal network from the public Internet. Types of firewall Packet filter A packet filter processes network traffic on a packet-by-packet basis. Its main job is to filter traffic from a remote IP host, so a router is needed to connect the internal network to the Internet. The router is known as a screening router, which screens packets leaving and entering the network. Stateful packet inspection In a stateful firewall the circuit-level gateway is a proxy server that operates at the network level of an Open Systems Interconnect (OSI) model and statically defines what traffic will be allowed. Circuit proxies