Glossary

Refers to mechanisms and policies that restrict access to computer resources. An access control list (ACL), for example, specifies what operations different users can perform on specific files and directories.

Active content refers to material that is downloaded that makes something happen, as opposed to static content, such as text or simple images that do nothing but get displayed. Active content includes such things as JavaScript animations, ActiveX controls, Java spreadsheets...anything that actually does something.

ActiveX is Microsoft's answer to the Java technology from Sun Microsystems. An ActiveX control is roughly equivalent to a Java applet. ActiveX is the name Microsoft has given to a set of "strategic" object-oriented program technologies and tools. The main thing that you create when writing a program to run in the ActiveX environment is a component, a self-sufficient program that can be run anywhere in your ActiveX network (currently a network consisting of Windows and Macintosh systems). This component is known as an ActiveX control.

An automated e-mail address directory that allows you to address your messages easily. Generally comes in personal and public versions.

With anti-replay service, each IP packet passing within the secure association is tagged with a sequence number. On the receiving end, each packet's sequence number is checked to see if it falls within a specified range. If an IP packet tag number falls outside of the range, the packet is blocked.

An API is the specific methodology by which a programmer writing an application program may make requests of the operating system or another application.

Application gateways look at data at the application layer of the protocol stack and serve as proxies for outside users, intercepting packets and forwarding them to the application. Thus, outside users never have a direct connection to anything beyond the firewall. The fact that the firewall looks at this application information means that it can distinguish among such things as telnet, file transfer protocol (FTP), or Lotus Notes traffic. Because the application gateway understands these protocols, it provides security for each application it supports.

An archive is a collection of computer files that have been packaged together for backup, to transport to some other location, for saving away from the computer so that more hard disk storage can be made available, or for some other purpose. An archive can include a simple list of files or files organized under a directory or catalog structure (depending on how a particular program supports archiving).

A protocol for mapping an Internet Protocol address (IP address) to a physical machine address that is recognized in the local network.

Asymmetric or public key cryptography is based on the concept of a key pair. Each half of the pair (one key) can encrypt information so that only the other half (the other key) can decrypt it. One part of the key pair, the private key, is known only by the designated owner; the other part, the public key, is published widely but is still associated with the owner.

A file that a user adds to an email message to transfer it to another user.

The process of determining the identity of a user that is attempting to access a network. Authentication occurs through challenge/response, time-based code sequences or other techniques. See CHAP and PAP.

The Authentication Header is a mechanism for providing strong integrity and authentication for IP datagrams. It might also provide non-repudiation, depending on which cryptographic algorithm is used and how keying is performed. For example, use of an asymmetric digital signature algorithm, such as RSA, could provide non- repudiation.

The process of determining what types of activities or access are permitted on a network. Usually used in the context of authentication: once you have authenticated a user, they may be authorized to have access to a specific service.

Generally speaking, bandwidth is directly proportional to the amount
of data transmitted or received per unit time. In digital systems, bandwidth is proportional to the data speed in bits per second (bps). Thus, a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps.

A specific host that is used to intercept packets entering or leaving a network. and the system that any outsider must ordinarily connect with to access a system or service that is inside the network's firewall. Typically the bastion host must be highly secured because it is vulnerable to attack due to its placement. See dual-homed gateway.

A buffer overflow attack works by exploiting a known bug in one of the applications running on a server. It then causes the application to overlay system areas, such as the system stack, thus gaining administrative rights. In most cases, this gives a hacker complete control over the system. Also referred to as stack overflow.

A CA (certificate authority) is an authority in a network that issues and manages security credentials and public keys for message encryption and decryption. As part of a public key infrastructure (PKI), a CA checks with a registration authority (RA) to verify information provided by the requestor of a digital certificate. If the RA verifies the requestor's information, the CA can then issue a certificate.

When a denial of service attack is aimed at the CGI, it is referred to as a CGI exploit. The CGI (common gateway interface) is a standard way for a Web server to pass a Web user's request to an application program and to receive data back to forward to the user.  It is part of the Web's HTTP protocol.

A common authentication technique whereby an individual is prompted (the challenge) to provide some private information (the response). Most security systems that rely on smart cards are based on challenge-response. A user is given a code (the challenge) which he or she enters into the smart card. The smart card then displays a new code (the response) that the user can present to log in.

An authentication technique where after a link is established, a server sends a challenge to the requestor. The requestor responds with a value obtained by using a one-way hash function. The server checks the response by comparing it its own calculation of the expected hash value. If the values match, the authentication is acknowledged otherwise the connection is usually terminated.

A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match, it's assumed that the complete transmission was received.

Circuit-level gateways run proxy applications at the session layer instead of the application layer. They can't distinguish different applications that run on the same protocol stack. However, these gateways don't need a new module for every new application, either. Circuit-level gateway is a firewall feature which can, when needed, serve as an alternative to packet filtering or application gateway functionality.

A client is the requesting program or user in a client/server relationship. For example, the user of a Web browser is effectively making client requests for pages from servers all over the Web. The browser itself is a client in its relationship with the computer that is getting and returning the requested HTML file.

The ability to block network traffic based on actual packet content.

The ability to review the actual information that an end user sees when using a specific Internet application. For example, the content of e-mail.

See data driven attack. Commonly protected against with a virus scanner.

Class of Service (CoS) is a way of managing traffic in a network by grouping similar types of traffic (for example, e-mail, streaming video, voice, large document file transfer) together and treating each type as a class with its own level of service priority.

A RedCreek hardware implementation that offloads the heavy computational load usually imposed by cryptographic tasks, freeing system resources and thus allowing rapid encryption.

A program that runs continuously and exists for the purpose of handling periodic service requests that a computer system expects to receive. The daemon program forwards the requests to other programs (or processes) as appropriate. Each server of pages on the Web has an HTTPD or Hypertext Transfer Protocol daemon that continually waits for requests to come in from Web clients and their users.

A form of intrusion in which the attack is encoded in seemingly innocuous data, and it is subsequently executed by a user or other software to actually implement the attack.

A widely-used method of data encryption using a private (secret) key that was judged so difficult to break by the U.S. government that it was restricted for exportation to other countries. There are 72,000,000,000,000,000 (72 quadrillion) or more possible encryption keys that can be used. For each given message, the key is chosen at random from among this enormous number of keys. Like other private key cryptographic methods, both the sender and the receiver must know and use the same private key.

A user or program takes up all the system resources by launching a multitude of requests, leaving no resources and thereby "denying" service to other users. Typically, denial-of-service attacks are aimed at bandwidth control.

The Diffie-Hellman Method For Key Agreement allows two hosts to create and share a secret key. VPNs operating on the IPSec standard use the Diffie-Hellman method for key management. Key management in IPSec begins with the overall framework called the Internet Security Association and Key Management Protocol (ISAKMP). Within that framework is the Internet Key Exchange (IKE) protocol. IKE relies on yet another protocol known as OAKLEY and it uses Diffie-Hellman.

Differential service mechanisms allow providers to allocate different levels of service to different users of the Internet. Broadly speaking, any traffic management or bandwidth control mechanism that treats different users differently - ranging from simple Weighted Fair Queuing to RSVP and per-session traffic scheduling - counts. However, in common Internet usage the term is coming to mean any relatively simple, lightweight mechanism that does not depend entirely on per-flow resource reservation.

A digital certificate is an electronic "credit card" that establishes your credentials when doing business or other transactions on the Web. It is issued by a certification authority (CA). It contains your name, a serial number, expiration dates, a copy of the certificate holder's public key (used for encrypting and decrypting messages and digital signatures), and the digital signature of the certificate-issuing authority so that a recipient can verify that the certificate is real.

A digital signature is an electronic rather than a written signature that can be used by someone to authenticate the identity of the sender of a message or of the signer of a document. It can also be used to ensure that the original content of the message or document that has been conveyed is unchanged. Additional benefits to the use of a digital signature are that it is easily transportable, cannot be easily repudiated, cannot be imitated by someone else, and can be automatically time-stamped.

A network added between a protected network and an external network in order to provide an additional layer of security. Sometimes called a perimeter network.

The Internet protocol for mapping host names, domain names and aliases to IP addresses.

Breaching the trust relationship by assuming the DNS name of another system. This is usually accomplished by either corrupting the name service cache of a victim system or by compromising a domain name server for a valid domain.

The unique name used to identify an Internet network.

A repository of addressing information for specific Internet hosts. Name servers use the domain name system to map IP addresses to Internet hosts.

A "downloadable" is a file that has been transmitted from one computer system to another, usually smaller computer system. From the Internet user's point-of-view, to download a file is to request it from another computer (or from a Web page on another computer) and to receive it.

A post office that communicates with a mail server through another post office or other post offices.

The Digital Signature Standard (DSS) is a cryptographic standard promulgated by the National Institute of Standards and Technology (NIST) in 1994. It has been adopted as the federal standard for authenticating electronic documents, much as a written signature verifies the authenticity of a paper document.

A proprietary technology that is patented and works in the following way. The operating system has a system call (or vector) table that contains memory address pointers for each system call. These pointers point to a location in memory where the actual kernel code of the system calls resides. DSX stores the address pointers for the security sensitive system calls and then redirects these pointers to the corresponding SECURED system call code, which is located elsewhere in memory.

A system that has two or more network interfaces, each of which is connected to a different network. In firewall configurations, a dual-homed gateway usually acts to block or filter some or all of the traffic trying to pass between the networks.

e-business" ("electronic business," derived from such terms as "e-mail" and "e-commerce") is the conduct of business on the Internet, not only buying and selling but also servicing customers and collaborating with business partners.

e-commerce (electronic commerce or EC) is the buying and selling of goods and services on the Internet, especially the World Wide Web. In practice, this term and e-business are often used interchangeably. For online retail selling, the term e-tailing is sometimes used.

An application from which users can create, send and read e-mail messages.

An application that controls the distribution and storage of e-mail messages.

A set of software that facilitates the electronic processing of business transactions using e-mail as an enabling technology.

Scrambling data in such a way that it can only be unscrambled through the application of the correct cryptographic key.

A security mode in which a Ravlin unit encrypts the IP packet's payload only (without encrypting the packet header). Because EIP does not require encryption of the IP header or encapsulation of the IP packet, overhead is lower and performance enhanced.

The Encapsulating Security Payload provides confidentiality for IP datagrams or packets, which are the message units that the Internet Protocol deals with and that the Internet transports, by encrypting the payload data to be protected. I

A local-area network (LAN) protocol developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology and supports data transfer rates of 100Mbps.

An executable is a file that contains a program - that is, a particular kind of file that is capable of being executed or run as a program in the computer.

An interface developed by Microsoft that provides messaging functions including addressing, sending, receiving and storing messages.

A set of ANSI protocols for sending digital data over fiber optic cable. FDDI networks are token-passing networks, and support data rates of up to 100 Mbps (100 million bits) per second. FDDI networks are typically used as backbones for wide-area networks.

A filter is a program or section of code that is designed to examine each input or output request for certain qualifying criteria and then process or forward it accordingly. .

A firewall is a program that protects the resources of one network from users from other networks. Typically, an enterprise with an intranet that allows its workers access to the wider Internet will want a firewall to prevent outsiders from accessing its own private data resources.

The firewall is specifically subjected to a denial-of-service attack.

FTP is the simplest way to exchange files between computers on the Internet. Like the Hypertext Transfer Protocol (HTTP), which transfers displayable Web pages and related files, and the Simple Mail Transfer Protocol (SMTP), which transfers e-mail, FTP is an application protocol that uses the Internet's TCP/IP protocols.

A gateway is a network point that acts as an entrance to another network. In a company network, a proxy server acts as a gateway between the internal network and the Internet. A gateway may also be any machine or service that passes packets from one network to another network in their trip across the Internet.

Terminals that are designed to be centrally-managed, configured with only essential equipment, and devoid of CD-ROM players, diskette drives, and expansion slots (and therefore lower in cost).

GSM is an open, non-proprietary system that is constantly evolving. One of its great strengths is the international roaming capability. This gives consumers seamless and same standardized same number contact ability in more than 159 countries. GSM satellite roaming has extended service access to areas where terrestrial coverage is not available.

Hacker is a term used by some to mean "a clever programmer" and by others, especially journalists or their editors, to mean "someone who tries to break into computer systems."

HDML is a language that allows the text portions of Web pages to be presented on cellular telephone and personal digital assistants (PDA) via wireless access. Developed by Unwired Planet, HDML is an open language offered royalty-free.

Control of a connection is taken by the attacker after the user authentication has been established.

HMAC is a hash function based message authentication code that was designed to meet the requirements of the IPSEC working group in the IETF, and is now a standard.

A standard set of commands used to structure documents and format text so that it can be used on the Web.

HTTP is the set of rules for exchanging files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. Relative to the TCP/IP suite of protocols (which are the basis for information exchange on the Internet), HTTP is an application protocol.

Intelligent Input/Output (I2O) is a hardware specification that describes a model for offloading I/O processing from the CPU. The model is after the style of what has been used in very large mainframes for years. It is not a replacement for the PCI architecture.

A standard protocol for accessing e-mail from your local server. IMAP (the latest version is IMAP4) is a client/server protocol in which e-mail is received and held for you by your Internet server. You (or your e-mail client) can view just the heading and the sender of the letter and then decide whether to download the mail. You can also create and manipulate folders or mailboxes on the server, delete messages, or search for certain parts or an entire note. IMAP requires continual access to the server during the time that you are working with your mail.

An attack originating from inside a protected network.

A hybrid protocol whose purpose is to negotiate, and provide authenticated keying material for, security associations in a protected manner. Processes which implement this protocol can be used for negotiating virtual private networks (VPNs) and also for providing a remote user from a remote site (whose IP address need not be known beforehand) access to a secure host or network.

Detection of break-ins or break-in attempts by reviewing logs or other information available on a network.

The Internet Protocol is the method or protocol by which data is sent from one computer to another on the Internet. Each computer (known as a host) on the Internet has at least one address that uniquely identifies it from all other computers on the Internet.

An attack where the attacker impersonates a trusted system by using its IP network address.

An attack where an active, established session is intercepted and taken over by the attacker. May take place after authentication has occurred which allows the attacker to assume the role of an already authorized user.

A developing standard for security at the network or packet processing layer of network communication. IPSec provides two choices of security service: Authentication Header (AH), which essentially allows authentication of the sender of data, and Encapsulating Security Payload (ESP), which supports both authentication of the sender and encryption of data as well.

Java is a programming language expressly designed for use in the distributed environment of the Internet. It was designed to have the "look and feel" of the C++ language, but it is simpler to use than C++ and enforces a completely object-oriented view of programming. Java can be used to create complete applications that may run on a single computer or be distributed among servers and clients in a network. It can also be used to build small application modules or applets for use as part of a Web page. Applets make it possible for a Web page user to interact with the page.

An authentication service developed at MIT based on a paper by Needham and Schoeder.

In cryptography, a key is a variable value that is applied using an algorithm to a string or block of unencrypted text to produce encrypted text. The length of the key generally determines how difficult it will be to decrypt the text in a given message.

The establishment and enforcement of message encryption and authentication procedures, in order to provide privacy-enhanced mail (PEM) services for electronic mail transfer over the Internet.

LDAP (Lightweight Directory Access Protocol) is an emerging software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network, whether on the Internet or on a corporate intranet. LDAP is a "lightweight" (smaller amount of code) version of DAP (Directory Access Protocol), which is part of X.500, a standard for directory services in a network.

Litigation protection is both the review and recording of Internet, intranet and extranet communications that is done in order to avoid litigation or the documentation of the communications parties and content in the event of litigation.

On a network, the MAC (Media Access Control) address is your computer's unique hardware number. The MAC address is used by the Media Access Control sublayer of the Data-Link Control (DLC) layer of telecommunication protocols. There is a different MAC sublayer for each physical device type. The Data-Link Layer is the protocol layer in a program that handles the moving of data in and out across a physical link in a network.

Macro viruses are small programs written using the internal programming language of a specific application program that replicate within documents created by the application program. Common examples of application programs that use macros include word processors such as Word and spreadsheets such as Excel.

Malicious code is any code added, changed, or removed from a software system in order to intentionally cause harm or subvert the intended function of the system. Traditional examples of malicious code include viruses, worms, Trojan Horses, and attack scripts, while more modern examples include Java attack applets and dangerous ActiveX controls.

The insertion of arbitrary streams of data without the user noticing it.

An interface developed by Microsoft that provides messaging functions including addressing, sending, receiving and storing messages. Simple MAPI includes some of these functions. Extended MAPI includes all of these functions.

A protocol used for transmitting documents with different formats via the Internet.

A view of individual user activity on a network, generally in real time. Provides administrators with the ability to view the content of user utilized applications.

A base technology for using label switching in conjunction with network layer routing and for the implementation of that technology over various link level technologies, which may include Packet-over-Sonet, Frame Relay, ATM, and Ethernet 

NAPT is a special case of NAT, where many IP numbers are hidden behind a number of addresses, but in contrast to the original  NAT this does not mean there can be only that number of connections at a time. In NAPT an almost arbitrary number of connections is multiplexed using TCP port information. The number of simultaneous connections is limited by the number of addresses multiplied by the number of TCP ports available.

A simplified IP addressing capability that eliminates the need to establish an intermediate IP address between a router and a firewall. Sometimes called Proxy-ARP. This feature allows the implementation of a firewall into an existing network without having to establish a new IP address scheme.

Network Address Translation allows your Intranet to use addresses that are different from what the outside Internet thinks you are using. It permits many users to share a single external IP address at the same time. The NAT provides what some people call "address hiding", which is, as it suggests, security through obscurity at best.

An organization with the mission to continually improve commercial computer security through certification of firewalls, anti-virus products and web sites. NCSA also shares and disseminates information concerning information security.

A high level, issue specific policy which defines those services that will be allowed or explicitly denied from a restricted network, the way in which these services will be used, and the conditions for exceptions to the policy.

NNTP (Network News Transfer Protocol) is the predominant protocol used by computers (servers and clients) for managing the notes posted on  newsgroups. NNTP replaced the original Usenet protocol, UNIX-to-UN

ODBC is a standard or open application programming interface (API) for accessing a database. By using ODBC statements in a program, you can access files in a number of different databases, including Access, dBase, DB2, Excel, and Text. In addition to the ODBC software, a separate module or driver is needed for each database to be accessed.

A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network. When any file (e-mail message, HTML file, GIF file, URL request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).

Packet filters keep out certain data packets based on their source and destination addresses and service type. Filters can be used to block connections from or to specific hosts, networks or ports. Packet filters are simple and fast. However, they make decisions based on a very limited amount of information.

Intercepting packets of information (including such things for example as a credit card number ) that are traveling between locations on the Internet.

A procedure used to validate a connection request. After the link is established, the requestor sends a password and an id to the server. The server either validates the request and sends back an acknowledgement, terminates the connection, or offers the requestor another chance.

An attack where repetitive attempts are made to duplicate a valid log-in and/or password sequence.

See DMZ.

A cryptographic product family that enables people to securely exchange  messages, and to secure files, disk volumes and network connections with both privacy and strong authentication.

A notorious exploit that (when first discovered) could be easily used to crash a wide variety of machines by overrunning the size limits in their TCP/IP stacks. The term is now used to refer to any nudge delivered by hackers over the network that causes bad things to happen on the system being nudged.

A PKI (public key infrastructure) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority.

An attack that is focuses on vulnerabilities in the operating system hosting the firewall.

Policy Enforced Networking defines and automates the creation, delivery and enforcement of business rules within an information network.

The Policy Management Zone protects communications between trusted parties and firewalls access to untrusted domains in an information network.

Polymorphic viruses encrypt the body of the virus in an attempt to hide its signature from anti-virus programs.

An e-mail protocol used to retrieve e-mail from a remote server over an Internet connection. POP3 is a client/server protocol in which e-mail is received and held for you by your Internet server. Periodically, you (or your client e-mail receiver) check your mail-box on the server and download any mail.

Point-to-Point Protocol (PPP) is a protocol for communication between two computers using a serial interface, typically a personal computer connected by phone line to a server.

Point-to-Point Tunneling Protocol (PPTP) is a network protocol that enables the secure transfer of data from a remote client to a private enterprise server by creating a virtual private network (VPN) across TCP/IP-based data networks. PPTP supports on-demand, multi-protocol, virtual private networking over public networks, such as the Internet.

In cryptography, a private or secret key is an encryption/decryption key known only to the party or parties that exchange secret messages. In traditional secret key cryptography, a key would be shared by the communicators so that each could encrypt and decrypt messages. The risk in this system is that if either party loses the key or it is stolen, the system is broken. A more recent alternative is to use a combination of public and private keys. In this system, a public key is used together with a private key.

A special set of rules for communicating that the end points in a telecommunication connection use when they send signals back and forth. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between the end points in communicating programs within the same computer or at different locations. Both end points must recognize and observe the protocol. Protocols are often described in an industry or international standard.

A

An agent that acts on behalf of a user, typically accepting a connection from a user and completing a connection on behalf of the user with a remote host or service. See also gateway and proxy server.

A proxy server is one that acts on behalf of one or more other servers, usually for screening, firewall, caching, or a combination of these purposes. Gateway is often used as a synonym for "proxy server." Typically, a proxy server is used within a company or enterprise to gather all Internet requests, forward them out to Internet servers, and then receive the responses and in turn forward them to the original requestor within the company.

A public key is a value provided by some designated authority as a key that, combined with a private key derived from the public key, can be used to effectively encrypt and decrypt messages and digital signatures. The use of combined public and private keys is known as asymmetric encryption. A system for using public keys is called a public key infrastructure (PKI).

On the Internet and in other networks, QoS is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance. QoS is of particular concern for the continuous transmission of high-bandwidth video and multimedia information.

An RA (registration authority) is an authority in a network that verifies user requests for a digital certificate and tells the certificate authority (CA) to issue it. RAs are part of a public key infrastructure (PKI), a networked system that enables companies and users to exchange information and money safely and securely.

RADIUS (Remote Authentication Dial-In User Service) is a client/server protocol and software that enables remote access servers to communicate with a central server to authenticate dial-in users and authorize their access to the requested system or service. RADIUS allows a company to maintain user profiles in a central database that all remote servers can share.

A feature built into Windows NT that enables users to log into an NT-based LAN using a modem, X.25 connection or WAN link. RAS works with several major network protocols, including TCP/IP, IPX, and NetBEUI.

The oldest routing protocol on the Internet and the most commonly used routing protocol on local area IP networks. Routers use RIP to periodically broadcast which networks they know how to reach.

On the Internet, an agent (also called an intelligent agent) is a program that gathers information or performs some other service without your immediate presence and on some regular schedule. Typically, an agent program, using parameters you have provided, searches all or some part of the Internet, gathers information you're interested in, and presents it to you on a daily or other periodic basis.

One of the fundamental encryption algorithms or series of mathematical actions developed in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman. The RSA algorithm is the most commonly used encryption and authentication algorithm and is included as part of the Web browsers from Netscape and Microsoft.

A computer software ratings system of Web site content developed by RSACI in response to the passage of US federal legislation prohibiting the transmittal of offensive, or indecent, materials over the Internet. RSACi was developed with the express intent of providing a simple, yet effective rating system for web sites which protect both children, by providing and empowering parents with detailed information about site content, and the rights of free speech of everyone who publishes on the World Wide Web.

Criteria that are used to organize and control incoming messages automatically. When you set up a rule, you designate the criteria that selects a specific class of messages and then you select one or more actions to handle the messages that meet the criteria.

A router configured to permit or deny traffic based on a set of permission rules installed by the administrator.

A Security Association (SA) is a relationship between two or more entities that describes how the entities will utilize security services to communicate securely. This relationship is represented by a set of information that can be considered a contract between the entities. The information must be agreed upon and shared between all the entities.

A one-way cryptographic function which takes a message produces a 160-bit message digest. A message digest is a value generated for a message or document that is unique to that message, and is sometimes referred to as a "fingerprint" of that message or data. Once a message digest is computed, any subsequent change to the original data will, with a very high probability, cause a change in the message digest, and the signature will fail to verify. This process is used to compress large data strings to a 20-byte length which is used in a cryptographic process. The reduced data length relieves computational requirements for data encryption.

In the Open Systems Interconnection (OSI)  communications model, the Session layer (sometimes called the "port layer") manages the setting up and taking down of the association between two communicating end points that is called a connection. A connection is maintained while the two end points are communicating back and forth in a conversation or session of some duration. Some connections and sessions last only long enough to send a message in one direction. However, other sessions may last longer, usually with one or both of the communicating parties able to terminate it.

SGML is the Standard Generalized Markup Language, the international standard for defining descriptions of the structure and content of different types of electronic documents.

A mailbox that stores messages for an entire domain that allows organizations with part-time Internet connections to exchange mail.

Viruses employ signatures by which they identify themselves to themselves and thereby avoid corrupting their own code. Standard viruses, including most macro viruses, use character-based signatures. More complex viruses, such as polymorphic viruses, use algorithmic signatures.

SLIP is a TCP/IP protocol used for communication between two machines that are previously configured for communication with each other.