Perl
PGP (Pretty Good Privacy)
POP (point-of-presence)
POP3 (Post Office Protocol 3)
Proxy Server

Perl (Practical Extraction and Reporting Language)

Perl is an interpreted programming language similar to the C language but including a number of popular UNIX facilities such as sed, awk, and tr. It is regarded as a good choice for developing Common Gateway Interface (CGI) programs because it has good text manipulation facilities (although it also handles binary files). It was invented by Larry Wall.

In general, Perl is easier to learn and faster to code in than the more structured, compiled C and C++ languages. Perl programs can, however, be quite sophisticated. Perl tends to have devoted adherents.

PGP (Pretty Good Privacy) is a popular program used to encrypt and decrypt e-mail over the Internet. It can also be used to send an encrypted digital signature that lets the receiver verify the sender's identity and know that the message was not changed en route. Available both as freeware and in a low-cost commercial version, PGP is the most widely used privacy-ensuring program by individuals and is also used by many corporations. Developed by Philip R. Zimmermann in 1991, PGP has become a de facto standard for e-mail security. PGP can also be used to encrypt files being stored so that they are unreadable by other users or intruders.

How It Works

PGP uses a variation of the public key system. In a public key system, each user has a publicly known encryption key and a private key known only to that user. You encrypt a message you send to someone else using their public key. When they receive it, they decrypt it using their private key. Since encrypting an entire message can be time-consuming, PGP uses a faster encryption algorithm to encrypt the message and then uses the public key to encrypt the shorter key that was used to encrypt the entire message. Both the encrypted message and the short key are sent to the receiver who first uses the receiver's private key to decrypt the short key and then uses that key to decrypt the message.

PGP comes in two public key versions - RSA and Diffie-Hellman. The RSA version, for which PGP must pay a license fee to RSA, uses the IDEA algorithm to generate a short key for the entire message and RSA to encrypt the short key. The Diffie-Hellman version uses the CAST algorithm for the short key to encrypt the message and the Diffie-Hellman algorithm to encrypt the short key.

For sending digital signatures, PGP uses an efficient algorithm that generates a hash code from the user's name and other signature information. This hash code is then encrypted with the sender's private key. The receiver uses the sender's public key to decrypt the hash code. If it matches the hash code sent as the digital signature for the message, then the receiver is sure that the message has arrived securely from the stated sender. PGP's RSA version uses the MD5 algorithm to generate the hash code. PGP's Diffie-Hellman version uses the SHA-1 algorithm to generate the hash code.

To use PGP, you download or purchase it and install it on your computer system. Typically, it contains a user interface that works with your customary e-mail program. You also need to register the public key that your PGP program gives you with a PGP public-key server so that people you exchange messages with will be able to find your public key. Network Associates maintains an LDAP/HTTP public key server that has 300,000 registered public keys. This server is mirrored at other sites around the world.

Where Can You Use PGP

Originally, the U.S. government restricted the exportation of PGP technology. Today, however, PGP encrypted e-mail can be exchanged with users outside the U.S if you have the correct versions of PGP at both ends. Unlike most other encryption products, the international version is just as secure as the domestic version.

The freely available PGP cannot legally be used for commercial purposes - for that, one must obtain the commercial version from Network Associates (formerly PGP, Inc.). There are several versions of PGP in use. Add-ons can be purchased that allow backwards compatibility for newer RSA versions with older versions. However, the Diffie-Hellman and RSA versions of PGP do not work with each other since they use different algorithms.

A POP (point-of-presence) is the location of an access point to the Internet. A POP necessarily has a unique Internet (IP) address. Your Internet service provider (ISP) or online service provider (OSP) has a point-of-presence on the Internet. The number of POPs that an ISP or OSP has is sometimes used as a measure of its size or growth rate.

A POP may actually reside in rented space owned by the telecommunications carrier such as AT&T or Sprint to which the ISP is connected. A POP usually includes routers, digital/analog call aggregators, servers, and frequently frame relay or ATM switches.

POP3 (Post Office Protocol 3) is the most recent version of a standard protocol for receiving e-mail. 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. POP3 is built into the Netmanage suite of Internet products and one of the most popular e-mail products, Eudora. It's also built into the Netscape browser.

An alternative protocol is IMAP (Interactive Mail Access Protocol). With IMAP, you view your e-mail at the server as though it was on your client computer. An e-mail message deleted locally is still on the server. E-mail can be kept on and searched at the server.

POP can be thought of as a "store-and-forward" service. IMAP can be thought of as a remote file server.

POP and IMAP deal with the receiving of e-mail and are not to be confused with SMTP, a protocol for transferring e-mail across the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient's behalf. Then the mail is read using POP or IMAP.

In an enterprise that uses the Internet, a proxy server is a server that acts as an intermediary between a workstation user and the Internet so that the enterprise can ensure security, administrative control, and caching service. A proxy server is associated with or part of a gateway server that separates the enterprise network from the outside network and a firewall server that protects the enterprise network from outside intrusion.

A proxy server receives a request for an Internet service (such as a Web page request) from a user. If it passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded Web pages. If it finds the page, it returns it to the user without needing to forward the request to the Internet. If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the server out on the Internet. When the page is returned, the proxy server relates it to the original request and forwards it on to the user.

To the user, the proxy server is invisible; all Internet requests and returned responses appear to be directly with the addressed Internet server. (The proxy is not quite invisible; its IP address has to be specified as a configuration option to the browser or other protocol program.)

An advantage of a proxy server is that its cache can serve all users. If one or more Internet sites are frequently requested, these are likely to be in the proxy's cache, which will improve user response time. In fact, there are special servers called cache servers. A proxy can also do logging.

The functions of proxy, firewall, and caching can be in separate server programs or combined in a single package. Different server programs can be in different computers. For example, a proxy server may in the same machine with a firewall server or it may be on a separate server and forward requests through the firewall.