Originally written by Philip Zimmerman in 1991, PGP freeware has gone through many generations, changed hands more than once and had features added then removed to be reserved for paid versions only. Personal philosophies exist about which freeware PGP version is best and why. Following are the most popular versions with a brief explanation of each.

The source code of each PGP version listed has been released to the public for purposes of peer review, to assure that "no back doors" exist. When downloading any PGP version, check to make sure that a digital signature is included to verify authenticity.

PGP version 2.6.2 (DOS/MacOS Command Line) freeware:

Purists maintain that the original command line PGP version is the safest to use, if not the most convenient. Third party graphic user interfaces (GUIs) exist that help automate the process to an extent, though third party software can introduce security bugs. Hence, most people drawn to PGP version 2.6.2 will most likely not use a GUI. Upside: Very secure, very small program. Downside: Requires manual, command line use. Availability: Archival websites.

In 1996 Zimmerman partnered with Viacrypt, which then became PGP Incorporated. Windows-compatible PGP versions 4x and 5x followed, but the next watermark version came after Zimmerman sold PGP in 1997 to Network Associates, Incorporated (NAI).

PGP version 6.5.8 freeware (NAI) included native email plug-ins for Outlook, Outlook Express, Netscape and Eudora. For other clients like Pegasus Mail or Bat!, free plug-ins were available from third parties. This version also included a Virtual Private Network (VPN) plugin. However, NAIs 6.5.8 was pre-XP and is no longer widely used.

After PGP version 6.5.8, NAI decided to discontinue the practice of releasing the source code. Zimmermann, who had been a consultant for NAI, left their employ over the disagreement. PGP version 7.x was unpopular and PGP languished. During this period, unaffiliated programmer Imad Faiad took it upon himself to use the source code from various 6.x releases to further develop PGP on his own. The result was the successful, albeit controversial, PGP version "6.5.8 CKT."

PGP version 6.5.8 CKT freeware (Windows 95/98/NT/2000/XP) by Imad Faiad:

Faiad wrote several builds of v6.5.8, but build 08 is most popular. He fixed the digital signing bug, made the version XP-compatible and added back PGP Disk -— the PGP utility for encrypting volumes that had been included in earlier 6.x versions. He also added several other enhancements including the ability to use very large keys. "CKT" stands for "Cyber Knights Templar" and identifies this version of PGP as separate from NAI's.

After NAI sold PGP rights to PGP Corporation in 2002, Faiad was asked to stop development. Zimmermann's dedication to provide PGP source code was for security reasons, not to invite 'competition' to official versions. Current source code is now released under these express conditions.

That said, PGP version 6.5.8 CKT remains widely used and many prefer it to official versions for the enhancements Faiad added. Upside: XP-compatible, with email plug-ins included for Outlook, Outlook Express, Eudora and Netscape; VPN plug-in included, with fixed digital signing bug; PGP Disk included along with other enhanced features including support for very large keys. Downside: Some people believe it unwise to support an unofficial PGP version. Availability: Archival websites including Zedz.net.

Currently, Zimmermann is an adviser to PGP Corporation. This brings us to current versions, starting with PGP version 8.x -- the first official release with Windows XP compatibility.

PGP version 8.1 (All Windows and MacOS) by PGP Corp.:

The program looks very much like an updated 6.5.8, but native email plug-ins are not functional without buying a license and a VPN plug-in is not included. PGP Disk has been replaced by PGP Whole Disk, available in licensed versions only. Free third-party plug-ins can be used with non-Microsoft email clients and there are freeware alternatives to PGP Disk for encrypting volumes and drives. PGP version 8.1 appears to be the most popular freeware PGP version as of mid-2005. Upside: Great look, feature enhancements, XP-compatible. Downside: Native email plug-ins are no longer free, though free third-party plug-ins can be found for virtually all popular (non-Microsoft) email clients. Availability: Unofficial archival websites.

The newest "freeware" PGP version at the time of writing is PGP Personal Desktop 9.x Trial Version. The most notable change is a switch to a transparent email proxy, replacing email plug-ins. The proxy scans mail ports for incoming or outgoing mail and encrypts/decrypts without user intervention. This fully automated feature works free for 30 days, after which the program reverts to the equivalent of PGP version 8.1 if a license is not purchased.

PGP version 9.x is a much bulkier download, weighing in at over 20 MB, as opposed to the approximate 7MB download of 8.1. Despite its enhanced desktop interface, it does not appear to be gaining popularity among PGP freeware enthusiasts over watermark PGP version 8.1.