Someone emailed me a PGP 1.0 manual, and it appears this program wasn't terribly different from the 2.x versions. It was for MS-DOS and SunOS 4.1.
Despite the fact that it did use the RSA public key encryption algorithm, its encryption scheme is completely incompatible with later versions.
Key generation options were:
The program was released by "Phil's Pretty Good Software", which I call "PPGS" in the chart to save space.
These versions of PGP used RSA encryption technology to produce basically unbreakable keys. Since more information is available on the net, I just want to concentrate on what versions will use these keys.
NO version of 2.x can read/write DH keys, except version 2.6.4.
PGP 2.3 apparently changed the way messages were signed; pre-2.3 version signatures are recognized by 2.3 through 4.5 but they 'whine' about it. PGP 5 is attempting to eliminate the pre-2.3 signatures: for example, it will ask you if you'd like them removed from your keyring.
This was the first version offered as a commercial product, by Viacrypt. They licensed the technology from Phil Zimmermann (founder of PGP, Inc.). It was the first version that could force the user to encrypt to both the sender and a company's key (for message recovery-see PGP 4.5 for Business for a more in-depth explanation of this). Bugfix versions existed, creating the .x revisions.
This version has an interesting history. The owners of the patent on the RSA algorithms allowed PGP to release this version with one implementation of their algorithms. (Free use of the algorithms, up to this point, was not clearly legitimate.) The condition on the agreement was simple: 2.5 set a flag (called "LEGAL_KLUDGE") that made it incompatible with previous versions of PGP.
These versions are still probably about as popular as some later versions. It was released free for all for non-commercial use only by Phil's Pretty Good Software (PPGS in the chart on the index page).
The Cyber-Knights Templar release of PGP 2.6.3, which supports huge RSA key sizes.
This version won't crash when you import DSS/DH keys or keys with DSS/DH signatures (previous versions will crash). Unfortunately, NAI never released it to the public. If you are looking for the ability to safely handle DSS/DH keys in a PGP 2.6.x flavor, see 2.64ui below.
This is a series of unofficial versions available here. They start with 2.6ui, then 2.62ui, 2.63ui, and the latest, 2.64ui.
The idea of this was that someone within a company could create a key and give it to the company so they might decrypt the messages, but the company would not have the ability to forge the employee's signature. Both the Personal and Business versions supported this feature.
Apparently, a message encrypted or signed with a single-function key is pretty much useless to any other version of PGP, at least up until this version and the "PGP 3.0" (see note below).
Note: the documentation mentions a "Freeware PGP 3.0" due to be released later "this year." I'm not sure what year that is, but I have received confirmation that PGP 3.0 did become PGP 5.0.
PGP 4.5 also contained plugins for Netscape 3.x and Eudora 3.x.