As mentioned in the Getting Started With PGP page, choosing a version of PGP can be a bit complex. With most software, you just get the newest version, but the versions are a bit more complex with PGP. For each revision level, there generally exist several different "feature set" versions of PGP. For various revision levels, there have been officially-released commercial, freeware, and trial (time-limited) versions. However, since (1) PGP is/was Open Source and (2) PGP was export-restricted encryption software, semi-official and unofficial International versions of PGP have also appeared on the Internet. These generally have better feature sets than the US Freeware versions. In some cases, the International versions have the full feature sets of the US Commercial versions.
One area of interest is the type and size of keys supported. If you want the greatest possible flexibility here, you should look at the CKT versions. The CKT versions are basically extensions to the semi-official International versions. The International versions all allow generation and use of both RSA and DH/DSS key types. Key sizes of the International versions are generally limited to the same sizes as the official versions -- 2048 for RSA, and 4096/1024 for DH/DSS. The CKT (Cyber Knights Templar) versions extend the maximum size to 16384 for RSA and 16384/1024 for DH/DSS. Additionally, the CKT versions allow more flexibility in combining public-key and conventional algorithms. PGP uses RSA and DH/DSS algorithms for public-key encryption; and CAST, IDEA and Triple-DES for conventional encryption. The CKT versions allow all valid combinations for public-key algorithm and conventional algorithm, while some of the Freeware and International versions do not. The 6.5.2 commercial version also supports all valid combinations, but does not support extended key sizes.
One cool, albeit minor, feature of the CKT versions is the ability to change the "Version ID" text that PGP inserts at the beginning of any encrypted, signed, or key text block. With the CKT versions, you can appear to others to be using any version of PGP you wish.
Sadly, the CKT versions of PGP are no longer being developed. The combination of relaxed US export restrictions on encryption and the decisions by NAI and PGP Inc. to not freely release source code on newer versions has effectively blocked new PGPi and CKT versions.
To learn more about PGP, start with the Help within PGP and the PGP Manual. Then follow some of the links on my main PGP page to find more information. Cryptography is a very complex science. There are many interesting and informative articles about PGP and other encryption software and algorithms on the Web. Some are by professionals, some by knowledgeable amateurs. There is much too much information for me to include here. While I may add information here from time to time, I heartily recommend browsing some of the other related sites to get a more complete picture.
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