PGP 5 Tips:

Nota Bene: I do not pretend that the solutions I recommend are the only ones available, just that they appear to be the best ones I know of, and I wanted to make sure to avoid as many points of confusion as possible.

1. Generating and using Keys:
   Use only RSA keys. This single item will avoid more compatibility problems with PGP 5 than anything else (specifically compatibility problems between PGP 5 and older versions of PGP).
   
   
   • If you're using the US freeware version or the US "payware" version 5.5 or later (which are unable to generate RSA keys), you've got three options:
     
     
     • Import RSA keys from a previous version of PGP
       
       
     • Upgrade to the full-featured "payware" (pre-5.5) version, if you can find it
       
       
     • "Upgrade" to the old version 2.6.x code, so that you maintain backward compatibility by using the old code
     
     I cannot recommend allowing a friend to generate the key for you using his RSA-capable version of PGP -- I don't trust anyone that much.
     
     
   • Assuming you take the first of the above three options, the proper way to import your keys into PGP 5 is to import the public key part first, re-assign trust models to it, then import the secret key part. Doing these steps in any other order will almost certainly cause you to lose the trust associations you set, and you'll end up having to do that work over again.
     
     
   • If you got PGP 5.x as a part of Eudora 3.x (or greater), the version you have is unable to support RSA keys in any fashion. You need to upgrade to the full-featured "payware" (pre-5.5) version. PGP, Inc may still be running a Eudora-specific $5.00 upgrade program, but it probably won't get you a pre-5.5 version. Just in case, see http://www.pgp.com/products/eudora.cgi.
   
   
   • PGP "payware" 5.5 and later are totally incapable of generating RSA keys. If you want to generate RSA keys, you have to use an older version -- either the 2.6.x code or the "payware" pre-5.5 versions.
   
   
2. HKP keyserver problems:
   If you're having problems getting through to one of the Horowitz Key Protocol (HKP) servers on port 11371 (e.g., pgpkeys.mit.edu, horowitz.surfnet.nl, etc...), you have two alternatives:
   
   
   • Talk to your security/firewall administrator and have them put up an HTTP proxy on port 11371. HKP is actually just a subset of HTTP, and existing proxies for HTTP should work just fine, if they listen to the right port.
     
     
   • Use one of the web-to-HKP servers (e.g., http://www.pgp.net/pgpnet/pks-commands.html or http://www.pgp.com/keyserver/pks-toplev.cgi; see also http://www.pgp.net/pgpnet/www-key.html), and deal with manually importing/exporting keys via the web.
   
   
3. Mixed RSA and DH/DSS recipients:
   If you have to send a message to several recipients, and some of them have RSA keys (PGP 5 and earlier) and some of them have Diffie-Hellman/DSS keys (PGP 5 and above), you should split the set of recipients up by the type of key they have and send two copies of it (one copy to only DH/DSS recipients and one copy to only RSA recipients). This way, any recipients who do not yet have PGP 5 will still be able to read your message.
   
   If you were to send just one copy of the message to a mixture of DH/DSS and RSA keyholders, you would run the risk that older versions of PGP will not know how to deal with the session keys that have been encrypted for the DH/DSS keyholders, and therefore they'll be unable to read the message.
   
   Fortunately, at least some versions of PGP 5.x are aware of this problem and will try to help you do "The Right Thing(tm)".
   
   
4. Message formatting with PGP/MIME:
   • Note that the most recent versions of many MUAs will default to using PGP/MIME format (see RFC 2015), and will typically request that you choose the signing/encrypting key and provide your passphrase at the time you connect to your mail server to deliver your mail message, as opposed to when you queue the message for later delivery. If you set things on automatic and walk away, this could cause the connection to time out and not deliver any messages after the first one that required your interaction at the keyboard.
     
     
   • Although PGP/MIME is a standards-track RFC, there are still a wide variety of programs that do not understand it at all, or provide only minimal support for it. If at all possible, configure your programs so that they generate what I like to call "PGP Classic" format (so that they have the "--- BEGIN PGP MESSAGE ---" and "--- END PGP MESSAGE ---" type headers and footers with integral signatures), but understand both "PGP Classic" and PGP/MIME.
     
     
   • If you use PGP/MIME, be aware that some mailing list mangers (e.g., Listserv) may munge your PGP-signed messages, unless they've set "Translate=No" for the list (this is a well-known problem and documented in section 3 of RFC 2015). Your options are:
     
     
     • Configure your MUA to use "PGP Classic" format instead of PGP/MIME
       
       
     • Figure out how to get your MUA to default to a Content-transfer-encoding of Quoted-Printable instead of US-ASCII (note that this may have some collateral problems, as there is good reason why the Europeans refer to quoted-printable as "quoted-unreadable")
   
   
5. Copyright and patent law issues:
   If you're in the US, don't think that by grabbing the International version and linking it against RSAREF that you've solved all your problems. This would solve the RSA licensing problems, but does not solve the problem that PGP is copyrighted in the US, and PGP, Inc. has released only certain specific programs for noncommercial freely available use in the US. Given the recent laws that have been passed regarding copyright and extremely stiff punishments for violations thereof, this is not a sort of activity that I can condone.
   
   
6. Version availability:
   Be advised that PGP 5.x is not yet available for MS-DOS or Microsoft Windows 3.x (and probably never will be, at least not in the US), and only beta releases of the International version are available for many forms of Unix. If you get the International version source code, you need to make sure that you compile it in a manner that will be legal for use in your country, both with respect to the RSA algorithms and their implementation, as well as the copyright issues of the PGP code itself.
   
   
7. Signing keys:
   • RSA Keys. Do not use DH/DSS keys to sign RSA keys. If the person whose RSA key you are signing doesn't have PGP 5.x, then the newly signed key will be unreadable and unusable to them -- they won't be able to import your DSS signature.
     
     
   • DH/DSS keys. While it is unlikely to mess anything up to sign a DH/DSS key with an RSA key, you should sign a DH/DSS key with another DH/DSS key, if at all possible. There may be a time in the future when all RSA support has been removed from all PGP products, and the presence of an RSA signature on your DH/DSS key may present problems.

Many thanks to the posters of comp.security.pgp.discuss for some of the above items.