What John says about the properties of Blum Blum Shub is correct, however
PGP does not use it. PGP encrypts messages using the IDEA block cipher,
and also uses IDEA in a loop, seeded off keyboard timings, to create
its random keys.
BTW the Foresight Exchange estimates a 25% chance that IDEA will be
broken by the year 2000! (I think this is way too high.) Some other
crypto related claims: 80%-90% that a 56 bit cipher (e.g. DES) will
be publically brute- force searched before the end of 1997; 30% that a
1024 bit RSA modulus will be factored by 2010. I don't see any claims
on quantum computers, surprisingly (maybe I missed something?).
Hal