The pgpk command in PGP 5.x on Unix systems uses an insufficiently random data source for non-interactive key pair generation, which may produce predictable keys.
http://www.securityfocus.com/bid/1251
http://www.cert.org/advisories/CA-2000-09.html
http://archives.neohapsis.com/archives/bugtraq/2000-05/0273.html