Although there are several ways to generate keys in JabRef, I would suggest to take a look at BibTool (http://www.gerd-neugebauer.de/software/TeX/BibTool.en.html): this provides a very flexible way to specify the numbers of characters of each word, which fields to use, etc. Apart from the predefined keys (and to maintain compatibility), the BibTool way could be added as a "user defined" keyin JabRef.
Best regards
Stefan
Updated link: http://www.gerd-neugebauer.de/software/TeX/BibTool/