generatig new unique string

Primary Keys should always be a numeric type (and make it auto-increment) if you have a choice. If you can still fix this, do it.
Some methods for creating unique strings can be found here (probably best)[^], here[^], here[^], and possibly some more here[^] ;)
The last method of the third one actually kind of way off creates strings of exactly length 8.
Hope it helps!