> Dallas DS1605

What is a DS1605 ? A search of Maxim returns no match so I'm
not sure if the following will be of use to you

> the PIC get on with generating the hash codes? This would
> increase power consumption quite significantly (80uA IIRC)...
>   BTW, the PIC I'm using is a 16F628.

I have a similar application and use an interrupt generated by
the chip from a cheap clock to wake the PIC every 2 seconds
to increment its RTC registers (a CMOS divider, eg 4040, could
be used to lengthen this period). The chip consumes very little
power (my meter reads 0.00uA at 1.5V) and is accurate to ~1
min/year. The last clock I bought to get a chip from was NZ$2

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