Erm no, read WanderingPhilosipher's reply above. Subtraction is good for clearing out values within a keyspace exponent, but for actually reducing the magnitude of the keyspace, division does the job.


Division deletes values on the number line. E.g something divided by 8 gives you 8 or 16 or 24... the 9-15 integers become "inaccessible", so it's possible the private key might be unreadable (because of impossible decimal remainder), meaning when you divide the keyspace by x number, not only do you have to search pubkey/x, you must also search (pubkey+1)/x, (pubkey+2)/x, ..., (pubkey+x-1)/x.