Digital Certificates Question:

Factoring has become easier over the last 15 years for two reasons: computer hardware has become more powerful, and better factoring algorithms have been developed.

Hardware improvement will continue inexorably, but it is important to realize that hardware improvements make RSA more secure, not less. This is because a hardware improvement that allows an attacker to factor a number two digits longer than before will at the same time allow a legitimate RSA user to use a key dozens of digits longer than before; a user can choose a new key a dozen digits longer than the old one without any performance slowdown, yet a factoring attack will become much more difficult. Therefore, although the hardware improvement does help the attacker, it helps the legitimate user much more. This general rule may fail in the sense that factoring may take place using fast machines of the future, attacking RSA keys of the past; in this scenario, only the attacker gets the advantage of the hardware improvement. This consideration argues for using a larger key size today than one might otherwise consider warranted. It also argues for replacing one's RSA key with a longer key every few years, in order to take advantage of the extra security offered by hardware improvements. This point holds for other public-key systems as well.