[cryptography] Password non-similarity?
solar at openwall.com
Mon Jan 2 22:12:14 EST 2012
On Mon, Jan 02, 2012 at 09:40:36PM -0500, Jonathan Katz wrote:
> Say passwords are chosen uniformly from a space of size N. If you never
> change your password, then an adversary is guaranteed to guess your
> password in N attempts, and in expectation guesses your password in N/2
> If you change passwords constantly, and an adversary guesses a random
> password (with replacement) each password-guessing attempt, then in
> expectation the adversary guesses your password in N attempts.
Not exactly. In N attempts, assuming that N is very large, their chance
will be more like 1-1/e, which is around 63%. For a 50% chance, I think
they need to try merely N*ln(2) passwords, or about 69% of N.
> Not much of an advantage.
Right. About 39% of extra effort for the attacker (50% to 69% of the
keyspace to test) for a 50% chance.
More information about the cryptography