Err, no, that's actually not true. Take the case of PBKDF2 -- if you apply 1000 rounds of PBKDF2 to password X to get X' then apply 1000 rounds to PBKDF2 to pa

Err, no, that's actually not true. Take the case of PBKDF2 -- if you apply 1000 rounds of PBKDF2 to password X to get X' then apply 1000 rounds to PBKDF2 to password X' to get X'', applying 2000 rounds of PBKDF2 to password X == X''. You could increase the cost factor (number of rounds) every time the password is used, for instance.