In a system which requires a user to have a password, as a quick workaround I wrote random junk as the bcrypt hash so that no one will be able to login to that user. However theoretically there probably is a password which matches this randomly selected hash value.
I was wondering if it could be possible in such a scenario to specially craft a hash value (possibly using a different supported algorithm other then bcrypt) for which it would be guaranteed that there is no preimage.
For bcrypt I suspect this wouldn't be possible, since if my math is correct the expected number of hashes with no 75 char password preimage is (1/e)^(2^416) also known as 0.
But perhaps for another common password hashing algorithm these values would not only be expected but there will also be a way to find such a value.