Re: Password encrypting
On Fri, 1 Jun 2001, Izak Burger wrote:
[cut]
> The "salt" is some random value that is used in the encryption
> algorithm, two-characters chosen from the set [a-zA-Z0-9./]. These
> two characters are then stored as the first two characters of the crypted
> password. That way, when you log in, the password you type along with the
> first two characters of the crypted password is handed to crypt(). If the
> resulting string is the same as the stored encrypted password, you are
> allowed to log in.
(nitpicking: it's technically not encrypting the password -- it's hashing)
As an additional comment, the reason for doing this is to make dictionary
attacks harder. Without the salt, an attacker could produce a database of
common words, names, etc. (whatever things people often use for
passwords) along with their hashed values, and then compare the database
contents with the contents of /etc/shadow. With the salt, the attacker
would need a database with his/her wordlist along with hashed values for
all possible salt values, which would be very expensive (computationally
and space-wise).
Of course, just because the system uses a salt doesn't mean you can feel
safe using a common word as your password.
Another reason for salting is that if two users happen to use the same
password (which shouldn't happen if they followed good password selection
rules) you wouldn't want either user to know about this by looking at
/etc/shadow.
--
Hubert Chan
Research Associate
Prediction in Interacting Systems (MITACS-PINTS)
University of Alberta
Office: CAB 522
Ph: 492-4394
e-mail: hubert@math.ualberta.ca
Reply to: