crypt function takes a password, key, as a string, and
a salt character array which is described below, and returns a
printable ASCII string which starts with another salt. It is believed
that, given the output of the function, the best way to find a key
that will produce that output is to guess values of key until the
original value of key is found.
The salt parameter does two things. Firstly, it selects which
algorithm is used, the MD5-based one or the DES-based one. Secondly, it
makes life harder for someone trying to guess passwords against a file
containing many passwords; without a salt, an intruder can make a
crypt on it once, and compare the result with all the
passwords. With a salt, the intruder must run
for each different salt.
For the MD5-based algorithm, the salt should consist of the string
$1$, followed by up to 8 characters, terminated by either
$ or the end of the string. The result of
will be the salt, followed by a
$ if the salt didn't end
with one, followed by 22 characters from the alphabet
./0-9A-Za-z, up to 34 characters total. Every character in the
key is significant.
For the DES-based algorithm, the salt should consist of two
characters from the alphabet
./0-9A-Za-z, and the result of
crypt will be those two characters followed by 11 more from the
same alphabet, 13 in total. Only the first 8 characters in the
key are significant.
The MD5-based algorithm has no limit on the useful length of the
password used, and is slightly more secure. It is therefore preferred
over the DES-based algorithm.
When the user enters their password for the first time, the salt
should be set to a new string which is reasonably random. To verify a
password against the result of a previous call to
the result of the previous call as the salt.