32.3 Encrypting Passwords

— Function: char * crypt (const char *key, const char *salt)
The 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 guess, run crypt on it once, and compare the result with all the passwords. With a 
salt, the intruder must run crypt once 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 another $ or the end of the string. The result of crypt 
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 crypt, pass the result of the previous call as the salt.

The following short program is an example of how to use crypt the first time a password is 
entered. Note that the salt generation is just barely acceptable; in particular, it is not 
unique between machines, and in many applications it would not be acceptable to let an 
attacker know what time the user's password was last set.*/


Function: char * crypt_r (const char *key, const char *salt, struct crypt_data * data)
The crypt_r function does the same thing as crypt, but takes an extra parameter which 
includes space for its result (among other things), so it can be reentrant. 
data->initialized must be cleared to zero before the first time crypt_r is called.

The crypt_r function is a GNU extension.
The crypt and crypt_r functions are prototyped in the header crypt.h.