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Preliminary: | MT-Unsafe race:crypt | AS-Unsafe corrupt lock heap dlopen | AC-Unsafe lock mem | See POSIX Safety Concepts.
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.
#include <stdio.h> #include <time.h> #include <unistd.h> #include <crypt.h> int main(void) { unsigned long seed[2]; char salt[] = "$1$........"; const char *const seedchars = "./0123456789ABCDEFGHIJKLMNOPQRST" "UVWXYZabcdefghijklmnopqrstuvwxyz"; char *password; int i; /* Generate a (not very) random seed. You should do it better than this... */ seed[0] = time(NULL); seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000); /* Turn it into printable characters from ‘seedchars’. */ for (i = 0; i < 8; i++) salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f]; /* Read in the user’s password and encrypt it. */ password = crypt(getpass("Password:"), salt); /* Print the results. */ puts(password); return 0; }
The next program shows how to verify a password. It prompts the user
for a password and prints “Access granted.” if the user types
GNU libc manual
.
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <crypt.h>
int
main(void)
{
/* Hashed form of "GNU libc manual". */
const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/";
char *result;
int ok;
/* Read in the user’s password and encrypt it,
passing the expected password in as the salt. */
result = crypt(getpass("Password:"), pass);
/* Test the result. */
ok = strcmp (result, pass) == 0;
puts(ok ? "Access granted." : "Access denied.");
return ok ? 0 : 1;
}
Preliminary: | MT-Safe | AS-Unsafe corrupt lock heap dlopen | AC-Unsafe lock mem | See POSIX Safety Concepts.
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.
Next: DES Encryption, Previous: getpass, Up: Cryptographic Functions [Contents][Index]