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File:,  Node: crypt,  Next: DES Encryption,  Prev: getpass,  Up: Cryptographic Functions

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.

     #include <stdio.h>
     #include <time.h>
     #include <unistd.h>
     #include <crypt.h>

       unsigned long seed[2];
       char salt[] = "$1$........";
       const char *const seedchars =
       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. */
       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

     #include <stdio.h>
     #include <string.h>
     #include <unistd.h>
     #include <crypt.h>

       /* 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;

 -- 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