man crypt

CRYPT(3)     Linux Programmer's Manual        CRYPT(3)

 

NAME
       crypt - password and data encryption

SYNOPSIS
       #define _XOPEN_SOURCE
       #include

       char *crypt(const char *key, const char *salt);

       Link with -lcrypt.

DESCRIPTION
       crypt() is  the password encryption function.  It is based on the Data
       Encryption Standard algorithm with  variations  intended  (among  other
       things) to discourage use of hardware implementations of a key search.

       key is a user's typed password.

       salt is a two-character string chosen from the set [a-zA-Z0-9./].  This
       string  is used to perturb the algorithm in one of 4096 different ways.

       By taking the lowest 7 bits of each of the first  eight characters  of
       the  key, a 56-bit key is obtained.  This 56-bit key is used to encrypt
       repeatedly a constant  string  (usually a  string  consisting  of  all
       zeros).  The returned value points to the encrypted password, a series
       of 13 printable ASCII characters (the first  two  characters  represent
       the salt itself).  The return value points to static data whose content
       is overwritten by each call.

       Warning: The key space consists of 2**56 equal 7.2e16 possible  values.
       Exhaustive searches of this key space are possible using massively par-
       allel computers.  Software, such as crack(1), is available  which  will
       search  the  portion of this key space that is generally used by humans
       for passwords.  Hence, password selection  should,  at  minimum,  avoid
       common words and names. The use of a passwd(1) program that checks for
       crackable passwords during the selection process is recommended.

       The DES algorithm itself has a few quirks which make  the  use  of  the
       crypt() interface  a very poor choice for anything other than password
       authentication. If you are planning on using the crypt() interface for
       a  cryptography project, don't do it: get a good book on encryption and
       one of the widely available DES libraries.

RETURN VALUE
       A pointer to the encrypted password is returned.   On  error,  NULL  is
       returned.

ERRORS
       ENOSYS The  crypt()  function  was not implemented, probably because of
       U.S.A. export restrictions.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001

NOTES
   Glibc Notes
       The glibc2 version of this function has the following  additional  fea-
       tures. If  salt is a character string starting with the three charac-
       ters "$1$" followed by at most eight characters, and optionally termi-
       nated  by  "$", then instead of using the DES machine, the glibc crypt
       function uses an MD5-based algorithm,  and  outputs  up to  34 bytes,
       namely  "$1$$",  where  "" stands for the up to 8
       characters following "$1$" in the salt, and "" is a further 22
       characters.   The characters in "" and "" are drawn from
       the set [a-zA-Z0-9./].  The entire key is significant here (instead  of
       only the first 8 bytes).

SEE ALSO
       login(1),    passwd(1),  encrypt(3),   getpass(3),   passwd(5),   fea-
       ture_test_macros(7)

COLOPHON
       This page is part of release 2.76 of the Linux  man-pages  project.   A
       description  of the project, and information about reporting bugs, can
       be found at .

 

      2001-12-23         CRYPT(3)