crypt

CRYPT(3)		      Library functions			     CRYPT(3)



NAME
       crypt - password and data encryption

SYNOPSIS
       #define _XOPEN_SOURCE
       #include <unistd.h>

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

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 con-
       tent 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
       parallel 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  recom-
       mended.

       The  DES	 algorithm  itself has a few quirks which make the use of the
       crypt(3) interface a very poor choice for anything other than password
       authentication.	 If  you are planning on using the crypt(3) 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.

GNU EXTENSION
       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 "$1$<string>$", where "<string>" stands for the up to 8 charac-
       ters following "$1$" in the salt, followed by 22 bytes chosen from the
       set [a–zA–Z0–9./].  The entire key is  significant  here	 (instead  of
       only the first 8 bytes).

       Programs using this function must be linked with -lcrypt.

CONFORMING TO
       SVID, X/OPEN, BSD 4.3, POSIX 1003.1-2001

SEE ALSO
       login(1), passwd(1), encrypt(3), getpass(3), passwd(5)



				  2001-12-23			     CRYPT(3)