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 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
       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 recommended.

       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 features.  If salt is a character string starting with the three charac-
       ters "$1$" followed by at most eight characters, and optionally terminated 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 characters
       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)