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



NAME
       crypt, crypt_r - password and data encryption

SYNOPSIS
       #define _XOPEN_SOURCE	   /* See feature_test_macros(7) */
       #include <unistd.h>

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

       #define _GNU_SOURCE	   /* See feature_test_macros(7) */
       #include <crypt.h>

       char *crypt_r(const char *key, const char *salt,
		     struct crypt_data *data);

       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.

       crypt_r() is a reentrant version of crypt().  The structure pointed  to
       by  data	 is  used  to  store  result data and bookkeeping information.
       Other than allocating it, the only thing that the caller should do with
       this  structure	is  to	set data->initialized to zero before the first
       call to crypt_r().

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

ERRORS
       EINVAL salt has the wrong format.

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

       EPERM  /proc/sys/crypto/fips_enabled  has  a  nonzero  value,  and   an
	      attempt was made to use a weak encryption type, such as DES.

ATTRIBUTES
       For   an	  explanation	of   the  terms	 used  in  this	 section,  see
       attributes(7).

       +----------+---------------+----------------------+
       |Interface | Attribute	  | Value		 |
       +----------+---------------+----------------------+
       |crypt()	  | Thread safety | MT-Unsafe race:crypt |
       +----------+---------------+----------------------+
       |crypt_r() | Thread safety | MT-Safe		 |
       +----------+---------------+----------------------+
CONFORMING TO
       crypt(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  crypt_r() is a  GNU
       extension.

NOTES
   Glibc notes
       The  glibc2  version  of	 this  function supports additional encryption
       algorithms.

       If salt is a character string starting with the characters "$id$"  fol-
       lowed by a string terminated by "$":

	      $id$salt$encrypted

       then  instead  of  using	 the DES machine, id identifies the encryption
       method used and this then determines  how  the  rest  of	 the  password
       string is interpreted.  The following values of id are supported:

	      ID  | Method
	      ---------------------------------------------------------
	      1	  | MD5
	      2a  | Blowfish (not in mainline glibc; added in some
		  | Linux distributions)
	      5	  | SHA-256 (since glibc 2.7)
	      6	  | SHA-512 (since glibc 2.7)

       So    $5$salt$encrypted	  is   an   SHA-256   encoded	password   and
       $6$salt$encrypted is an SHA-512 encoded one.

       "salt" stands for the up to 16 characters following "$id$" in the salt.
       The  encrypted part of the password string is the actual computed pass-
       word.  The size of this string is fixed:

       MD5     | 22 characters
       SHA-256 | 43 characters
       SHA-512 | 86 characters

       The characters in  "salt"  and  "encrypted"  are	 drawn	from  the  set
       [a-zA-Z0-9./].	In  the	 MD5 and SHA implementations the entire key is
       significant (instead of only the first 8 bytes in DES).

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

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



				  2015-08-08			      CRYPT(3)