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

NAME

       crypt, crypt_r - password and data encryption

SYNOPSIS

       #define _XOPEN_SOURCE
       #include <unistd.h>

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

       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
       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() 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

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

CONFORMING TO

       crypt(): SVr4, 4.3BSD, POSIX.1-2001.  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 "$id$" followed 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
       password.  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 sig-
       nificant (instead of only the first 8 bytes in DES).

SEE ALSO

       login(1), passwd(1), encrypt(3), getpass(3), passwd(5), feature_test_macros(7)

COLOPHON

       This page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

								    2010-06-20								  CRYPT(3)