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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
       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
       significant (instead of only the first 8 bytes in DES).

SEE ALSO

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

COLOPHON

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                                  2011-09-15                          CRYPT(3)