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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
       ENOSYS The  crypt()  function  was not implemented, probably because of
              U.S.A. export restrictions.
ATTRIBUTES
   Multithreading (see pthreads(7))
       The crypt() function is not thread-safe.
       The crypt_r() function is thread-safe.
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 "$id$" 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 3.53 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/.

                                  2013-06-21                          CRYPT(3)