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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 varia-
       tions 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 char-
       acters 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 cryptog-
       raphy 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  struc-
       ture 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 implementa-
       tions 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), feature_test_macros(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                  2010-06-20                          CRYPT(3)