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
   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

   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 "$id$"
   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

   This  page  is  part of release 4.09 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)

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