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

       crypt, crypt_r - password and data encryption

       #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.

       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().

       On success, a pointer to the encrypted password is returned.  On error,
       NULL is returned.

       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.

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface | Attribute     | Value                |
       |crypt()   | Thread safety | MT-Unsafe race:crypt |
       |crypt_r() | Thread safety | MT-Safe              |
       crypt(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  crypt_r() is a  GNU

   Glibc notes
       The  glibc2  version  of  this  function supports additional encryption

       If salt is a character string starting with the characters "$id$"  fol-
       lowed by a string optionally terminated by "$", then the result has the


       id identifies the encryption method used instead of DES 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)

       Thus, $5$salt$encrypted  and  $6$salt$encrypted  contain  the  password
       encrypted with, respectively, functions based on SHA-256 and SHA-512.

       "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).

       Since  glibc  2.7,  the  SHA-256  and SHA-512 implementations support a
       user-supplied number of hashing rounds, defaulting  to  5000.   If  the
       "$id$"  characters in the salt are followed by "rounds=xxx$", where xxx
       is an integer, then the result has the form


       where yyy is the number of hashing rounds actually used.  The number of
       rounds actually used is 1000 if xxx is less than 1000, 999999999 if xxx
       is greater than 999999999, and is equal to xxx otherwise.

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

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                                  2017-09-15                          CRYPT(3)
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