This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       Historical implementations of the encrypt() function used a rather primitive encoding algorithm.

       In  some environments, decoding might not be implemented. This is related to some Government restrictions
       on encryption and decryption routines. Historical practice has been to ship a different  version  of  the
       encryption  library without the decryption feature in the routines supplied. Thus the exported version of
       encrypt() does encoding but not decoding.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                       ENCRYPT(3POSIX)

       The  encrypt()  function  shall  provide  access to an implementation-defined encoding algorithm. The key
       generated by setkey() is used to encrypt the string block with encrypt().

       The block argument to encrypt() shall be an array of length 64  bytes  containing  only  the  bytes  with
       values  of  0 and 1. The array is modified in place to a similar array using the key set by setkey().  If
       edflag is 0, the argument is encoded. If edflag is 1, the argument may be decoded  (see  the  APPLICATION
       USAGE section); if the argument is not decoded, errno shall be set to [ENOSYS].

       The  encrypt()  function  shall  not change the setting of errno if successful. An application wishing to
       check for error situations should set errno to 0 before calling  encrypt().   If  errno  is  non-zero  on
       return, an error has occurred.

       The encrypt() function need not be thread-safe.

       The encrypt() function shall fail if:

       ENOSYS The functionality is not supported on this implementation.

       Thefollowingsectionsareinformative.

       None.

       A future version of the standard may mark this interface as obsolete or remove it altogether.

       encrypt — encoding function (CRYPT)

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       The encrypt() function shall not return a value.

crypt(), setkey()

       The Base Definitions volume of POSIX.1‐2017, <unistd.h>

       #include <unistd.h>

       void encrypt(char block[64], int edflag);