PROLOG

       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.

NAME

       crypt — string encoding function (CRYPT)

SYNOPSIS

       #include <unistd.h>

       char *crypt(const char *key, const char *salt);

DESCRIPTION

       The crypt() function is a string encoding function. The algorithm is implementation-defined.

       The  key  argument  points to a string to be encoded. The salt argument shall be a string of at least two
       bytes in length not including the null character chosen from the set:

           a b c d e f g h i j k l m n o p q r s t u v w x y z
           A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
           0 1 2 3 4 5 6 7 8 9 . /

       The first two bytes of this string may be used to perturb the encoding algorithm.

       The return value of crypt() points to static data that is overwritten by each call.

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

RETURN VALUE

       Upon successful completion, crypt() shall return a pointer to the encoded string. The first two bytes  of
       the returned value shall be those of the salt argument. Otherwise, it shall return a null pointer and set
       errno to indicate the error.

ERRORS

       The crypt() function shall fail if:

       ENOSYS The functionality is not supported on this implementation.

       The following sections are informative.

EXAMPLES

   Encoding Passwords
       The following example finds a user database entry matching a particular user name and changes the current
       password to a new password. The crypt() function generates an encoded version of each password. The first
       call to crypt() produces an encoded version of the old password; that encoded password is  then  compared
       to  the  password stored in the user database. The second call to crypt() encodes the new password before
       it is stored.

       The putpwent() function, used in the following example, is not part of POSIX.1‐2008.

           #include <unistd.h>
           #include <pwd.h>
           #include <string.h>
           #include <stdio.h>
           ...
           int valid_change;
           int pfd;  /* Integer for file descriptor returned by open(). */
           FILE *fpfd;  /* File pointer for use in putpwent(). */
           struct passwd *p;
           char user[100];
           char oldpasswd[100];
           char newpasswd[100];
           char savepasswd[100];
           ...
           valid_change = 0;
           while ((p = getpwent()) != NULL) {
               /* Change entry if found. */
               if (strcmp(p->pw_name, user) == 0) {
                   if (strcmp(p->pw_passwd, crypt(oldpasswd, p->pw_passwd)) == 0) {
                       strcpy(savepasswd, crypt(newpasswd, user));
                       p->pw_passwd = savepasswd;
                       valid_change = 1;
                   }
                   else {
                       fprintf(stderr, "Old password is not valid\n");
                   }
               }
               /* Put passwd entry into ptmp. */
               putpwent(p, fpfd);
           }

APPLICATION USAGE

       The values returned by this function need not be portable among XSI-conformant systems.

       Several implementations offer extensions via characters  outside  of  the  set  specified  for  the  salt
       argument  for  specifying  alternative  algorithms; while not portable, these extensions may offer better
       security. The use of crypt() for anything other than password hashing is not recommended.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       encrypt(), setkey()

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

COPYRIGHT

       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 .