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

       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.

       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                                         CRYPT(3POSIX)

       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.

       The crypt() function shall fail if:

       ENOSYS The functionality is not supported on this implementation.

       Thefollowingsectionsareinformative.

EncodingPasswords
       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);
           }

       None.

       crypt — string 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.

       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.

encrypt(), setkey()

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

       #include <unistd.h>

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