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.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       encrypt(), setkey()

       The Base Definitions volume of POSIX.1‐2008, <unistd.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2013 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX),  The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .