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CRYPT(3P)                  POSIX Programmer's Manual                 CRYPT(3P)
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 Electri-
       cal 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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            CRYPT(3P)