CRYPT(3)                   POSIX Programmer's Manual                  CRYPT(3)



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

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 .



IEEE/The Open Group                  2017                             CRYPT(3)
