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



PPRROOLLOOGG
       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.

NNAAMMEE
       crypt -- string encoding function (CCRRYYPPTT)

SSYYNNOOPPSSIISS
       #include <unistd.h>

       char *crypt(const char *_k_e_y, const char *_s_a_l_t);

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

       The _k_e_y argument points to a string to be encoded. The _s_a_l_t 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 _c_r_y_p_t() points to static data that is overwritten
       by each call.

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

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

EERRRROORRSS
       The _c_r_y_p_t() function shall fail if:

       EENNOOSSYYSS The functionality is not supported on this implementation.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
   EEnnccooddiinngg PPaasssswwoorrddss
       The following example finds a user database entry matching a particular
       user name and changes the current password to a new password. The
       _c_r_y_p_t() function generates an encoded version of each password. The
       first call to _c_r_y_p_t() 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 _c_r_y_p_t() encodes the new password
       before it is stored.

       The _p_u_t_p_w_e_n_t() 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);
           }

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       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 _s_a_l_t argument for specifying alternative
       algorithms; while not portable, these extensions may offer better
       security. The use of _c_r_y_p_t() for anything other than password hashing
       is not recommended.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _e_n_c_r_y_p_t(), _s_e_t_k_e_y()

       The Base Definitions volume of POSIX.1-2017, <<uunniissttdd..hh>>

CCOOPPYYRRIIGGHHTT
       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)
