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

       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.

       crypt — string encoding function (CRYPT)

       #include <unistd.h>

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

       The crypt() function is a string encoding function. The algorithm is

       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

       The return value of crypt() points to static data that is overwritten
       by each call.

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

       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.

       The crypt() function shall fail if:

       ENOSYS The functionality is not supported on this implementation.

       The following sections are informative.

   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

           #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);

       The values returned by this function need not be portable among XSI-
       conformant systems.



       encrypt(), setkey()

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

       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 .

       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 .

IEEE/The Open Group                  2013                            CRYPT(3P)