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

PROLOG         top

       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         top

       crypt — string encoding function (CRYPT)

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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 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         top

       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         top

       The crypt() function shall fail if:

       ENOSYS The functionality is not supported on this implementation.

       The following sections are informative.

EXAMPLES         top

   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);


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

RATIONALE         top




SEE ALSO         top

       encrypt(3p), setkey(3p)

       The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)

COPYRIGHT         top

       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)

Pages that refer to this page: unistd.h(0p)encrypt(3p)setkey(3p)