man crypt – Man page for crypt

June 1, 2007 – 1:23 am

CRYPT

Section: POSIX Programmer’s Manual (P)
Updated: 2003
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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 is a string 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 characters 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 reentrant. A function that
is not required to be reentrant is not required to be
thread–safe.

RETURN VALUE

Upon successful completion, crypt() shall return a pointer to
the encoded string. The first two characters 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 IEEE Std 1003.1–2001.


#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.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
–– Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001–2003 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 .