In combination with a block cipher such as DES or IDEA, you can encrypt and decrypt messages of arbitrarily long length.

use Crypt::Blowfish
my $file = "your file name here";
my $key  = "magick";
open(my $in,  "<", $file        ) or die "Couldn't open '$file' for re
+ad: $!";
open(my $out, ">", "$file.crypt") or die "Couldn't open '$file.crypt' 
+for write: $!";

my $cipher = Crypt::Blowfish->new($key);
my $buffer;
while( read($in, $buffer, 1024) ) {
   print $out $cipher->encrypt($buffer);
}
[download]

Two things:

1. The OP isn't saying "only core modules"; rather only "standard" modules. That might be those that come with his distribution, but I bet it means only those available on CPAN.
2. Crypt::Blowfish can only deal with 8-byte blocks. That means, you need code like this:

   my $cipher = Crypt::Blowfish->new($key);
   my $buffer;
   while( read($in, $buffer, 8) ) {
      print $out $cipher->encrypt($buffer);
   }
   [download]

   The OP seems to be asking about dealing with the last few bytes of the file, since the Crypt::Blowfish module requires exactly 8 bytes to operate.

The answer, OP, is to pad the last chunk. Something like this:

my $cipher = Crypt::Blowfish->new($key);
my $buffer;
while( read($in, $buffer, 8) ) {
   if ( length($buffer) < 8 ) {
      for (1..8-length($buffer)) { $buffer.=chr(0) }
   }
   print $out $cipher->encrypt($buffer);
}
[download]

However, the best way to take care of the issue is to do as others have suggested and use Crypt::CBC to handle the mechanics for you.

<-radiant.matrix->
A collection of thoughts and links from the minds of geeks
The Code that can be seen is not the true Code
"In any sufficiently large group of people, most are idiots" - Kaa's Law

Thanks to anybody replyed to my question :)

Indeed, i have to use core modules except the crypt::Blowfish.
This is the only module additional installed on the machine.

The way radiant.matrix mentioned works fine.

Except one little problem:
The decrypted file contains some characters additional to the original file: ^@
I tried various ways to analyse the problem, but get only a change of the amount of ^@

So please give me a hint: what am i doing wrong?

crypting:



use Crypt::Blowfish;

my $key = 'ThisIsMyKey';
my $cipher = Crypt::Blowfish->new($key);
my $buffer;
my $file = 'TestFile.txt';
open(my $in,  "<", $file) or die "Couldn't open '$file' for read: $!";
open(my $out, ">", "$file.crypt") or die "Couldn't open '$file.crypt' 
+for write: $!";
while( read($in, $buffer, 8) ) {
        if ( length($buffer) < 8 ) {
                for (1..8-length($buffer)) {
                        $buffer.=chr(0);
                        print length($buffer);
                }
        }
        print $out $cipher->encrypt($buffer);
}
[download]

decrypting:

my $key = 'ThisIsMyKey';
my $cipher = Crypt::Blowfish->new($key);
my $buffer;
my $file = 'TestFile.txt';
open(my $in,  "<", "$file.crypt") or die "Couldn't open '$file' for re
+ad: $!";
open(my $out, ">", "$file.decrypt") or die "Couldn't open '$file.decry
+pt' for write: $!";
while( read($in, $buffer, 8) ) {
        print $out $cipher->decrypt($buffer);
}
[download]

Thanks a lot, PeterE

I see, however, that you've place a seemingly arbitrary restriction on using core modules

Actually, he placed a seemingly arbitrary restriction on using non-core modules. Some may argue that that's more understandable... but then one wonders whether he would be allowed to write a module himself and install it. Once you realize that you could just copy and paste code into your program, restrictions like that do indeed seem arbitrary, at least when simply toolsmithing. They do make sense, however, when creating an application for commercial distribution where you may be trying to avoid licensing pollution, reduce footprint, and/or ease installation.

-sauoq
"My two cents aren't worth a dime.";

My mistake...I mis-spoke. What I meant was that the OP placed a restriction that only core modules be used.

thor

Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come

That's entirely false. Crypt::Blowfish only encrypts blocks of exactly 8 bytes. Passing an entire file will result in an error (unless, of course, the file happens to be 8 bytes long).

if (input_len != 8)
    croak("input must be 8 bytes long");
[download]

The Pure Perl version doesn't croak, but it only encrypts the first 8 bytes of data passed to it.

Crypt::CBC is the solution to this, as mentioned in the Crypt::Blowfish docs.