Well, you need a program to encrypt your doc, using a method that allow the decrypt. I made some test for fun with encrypters some time ago.
I'm sending a script that you can use in your docs. This use a key of numbers to move the bits to another position, bit by bit. The code was fully on Perl, can be slow in big files, since an encrypter normally is writed in C.
When you encrypt a doc NEVER lose the key number!!! Or you can decrypt it anymore! You can change some flags of the program, like the block size, and the key number will only work with the same flags used in the main encrypter.
This program was based in the idea (move bits) of ABC1 (A Bit Cypher program) published in IBM AlphaWorks. And I put the name James Bond, don't laughs! ;-P
This script is only for educational purpose, don't use it for malicious ways. And you can find better things on the Internet too...
"The creativity is the expression of the liberty".
To run just type: perl bond.pl
and fallow the promp, or use the -h flag for help.
SOURCE follows...
| [reply]
[d/l] |
...but your code has been broken. Below is a simple known plaintext
attack which "nearly" recovers the encryption key, given a block of
encrypted data and a few known plaintext bytes. It's generally not
difficult to get ahold of the known plaintexts, especially if you're
encrypting files like Word docs that have a fixed header.
You say your program was "based on the ABC1 idea." I don't know how
close the resemblence is, but I'd like to point out that changing a
cipher even slightly can destroy its security.
Here's the sample output. Notice that, for each key byte, the
correct value is one of the given possibilities.
Secret key is 'Shaken'
Guessing key bytes
key byte 0
'S' : 10
key byte 1
'#' : 9
'h' : 10
key byte 2
'/' : 10
'3' : 9
'F' : 10
'N' : 9
'a' : 11
key byte 3
'k' : 11
'o' : 9
key byte 4
'F' : 9
'[' : 9
'e' : 11
'o' : 9
key byte 5
'a' : 9
'n' : 10
And here's the code. It needs to know the block size and the length
of the encryption key. That information might not be available, but
it's easy enough to find by brute force.
#!/usr/bin/perl
use strict;
use warnings;
# this information is known to the attacker
my ($ciphertext, $known_plaintext, $keylen);
{ # secret stuff in here
my $key = "Shaken";
print "Secret key is '$key'\n";
$keylen = length($key);
open F, $0 or die; # read test data from this script - why not?
my $plaintext;
read F, $plaintext, 1024 or die;
close F;
$known_plaintext = substr($plaintext, 0, 8);
$ciphertext = encrypt_blk($plaintext, $key);
} # secret stuff in here
print "Guessing key bytes\n";
my $plain_bits = length($known_plaintext)*8;
my $cipher_bits = length($ciphertext)*8 - 1;
foreach my $keypos (0 .. $keylen-1) {
print "key byte $keypos\n";
my $first = $keypos > 1 ? $keypos - 1 : $keylen + $keypos - 1;
foreach my $keybyte (0x20 .. 0x7e) {
my $keyval = ($keybyte >= 0x30 && $keybyte <= 0x39)
? $keybyte-0x2f : $keybyte+1;
my $hits = 0;
my $trials = 0;
for (my $bitpos = $first; $bitpos < $plain_bits; $bitpos += $keyle
+n) {
my $pos = ($bitpos * $keyval) % $cipher_bits;
$trials++;
$hits++ if vec($known_plaintext,$bitpos,1) == vec($ciphertext,$p
+os,1);
}
my $thresh = int($trials * .8);
print "'",chr($keybyte),"' : $hits\n" if $hits > $thresh;
}
}
sub encrypt_blk {
my ($inblk, $key) = @_;
my @key = map { (/\d/ ? $_ : ord) + 1 } split(//, $key);
my $blkbytes = length($inblk);
my $blkbits = $blkbytes * 8;
my $outblk = "\0" x $blkbytes;
my $used = "\0" x $blkbytes;
my $keypos = 0;
my $newpos;
for (my $blkpos = 0; $blkpos < $blkbits; $blkpos++) {
++$keypos >= @key and $keypos = 0;
$newpos = ($key[$keypos] * $blkpos) % ($blkbits - 1);
while (vec($used, $newpos, 1)) {
++$newpos >= $blkbits and $newpos = 0;
}
vec($used, $newpos, 1) = 1;
vec($outblk, $newpos, 1) = vec($inblk, $blkpos, 1);
}
return $outblk;
} # encrypt_blk
Update: The DMCA can byte me. Show me where this
cipher is used to "effectively control access to a
copyrighted work." Cryptography is perfectly legal, and
I'm getting tired of hearing from you fear-mongers. | [reply]
[d/l] |
Cool! I liked what you made, and you are right, documents like word, pdf, have the same header, and using this information we can use brute force to get the headers, and geting the headers we can get the rest. But if we use another block size (not 1024, the default) and a key with 20+ digits will be very difficult. To make a better cipher we can just add something to mask the headers too, or encrypt again the data!
The James Bond cipher are not based in the source of the ABC1, just in the idea to move bits.
But remember, I made this for fun, this is the only cipher that I made! I never used this for real, because I don't have top secret things! Every thing that I made is for Open Source. Why I will encrypt what I will publish for the world!?
"The hardest door to breack is the door that we don't need to open".
And "no_slogan", DMCA can't kill our liberty of expression. Nice code! And Cryptography is legal, because without it internet can't exist.
"The creativity is the expression of the liberty".
| [reply] |
thank for this! though it is little weighty, i am still in my perl diapers, but i look forward to setting this up! | [reply] |