...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.
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