...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

[download]
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.

In reply to I'm sorry, Mr. Bond by no_slogan
in thread password protect Word, PDF, etc documents by wellsayd

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