Date: Tue, 6 Jul 2004 00:30:04 +0300 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: Joe Schmoe <non_secure@yahoo.com> Cc: freebsd-questions@freebsd.org Subject: Re: making files opposite from themselves (100% change) Message-ID: <20040705213004.GC4560@gothmog.gr> In-Reply-To: <20040705205500.30396.qmail@web53306.mail.yahoo.com> References: <20040705205500.30396.qmail@web53306.mail.yahoo.com>
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On 2004-07-05 13:55, Joe Schmoe <non_secure@yahoo.com> wrote:
> So the question is, how do I take a given file and make it 100%
> different from itself (but maintain its size and place on disk) ?
> I could just output /dev/zero to it, but that would leave unchanged
> all the bits that were aleady zero.
Use an algorithm similar to the one shown below as a Perl script, to
pick a certain percentage of the bytes within a file, and at those
offsets chosen by this algorithm, use XOR with 0xFF or a random value to
alter the value of only the given percentage of bytes.
: #!/usr/bin/perl -w
:
: use strict;
: my ($filesize, $percent, $k, $nparts, $partlen);
:
: die "usage: foo.pl FILESIZE PERCENT"
: unless ($#ARGV == 1);
:
: $filesize = $ARGV[0];
: $percent = $ARGV[1] % 100;
:
: $nparts = int(($percent * $filesize) / 100);
: $partlen = int($filesize / $nparts);
:
: srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
:
: print "offsets:";
: for ($k = 0; $k < $nparts; $k++) {
: my $tmp = int(rand($partlen));
: my $nbyte = ($tmp + $k * $partlen);
: print " $nbyte"
: }
: print "\n";
> So how do I flip the bits of an entire file ?
That's even easier:
: for each byte:
: xor(byte, 0xFF);
HTH,
Giorgos
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