Randomly mangle a binary file

A companion script to the one in Finding differences in binary files, written for testing purposes and posted here for the same reasons.

Alterates randomly a number (currently hardcoded, it's the value of $LEN) of consecutive bytes in a binary file:

#!/usr/bin/perl

use strict;
use warnings;

die "Usage: $0 file\n" unless @ARGV == 1;
my $fn=shift;
my $LEN=3;

my $cont = do {
    open my $fh, '<:raw', $fn or die "$fn: D'Oh! $!\n";
    local $/;
    <$fh>;
};

open my $fh, '>:raw', $fn or die "$fn: D'Oh! $!\n";

print $fh $cont ^ do { 
    "\0" x rand(-$LEN + length $cont) .
      join '', map { chr rand 0x100 } 1 .. $LEN;
};

__END__
[download]

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Comment on Randomly mangle a binary file Select or Download Code

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Re: Randomly mangle a binary file by jwkrahn (Abbot) on Sep 01, 2008 at 12:33 UTC
You don't have to read the whole file just to modify three bytes: `#!/usr/bin/perl use strict; use warnings; use Fcntl ':seek'; die "Usage: $0 file\n" unless @ARGV == 1; my $fn = shift; my $LEN = 3; open my $fh, '+<:raw', $fn or die "$fn: D'Oh! $!\n"; my $offset = rand( -$LEN + -s $fh ); seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n"; read $fh, my $cont, $LEN; seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n"; print $fh $cont ^ join '', map chr( rand 0x100 ), 1 .. $LEN; __END__` [download]	[reply] [d/l]
Re^2: Randomly mangle a binary file by blazar (Canon) on Sep 01, 2008 at 13:04 UTC
You don't have to read the whole file just to modify three bytes: Actually, I knew: I didn't use "your" technique, because I'm less familiar with it and the whole point of the program was to create a test case that were "small enough." (I just wanted to... get it done, if you know what I mean!) Indeed, not only is your approach more efficient, but it is even simpler as a whole: well done mentioning it! `seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n"; read $fh, my $cont, $LEN; seek $fh, $offset, SEEK_SET or die "$fn: D'Oh! $!\n"; print $fh $cont ^ join '', map chr( rand 0x100 ), 1 .. $LEN;` [download] (Incidentally, there's a typo that makes for a syntax error: missing parens around `my $cont`.) It can be even easier than that: given that we're xoring with completely random bytes (as opposed, say, to bytes with a prescribed number of bits on) we may avoid doing so altogether and just print them, thus: `#!/usr/bin/perl use strict; use warnings; use Fcntl ':seek'; die "Usage: $0 file\n" unless @ARGV == 1; my $fn = shift; my $LEN = 3; open my $fh, '+<:raw', $fn or die "$fn: D'Oh! $!\n"; seek $fh, rand(-$LEN + -s $fh), SEEK_SET or die "$fn: D'Oh! $!\n"; print $fh map chr rand 0x100, 1 .. $LEN; __END__` [download] Oh my! `--` ~~If you can't understand the incipit, then please check the IPB Campaign.~~	[reply] [d/l] [select]