From: Robert Klemme <shortcutter@...>
Date: 2009-11-19T18:20:04+09:00
Subject: Re: open a binary file-read bit-change bit-update same file

2009/11/19 Sniper Abandon <sathish.salem.1984@gmail.com>:
> my Aim is i want to encrypt a file
>
> i want to open a binary file
>
> i want to read the 1-st bit
> if the bit is "1" then update that position bit to "0" instantly
> if the bit is "0" then update that position bit to "1" instantly
>
> then i have to read 2-nd bit
> do the same thing like above conversion
> .......
> .......
> like i want to do this for all bit of that file
>
> ---------------------
>
>
> i know we can read the file into a array then we can do the
> modification,etc
> but i want to update on that same file object, and the file size may be
> more then 1 GB
>
> is it possible?

Anything is possible.  But doing it bitwise is extremely inefficient.
You should rather do it in chunks of, say, 1025 or 512 bytes or - even
better - use the cluster size of your file system.

XOR will be useful

irb(main):004:0> a = 11
=> 11
irb(main):005:0> sprintf "%08b -> %08b", a, a ^ 0xFF
=> "00001011 -> 11110100"


Kind regards

robert


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