Re: Homework help - File encrpytion/decryption problem...

From:

Juha Nieminen <nospam@thanks.invalid>

Newsgroups:

comp.lang.c++

Date:

Thu, 10 May 2012 06:49:50 +0000 (UTC)

Message-ID:

<jofoee$o11$1@speranza.aioe.org>

osmium <r124c4u102@comcast.net> wrote:

"Juha Nieminen" wrote:

Chad <cdalten@gmail.com> wrote:

   //encrpyt the file
   while (IS.read(&c, 1))
   {
         c = value ^ c;
         OS << c;
   }

As a side note, I hope you understand that this is approximately the
poorest form of "encryption" possible.

To the OP:
When you get this working perhaps you could post a brief message and Juha
could break it for us.thus demonstrating just how poor the method is.

  Well, it's completely trivial because there are only 256 possible
encryption keys. Just "decrypt" the message with all of them and look
which one results in clear text. (If the message were very large, you
wouldn't even need to decrypt it all. It would be enough to decrypt
the first few dozens of bytes, and see if it forms readable text.)

  Even if that weren't possible, it would still be quite easy in another
way: Since a space is the most common character in a written piece of text,
just look at a value that repeats in average each 3 to 6 bytes in the
"encrypted" text, calculate its XOR with the character ' ' and then use
that to decrypt the entire message.

  This form of "encryption" is incredibly poor.