[RC5] What does finding the contest key prove?

Greg Wooledge wooledge at kellnet.com
Fri Apr 16 20:12:38 EDT 1999

Matthew_Webber at cca-int.com (Matthew_Webber at cca-int.com) wrote:

> What does this tell us about a read-world cracking example, where we do NOT
> know any part of the plaintext message (we may not even know what language
> or characterset it is in)?

The answer to this is complex.

First, note that if we're going to be cracking some data, we probably
have a pretty good idea what it is.  Otherwise, why bother?  So, let's
suppose we're trying to crack a letter that was sent from the president
of a rival company to the VPs of that same company.  We probably know
what human language it's written in.  We can probably also guess some
other features -- if that company uses MS Word 97 for all its internal
communications, then it's probably an MS Word 97 file and has appropriate
header information.

Second, note that most commnuications in general must follow standard
protocols in order to be understood.  If we're trying to crack
an encrypted e-mail session, what are the chances that the first 4
bytes of the response are "220 " (the standard SMTP greeting prefix)?
Pretty good, right?  Once we find a key that decrypts the SMTP session
in which the message is wrapped, we might be able to read the message.

Putting this all together, remember that code-breaking doesn't happen
in a vacuum.  There are usually many pieces of information available to
the cracker to help.

Greg Wooledge                    | Distributed.NET http://www.distributed.net/
wooledge at kellnet.com             | because a CPU is a terrible thing to waste.
http://www.kellnet.com/wooledge/ |

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