Re: decrypting a XOR of two plaintexts

From: Joe Peschel (jpeschel_at_no.spam.org)
Date: 04/01/04 

Date: Wed, 31 Mar 2004 23:51:59 -0000

Korejwa <korejwa@tiac.net> wrote in
news:opr5q1f7ayyzibhw@news.west.earthlink.net:

> On Wed, 31 Mar 2004 20:26:04 -0000, Joe Peschel <jpeschel@no.spam.org>
> wrote:
>>
>> If you have the plaintext and the associated ciphertext, you can XOR
>> the plaintext against the ciphertext and recover the keystream. If
>> the keystream is the same for every message, you can XOR the
>> keystream against
>> the incoming ciphertext traffic and recover the plaintext.
>>
>> If you have two ciphertext messages encrypted by the same keystream,
>> you can XOR the two together. Take the resultant message and drag a
>> crib through it to recover one plaintext message. Then XOR one of the
>> recovered
>> plaintext messages with the ciphertext to get the keystream.
>>
>
> Joe Peschel:
>
> If you have two plaintext messages XORed together, how do you decrypt
> them?

If you have two plaintext messages, there is no need to decrypt them. They
are already decrypted. Maybe I don't understand your question.

>
> I have no idea what you mean by "drag a crib through it." Could you
> explain this a little better?
>

Crib-dragging is akin to known-plaintext attacks, except the plaintext
isn't actually known. It's guessed. You assume probable words in the
plaintext, for instance, _the_ and _that_, and then move them along the
ciphertext. In this case, you XOR them with the ciphertext. If your cribs
are good, portions of plaintext will begin to emerge.

J 

-- 
__________________________________________
When will Bush come to his senses?
Joe Peschel 
D.O.E. SysWorks                                 
http://members.aol.com/jpeschel/index.htm
__________________________________________

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