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From: Dane Smith on 8 Mar 2007 16:36
Just out of my own curiosity, how do people feel about a block cipher
which encrypts the same message using the same key to any one of many
possible ciphertexts. I'm not entirely sure this has ever been done to
any great length, though, it is quite possible I have missed something.
Does anyone believe that it is even necessarily possible?

Thanks for the input,
Dane Smith
From: Dane Smith on 8 Mar 2007 17:17
Sebastian Gottschalk wrote:
> Dane Smith wrote:
>
>> Just out of my own curiosity, how do people feel about a block cipher
>> which encrypts the same message using the same key to any one of many
>> possible ciphertexts. I'm not entirely sure this has ever been done to
>> any great length, though, it is quite possible I have missed something.
>> Does anyone believe that it is even necessarily possible?
>
> It's called and IV (initialization vector) and is supposed to protect
> against exactly your scenario: "Hey, that's the same ciphertext. It must
> have been the same plaintext encrypted with he same key."

But, assuming you have the same IV, then you still have the same output.
IV's are normally associated with CBC mode in block ciphers, assuming Im
not mistaken. What I am trying to describe is something that with the
same exact input, key, IV, plaintext, would still be able to vary in output.

Dane
From: Dane Smith on 8 Mar 2007 17:22
Dane Smith wrote:
> Sebastian Gottschalk wrote:
>> Dane Smith wrote:
>>
>>> Just out of my own curiosity, how do people feel about a block cipher
>>> which encrypts the same message using the same key to any one of many
>>> possible ciphertexts. I'm not entirely sure this has ever been done to
>>> any great length, though, it is quite possible I have missed something.
>>> Does anyone believe that it is even necessarily possible?
>> It's called and IV (initialization vector) and is supposed to protect
>> against exactly your scenario: "Hey, that's the same ciphertext. It must
>> have been the same plaintext encrypted with he same key."
>
> But, assuming you have the same IV, then you still have the same output.
> IV's are normally associated with CBC mode in block ciphers, assuming Im
> not mistaken. What I am trying to describe is something that with the
> same exact input, key, IV, plaintext, would still be able to vary in output.
>
> Dane
Please allow me to correct myself. An IV would accomplish this, assuming
your using a mode other than ECB. What I am proposing is something where
each independent block encrypts to something different every time, even
in ECB mode where there is no IV.

Dane
From: Will Dickson on 8 Mar 2007 17:32
On Thu, 08 Mar 2007 16:36:23 -0500, Dane Smith wrote:

> Just out of my own curiosity, how do people feel about a block cipher
> which encrypts the same message using the same key to any one of many
> possible ciphertexts. I'm not entirely sure this has ever been done to
> any great length, though, it is quite possible I have missed something.
> Does anyone believe that it is even necessarily possible?

How would you decrypt it?


Will.

From: Kristian Gj�steen on 8 Mar 2007 17:58
Dane Smith <dasmith6(a)cse.buffalo.edu> wrote:
>What I am proposing is something where
> each independent block encrypts to something different every time, even
>in ECB mode where there is no IV.

It is easy to see that the output must be bigger than the input for this
to be possible. This means that you will be using more space for your
randomness that you do in CBC mode (where the IV is the overhead).

--
Kristian Gj�steen
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