Question on CFB variant with c[i-N]

[David Honig]

At 11:22 AM 12/22/97 -0700, Johnson, Michael P (Mike) wrote:
>>David Honig <honig at otc.net> wrote:
>>At 06:46 PM 12/20/97 -0700, Johnson, Michael P (Mike) wrote:
>>>>
>>>>
>>>>>>              cfb    Ciphertext feeback mode
>>>>>>                     c[i] = f1(K, c[i-1]) ^ p[i]
>>>>>>                     p[i] = f1(K, c[i-1]) ^ c[i]
>>>>
>>>
>>>
>>>Suppose instead of c[i-1] you use c[i-N] where N is say 10. 
>>>How would you prove that this has no security implications?
>>>That 10-way interleaved cfb streams are security-equivalent to
>>>a single cfb stream interleaved with the immediately previous block?
>
>That would make it harder to get the process started, since you would
>need 10 initialization vector blocks instead of 1, so it would bloat
>your messages more.
>
>How about this mode:
>    c[i] = e(K1, e(k2, c[i-1]) ^ p[i-1]) ^ p[i]
>    p[i] = e(K1, e(k2, c[i-1]) ^ p[i-1]) ^ p[i]
>
>The feedback possibilities are literally endless. The analysis of the
>effects on security, speed, error propagation, etc., are left as an
>exercise for the reader. <grin>


In case you think I came up with this question
with the goal of proposing a stronger form of feedback:

I did not; I am not qualified to attempt such a thing. (Also, my intuition
matches Bruce's, there's no cryptostrength difference). Instead, the question
came up in an implementation context, where I wanted to know whether the 
more knowledgable community would seriously question such a variant on 
a well-known feedback mode if it were necessary in this implementation.

Thanks






------------------------------------------------------------
      David Honig                   Orbit Technology
     honig at otc.net                  Intaanetto Jigyoubu

"Windows 95 is a technologically complex product that is best left alone by
the government..."
 ---MSFT Atty B. Smith