cryptographic ergodic sequence generators?

[Greg Rose]

At 02:09 PM 9/6/2003 -0400, Perry E. Metzger wrote:
>For making things like IP fragmentation ids and other similar protocol
>elements unpredictable, it would be useful to have what I'll call a
>cryptographic ergodic sequence generator -- that is, a generator that
>will produce a sequence of n bit numbers such that there are no
>repeats until you pass the 2^nth number in the sequence (that is, the
>sequence is a permutation of all 2^n bit numbers) and such that it is
>very difficult to predict what the next number in the sequence might
>be beyond the fact that it will not be one of the numbers seen earlier
>in the sequence. It is also rather important that the generator be
>computationally inexpensive.

The characteristic you ask for is exactly that of an n-bit block cipher in 
Counter Mode. For example, that's exactly why we developed Skip32, which is 
on our web page; we needed an unpredictable but non-repeating 32 bit nonce.

If you aren't prepared to accept the cost of a (scaled down) block cipher, 
then you'll have to restate your requirements.

Greg.

Greg Rose                                       INTERNET: ggr at qualcomm.com
Qualcomm Australia          VOICE:  +61-2-9817 4188   FAX: +61-2-9817 5199
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