Subject: Re: rpc xid randomness
To: None <>
From: Jun-ichiro itojun Hagino <>
List: tech-userlevel
Date: 09/09/2003 07:00:55
> On Mon, Sep 08, 2003 at 07:50:58PM +0900, Jun-ichiro itojun Hagino wrote:
> > 	to summarize,
> > 	- the currently-committed code is not good.  it is not resistant to
> > 	  number reuse/duplication.
> > 	- sequential number with time.tv_sec initialization is resistant to
> > 	  number reuse/duplication, if we don't set date(1).
> > 	- niels' generator is resistant to number reuse/duplcation, and probably
> > 	  there's no chance for duplication on reboot (due to the use of random
> > 	  number as initialization)
> > 
> > 	now, may i commit?
> IMHO this generator is too expensive for RPC xids, and I'm not sure
> that it is good enough for anything that needs randomness.
> It looks as though the value is calculated from:
> 	a ** b mod c
> where 'a' changes for each block of numbers, 'b' sequences through
> terms of a LCG whenever a value is wanted (missing 0 to 7 each time)
> and 'c' is constant.
> Some notes I have on the security of RSA (where 'a' would be the message
> and 'b' and 'c' the key) say that you should not use different values of
> 'b' with the same 'c' - otherwise recovering the key is trivial if the
> same message (ie 'a') is encypted with both.
> This use of the equation seems to be going out of its way to make it easy
> to break!

	take a look at the comment on sys/netinet/ip_id.c more carefully.
	X[n] sequence is generated by a ** b mod m, but the actual id
	is seed xor (g^X[n] mod n).  and seed is random.  i don't think
	it emit predictable sequence of number.  it just avoids number
	collision in the sequence.