Non-determinism

[Berend Tober]

On 18 Apr 2002 at 0:12, Martin Pool wrote:

> 
> The analysis is done to a reasonable extent in tridge's thesis.
> MD4 (and MD5) is no longer considered cryptographically strong, 
> but we're not contending against an intelligent adversary here,
> only random chance.
> 
> You might like to look at Schneier's /Applied Cryptography 2nd ed/ for
> details on MD4.  It produces a 128-bit hash; I am fairly sure that the
> way it's used in rsync that means there is a 2^-128 chance of an
> undetected failure.
> 
> Sure, it's only probabilistic.  Most aspects of computer systems
> are:
> 

That was my point about comparing rsync to sending the entire file 
using say, ftp or cp. That is, one might think that sending the 
entire file via ftp or cp will produce a exact file copy, however the 
actual transmission of the data takes the form of electrical signals 
on a wire that must be detected at the receiving end. The detection 
process must have some probablilty of false alarm/missed detection 
characteristic and so there must be some estimate of the probability 
of ftp and cp failing to produce a reliable copy. So while the 
software algorithm of ftp and cp are deterministic, there must be 
some quantifiable probablity of failure non-the-less. The difference 
with rsync is that not only are the same effects of data corruption 
at work as with ftp and cp, but the algorithm itself introduces non-
determinism.

I still think rsync as in incredible tool, however, despite me 
expression of this reservation.
Regards,
Berend Tober