From: "Mark M." <markm@voicenet.com>
Date: Mon, 25 Nov 1996 13:28:38 -0800 (PST)
To: inssdl@dstn21.dct.ac.uk>
Subject: Re: A source of entropy?
In-Reply-To: <9611250834.AA11220@dstn21.dct.ac.uk>
Message-ID: <Pine.LNX.3.95.961125162439.520A-100000@gak.voicenet.com>
MIME-Version: 1.0
Content-Type: text/plain


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On Mon, 25 Nov 1996, inssdl wrote:

> Last weekend I spent time formatting a little over 100 new floppies. When 
> I was staring at the monitor between changes, I started looking at the 
> volume serial number that was being thrown up for each disk.
> 
> These *appeared* to be unpredictable from the previous serial number given.
> 
> If the serial number is represented as xxxx-yyyy then sometimes yyyy 
> would be one less than the previous yyyy but xxxx always seemed to be 
> "random".

The serial number is derived from the time of day.  This is used by a lot of
programs to get a few bits of entropy, so the disk serial number will probably
not add much entropy.  Here's a post I saved from alt.hackers that explains the
exact algorithm:

From: jsl2228@acf4.nyu.edu (jsl2228)
Newsgroups: alt.hackers
Subject: Re: Disk Serial Numbers.
Date: 17 May 1995 04:36:00 GMT


MICHAEL PAUL DANIEL (mdanie@wilbur.mbark.swin.oz.au) wrote:
: Just a quick question.. How does MS-DOS determine the serial number when
: you format a disk??

        It's not a pesudo-randum number #, it actually has a formula.  It's
based on the current date & time.

unsigned long int NS;  (Serial numbers are 32 bits)

NS = (  ( ( ((seconds << 8) + hundreth) ) +
            ((day << 8) + month) ) << 16 ) +
       (   ((hour << 8) + minutes) + year);

        Try this: get a disk that is already formatted (but whose data you
don't care about:)

>FORMAT A:/Q/U/V:""

        Format the disk once, note the serial number, then immediately
format it again.  Note that the second serial number only differs in
a few digits from the first.
[rest deleted]

Mark
- -- 
finger -l for PGP key
PGP encrypted mail prefered.
0xf9b22ba5 now revoked

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