[hashcash] Re: example v1 stamp

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On Wed, Apr 28, 2004 at 04:34:55PM -0400, Adam Back wrote:
>Well I coded a little bit, and can now create and verify v1 stamps.
>
>Here's a real example:
>
>1:20:040428:foo::0298b26853c8ebb3:beca0
>
>which is broken down by : separated field:

So, I don't remember the verdict from before.  Is ':' always a separator?
How do we put a ':' in a field (some kind of escape)?  I guess I'm asking
what's the right way to pull this thing into its component parts.

I made a pretty simple change in hashcash-sendmail to deal with the new
format.  I haven't published the change yet, since I haven't tested it at
all.

diff -r1.14 hashcash-sendmail
773c773,777
<       $expiry = $1 if ( $tok =~ /X-Hashcash: 0:(\d+):/ );
- - ---
>       # v0 = 0:date
>       # v1 = 1:bits:date
>       if ( $tok =~ /X-Hashcash: (?:0|1:\d+):(\d+):/ ) {
>           $expiry = $1;
>       }

I think it's simple enough I don't need a test, but I've been wrong
before.  8-)

>the random field is to avoid collisions; the counter starts at 0 and
>increases.  For small numbers of bits you expect a short counter (on
>average) and for larger numbers of bits you expect a larger counter.
>
>This arrangement with separate randomness part counter field makes the
>probability of collision collision independent of size in bits.

I'm missing something.  I don't understand what you gain from separating
the random part and the count.  You have <random>:<count> (e.g., 123:456).
Is that very different from <random><count> (e.g., 123456) or
<random+count> (e.g., 579)?  I guess I'm thinking that they all collide the
same way, with the same frequency.

You seem to be saying this has something to do with the size in bits, but I
don't get it.  We expect a low bit stamp to have a smaller counter, I
guess, but I'm not sure what that has to do with anything.

>You can also see directly in the stamp btw how unlucky the generator
>got.  So for a 20 bit stamp you expect 2^20 tries which is 1
>mega-tries, or 100000 in hex.  You can see it took only BECA0 tries or
>about BE/100 = 74% of expected time.

I'm going to rewrite my generator to start the counter at 2^(bits+2).
It'll be the unluckiest generator on the net.  8-)
- -- 
Kyle Hasselbacher              TV is chewing gum for the eyes.
kyle@xxxxxxxxxxx                          -- Frank Lloyd Wright
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