Re: [Asrg] 6. Email Path Verification

> You might want to check out the camram.org project.
>
>         http://camram.org/
(Above URL is broken, use http://www.camram.org/ instead.)

I like the idea of hashcash as an increase-spammer-costs strategy, but 
there's one big fly in the ointment.  The computational requirements 
effectively make a hashcash-based system impractical for small, 
ultra-portable or autonomous devices.
A modern PC might only take a few seconds to generate a 20-bit 
hashcash, but an ARM-based PDA or mobile phone might take a lot longer 
(burning battery power all the while), and a toaster would take 
forever.  Not to forget the old Macs (usually 68030 based) that are 
commonly resurrected to help elderly, disadvantaged or disabled people 
establish lines of contact.  (Yes, I know someone who actually does 
this.  Old Macs are amazingly durable.)
IIRC, the '030 (which is a contemporary of the i386) is neither 
superscalar nor pipelined, usually has a pitifully low clock speed, and 
is therefore hopelessly slow by modern standards.  Even simple 
image-compression algorithms are hard work for such a machine.  How 
long is it acceptable for a hashcash algorithm to take on such a 
low-end box?  That's the limiting factor when deciding how much each 
mail will "cost".
Finally, I should point out that it is possible to implement most 
crypto algorithms in hardware, resulting in a much greater hashcash 
availability to a determined organisation (such as a spammer ring) 
after some capital investment.  The EFF showed that it was possible for 
3DES with "Deep Crack", and while it cost $250K to build Deep Crack, an 
equivalent machine would be much cheaper today because of the 
availability of inexpensive FPGAs.
A 20-kilogate FPGA development kit costs $300, individual 20Kgate FPGAs 
cost in the $tens, and a set of custom PCBs can be had for $hundreds at 
worst.  So, let's say the incremental cost is of the order of $25 per 
FPGA, giving perhaps 25 hashcash tokens per second on average (yes, I'm 
being faintly optimistic).  Even allowing for one-off development and 
equipment-manufacture costs, spam would appear to be worth enough to 
make such a system affordable.  That puts Granny and her long-suffering 
Mac*[1] at even more of a disadvantage.
However, a system that uses hashcash as a fallback for other 
authentication methods would be an interesting concept.  For example, 
use hashcash only to initiate "stranger" communications, and use a 
strongly-authenticated whitelist otherwise.  Reading the rest of that 
site, I see that's in fact what it suggests.  It still doesn't help 
Granny if it takes her (say) 4 hours to make hashcash, even if she only 
needs to do that once in a blue moon.
However, there's another problem with CAMRAM, though it's less obvious. 
 Users will build up a list of public keys over time, presumably stored 
on their own machine.  Their private keys will also presumably be 
stored locally.  It's reasonably trivial for a worm to pick up a 
selection of these keys as it propagates, and deliver them to the 
black-hats by any of a variety of means.
Suddenly, they have the means to send spam to a lot of people - and 
even better (in the case of a mass-mailing attachment-based worm), 
these are mostly the clueless types who are most likely to respond 
positively to spam and scams.  Oh dear.
This second problem is the main reason why any mainstream 
authentication solution requires the authentication to be done via 
third-party, (semi-)trusted servers.  It's much harder to pick keys off 
a dedicated server than some newbie's desktop, and if they are, that 
server can be de-listed quite quickly until it fixes the problem.
*[1] - Mac IIcx, 16MHz 68030+68882, 5MB 120ns RAM, 40MB HD, 640x480, 
monochrome.  I pulled one of these out of a skip last year, and after 
drying it out I found that it still works.  It has similar capabilities 
to Macs I've heard of being distributed, free of charge, to 
disadvantaged folks in the American desert.  So, if someone wants to 
know exactly how long a given algorithm will take on that machine, 
there's at least half a chance I'll run it and let them know.
--------------------------------------------------------------
from:     Jonathan "Chromatix" Morton
mail:     chromi(_at_)chromatix(_dot_)demon(_dot_)co(_dot_)uk
website:  http://www.chromatix.uklinux.net/
tagline:  The key to knowledge is not to rely on people to teach you it.


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