DKIM is hardly a PoW; signing is cheap and gets cheaper all the time. I
used to work in the email business and big bulk mailers all spent far more
CPU time on other aspects of their business, the overhead of DKIM is

PoW didn't work in the anti spam world because it (amongst other problems)
mixes up bulk mail and spam, which are not the same thing. Very common
conceptual error though.

> humans also don't care if their patience is put to the test by having to
> wait until one Tor exit node is finally unbanned, or by waiting for
> the connection PoW to finish because it temporarily got harder due to
> an attack.

They don't? This is news to me. Humans always care. One of the surest ways
to hurt your online business is to have a slow website because lots of
users will give up rather than tolerate a few seconds of latency. At Google
we actually had formulas that could relate a change in web search latency
to revenue impact.

So humans very much care! I actually doubt that any reasonable mobile
wallet will use the new Tor support bitcoinj by default, for example,
because it imposes quite some startup cost when the downloaded consensus
isn't fresh, and slow startup is painful. It could be optimised but nobody
has done that. For long running desktop wallets where startup time can be
amortised over hours or days, I guess it makes more sense.

I agree that PoW tokens might make sense as a last resort if nodes can't
even put a connection at the bottom of a priority queue and you're right
that it may be a useful tool in a shared toolbox. However if we reach the
point where users are all being PoWd then we're already pretty hosed and
it's probably close to game over :(

I'd say, better have a few Tor-based users realize that they
> should look for a fixed update because their client has to do PoW for
> connecting, rather than having all Tor-based users locked out.

I think Tor is a separate issue. If an attacker wants to either force all
users off Tor, or force them via a handful of exits, then this attack is
quite detectable already and wallets could already decide to simply give up
on Tor at that point automatically. No PoW needed. Well, ideally, nodes
would disconnect a banned IP with some kind of notice saying why it was
banned, but that's a small improvement.

Still, users should be notified that something is unusual.

If we're talking mainstream success then users by and large do not care
about technical mumbo jumbo like peer to peer networks or Tor ("that's the
thing drug dealers and pedos use???"). They just want the damn thing to
work reliably. So notifying them is unhelpful - it's not actionable. They
would just see a message like

   "The wizzle sprocket is kaput - keep working? YES NO"

and then everyone presses yes.

Stuff like Tor plays well in the crypto community but it's very hard to
actually switch on by default, because it needs to have absolutely no cost
at all, otherwise you'll just annoy the vast majority who don't want to pay
for very abstract and hard to quantify privacy benefits.

So I think it's worth considering the DoS problem and Tor somewhat
separately, even though they're related. The solution to a crafty
privacy-attacking DoS that tries to make exits useless is don't use Tor at
all. The solution to "the entire Bitcoin network is under attack" is much
harder. It's unclear to me we can ever solve it convincingly - banks don't
connect together using private networks in which anonymity is forbidden
because they're stupid. They do it because it solves DoS attacks in one
solid move and they feel it's worth the high cost.
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