Subject: Re: [bitcoin-dev] Time to worry about 80-bit collision attacks or not?

From: Pieter Wuille <[email-masked]>
Date: Fri, 8 Jan 2016 00:52:27 +0100

> "The problem case is where someone in a contract setup shows you a
script, which you accept as being a payment to yourself. An attacker could
use a collision attack to construct scripts with identical hashes, only one
of which does have the property you want, and steal coins.
>
> So you really want collision security, and I don't think 80 bits is
something we should encourage for that. Normal pubkey hashes don't have
that problem, as they can't be constructed to pay to you."
>
> ... but I'm unconvinced:
>
> "But it is trivial for contract wallets to protect against collision
attacks-- if you give me a script that is "gavin_pubkey CHECKSIG
arbitrary_data OP_DROP" with "I promise I'm not trying to rip you off, just
ignore that arbitrary data" a wallet can just refuse. Even more likely, a
contract wallet won't even recognize that as a pay-to-gavin transaction.
>
> I suppose it could be looking for some form of "gavin_pubkey
somebody_else_pubkey CHECKMULTISIG ... with the attacker using
somebody_else_pubkey to force the collision, but, again, trivial contract
protocol tweaks ("send along a proof you have the private key corresponding
to the public key" or "everybody pre-commits pubkeys they'll use at
protocol start") would protect against that.

Yes, this is what I worry about. We're constructing a 2-of-2 multisig
escrow in a contract. I reveal my public key A, you do a 80-bit search for
B and C such that H(A and B) = H(B and C). You tell me your keys B, and I
happily send to H(A and B), which you steal with H(B and C).

Sending along a proof does not help, you can't prove that you do not know
of a collision. Pre-committing does help, but is a very non-obvious
security requirement, something I strongly believe is far riskier in
practice.

Bitcoin does have parts that rely on economic arguments for security or
privacy, but can we please stick to using cryptography that is up to par
for parts where we can? It's a small constant factor of data, and it
categorically removes the worry about security levels.

-- 
Pieter