From: Rusty Russell <rusty@rustcorp.com.au>
To: Matt Corallo <lf-lists@mattcorallo.com>,
	Rusty Russell via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>,
	Pieter Wuille <pieter.wuille@gmail.com>,
	Gavin Andresen <gavinandresen@gmail.com>
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	<CAPg+sBhH0MODjjp8Avx+Fy_UGqzMjUq_jn3vT3oH=u3711tsSA@mail.gmail.com>
	<8760z4rbng.fsf@rustcorp.com.au>
	<C4B5B9F1-9C53-45BC-9B30-F572C78096E3@mattcorallo.com>
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Date: Fri, 08 Jan 2016 22:32:01 +1030
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Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Time to worry about 80-bit collision
	attacks	or	not?
Precedence: list

Matt Corallo <lf-lists@mattcorallo.com> writes:
> Indeed, anything which uses P2SH is obviously vulnerable if there is
> an attack on RIPEMD160 which reduces it's security only marginally.

I don't think this is true?  Even if you can generate a collision in
RIPEMD160, that doesn't help you since you need to create a specific
SHA256 hash for the RIPEMD160 preimage.

Even a preimage attack only helps if it leads to more than one preimage
fairly cheaply; that would make grinding out the SHA256 preimage easier.
AFAICT even MD4 isn't this broken.

But just with Moore's law (doubling every 18 months), we'll worry about
economically viable attacks in 20 years.[1]

That's far enough away that I would choose simplicity, and have all SW
scriptPubKeys simply be "<0> RIPEMD(SHA256(WP))" for now, but it's
not a no-brainer.

Cheers,
Rusty.

[1] Assume bitcoin-network-level compute (collision in 19 days) costs
    $1B to build today.  Assume there will be 100 million dollars a day
    in vulnerable txs, and you're on one end of all of them (or can MITM
    if you find a collision), *and* can delay them all by 10 seconds,
    and none are in parallel so you can attack all of them.  IOW, just
    like a single $100M opportunity for 3650 seconds each year.

    Our machine has a 0.11% chance of finding a collision in 1 hour, so
    it's worth about $110,000.  We can build it for that in about 20
    years.