On Sat, Feb 25, 2017 at 11:10:02AM -0500, Ethan Heilman via bitcoin-dev wrote:
> >SHA1 is insecure because the SHA1 algorithm is insecure, not because
> 160bits isn't enough.
> 
> I would argue that 160-bits isn't enough for collision resistance. Assuming
> RIPEMD-160(SHA-256(msg)) has no flaws (i.e. is a random oracle), collisions

That's something that we're well aware of; there have been a few discussions on
this list about how P2SH's 160-bits is insufficient in certain use-cases such
as multisig.

However, remember that a 160-bit *security level* is sufficient, and RIPEMD160
has 160-bit security against preimage attacks. Thus things like
pay-to-pubkey-hash are perfectly secure: sure you could generate two pubkeys
that have the same RIPEMD160(SHA256()) digest, but if someone does that it
doesn't cause the Bitcoin network itself any harm, and doing so is something
you choose to do to yourself.

In any case, segwit will provide a 256-bit pay-to-witness-script-hash(1), which
provides a 128-bit security level against collision attacks.

1) https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki#Native_P2WSH

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https://petertodd.org 'peter'[:-1]@petertodd.org