Re: [bitcoin-dev] SHA1 collisions make Git vulnerable to attakcs by third-parties, not just repo maintainers

From: Shin'ichiro Matsuo <matsuo@mac•com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Cc: Steve Davis <steven.charles.davis@gmail•com>
Subject: Re: [bitcoin-dev] SHA1 collisions make Git vulnerable to attakcs by third-parties, not just repo maintainers
Date: Sat, 25 Feb 2017 09:45:36 -0800	[thread overview]
Message-ID: <81BE82BA-EDFC-45D1-84DE-D65EC24C9F41@mac.com> (raw)
In-Reply-To: <CAEM=y+WkgSkc07ZsU6APAkcu37zVZ7dwSc=jAg1nho31S5ZyxQ@mail.gmail.com>

We should distinguish collision resistance from 2nd pre-image resistance, in general.

As previously written, we should care both hash output length and algorithm itself. The weakness of SHA-0 (preliminary version of SHA-1) was reported in 2004, then many research on the structure of SHA-1 were conducted. In the case of SHA-2, it is harder than SHA-1 to find collisions.

Existing security consideration and evaluation criteria were extensively discussed in the NIST SHA-3 competition. Please see the following sites.

https://ehash.iaik.tugraz.at/wiki/The_SHA-3_Zoo
https://ehash.iaik.tugraz.at/wiki/Cryptanalysis_Categories

We need similar analysis on RIPEMD160 and impacts of attacks on (RIPEMD160(SHA2(msg)). 

We can also refer the security assumption of hash chain in Asiacrypt 2004 Paper. 
https://home.cyber.ee/~ahtbu/timestampsec.pdf

In the discussion of SHA3 competition, we choose another hash design structure, so called "sponge structure." This leads diversity of design principles of hash function and gives resilience even when one hash design structure becomes vulnerable. As Peter Todd wrote, discussion on design structure and algorithm is important. Discussions on all of algorithm, output length and security requirements are needed.

At some future moment, we should think about transition of underlying hash functions. I’m working on this subject and will present an idea at IEEE S&B.

Shin’ichiro Matsuo

> On Feb 25, 2017, at 8:10 AM, Ethan Heilman via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> 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 can be generated in 2^80 queries (actually detecting these collisions requires some time-memory additional trade-offs). The Bitcoin network at the current hash rate performs roughly SHA-256 ~2^78 queries a day or 2^80 queries every four days. Without any break in RIPEMD-160(SHA-256(msg)) the US could build an ASIC datacenter and produce RIPEMD-160 collisions for a fraction of its yearly cryptologic budget.
> 
> The impact of collisions in RIPEMD-160(SHA-256(msg)) according to "On Bitcoin Security in the Presence of Broken Crypto Primitives"(https://eprint.iacr.org/2016/167.pdf):
> 
> >Collisions are similar, though in this case both public keys are under the adversary’s control, and again the adversary does not have access to the private keys. In both scenarios, there is a question of nonrepudiation external to the protocol itself: by presenting a second pre-image of a key used to sign a transaction, a user/adversary can claim that his coins were stolen. 
> 
> How would such an event effect the price of Bitcoin when headlines are "Bitcoin's Cryptography Broken"? How much money could someone make by playing the market in this way? 
> 
> For both reasons of credibility and good engineering (safety margins) Bitcoin should strive to always use cryptography which is beyond reproach.
> 
> 
> On Sat, Feb 25, 2017 at 9:50 AM, Leandro Coutinho via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
> Google recommeds "migrate to safer cryptographic hashes such as SHA-256 and SHA-3"
> It does not mention RIPEMD-160
> 
> https://security.googleblog.com/2017/02/announcing-first-sha1-collision.html?m=1
> 
> 
> Em 25/02/2017 10:47, "Steve Davis via bitcoin-dev" <bitcoin-dev@lists•linuxfoundation.org> escreveu:
> 
> > On Feb 24, 2017, at 7:01 PM, Peter Todd <pete@petertodd•org> wrote:
> >
> > On Fri, Feb 24, 2017 at 05:49:36PM -0600, Steve Davis via bitcoin-dev wrote:
> >> If the 20 byte SHA1 is now considered insecure (with good reason), what about RIPEMD-160 which is the foundation of Bitcoin addresses?
> >
> > SHA1 is insecure because the SHA1 algorithm is insecure, not because 160bits isn't enough.
> >
> > AFAIK there aren't any known weaknesses in RIPEMD160,
> 
> …so far. I wonder how long that vacation will last?
> 
> > but it also hasn't been
> > as closely studied as more common hash algorithms.
> 
> ...but we can be sure that it will be, since the dollar value held in existing utxos continues to increase...
> 
> > That said, Bitcoin uses
> > RIPEMD160(SHA256(msg)), which may make creating collisions harder if an attack
> > is found than if it used RIPEMD160 alone.
> 
> Does that offer any greater protection? That’s not so clear to me as the outputs (at least for p2pkh) only verify the public key against the final 20 byte hash. Specifically, in the first (notional) case the challenge would be to find a private key that has a public key that hashes to the final hash. In the second (realistic) case, you merely need to add the sha256 hash into the problem, which doesn’t seem to me to increase the difficulty by any significant amount?
> 
> 
> /s
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