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From: Johnson Lau <jl2012@xbt.hk>
To: Russell O'Connor <roconnor@blockstream.io>
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] SIGHASH2 for version 1 witness programme
Date: Sat, 2 Jun 2018 01:03:05 +0800	[thread overview]
Message-ID: <C3ED56D2-CB1F-4DE5-AB43-F826705806FB@xbt.hk> (raw)
In-Reply-To: <CAMZUoKms85DhtS1mN70nq4LSY7QtXym6E4_yvQk5Q0tizkVwEQ@mail.gmail.com>

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> On 1 Jun 2018, at 11:03 PM, Russell O'Connor <roconnor@blockstream.io> wrote:
> 
> 
> 
> On Thu, May 31, 2018 at 2:35 PM, Johnson Lau via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org <mailto:bitcoin-dev@lists.linuxfoundation.org>> wrote:
> 
>   Double SHA256 of the serialization of:
> 
> Should we replace the Double SHA256 with a Single SHA256?  There is no possible length extension attack here.  Or are we speculating that there is a robustness of Double SHA256 in the presence of SHA256 breaking?
> 
> I suggest putting `sigversion` at the beginning instead of the end of the format.  Because its value is constant, the beginning of the SHA-256 computation could be pre-computed in advance.  Furthermore, if we make the `sigversion` exactly 64-bytes long then the entire first block of the SHA-256 compression function could be pre-computed.
> 
> Can we add CHECKSIGFROMSTACK or do you think that would go into a separate BIP?

I think it’s just a tradition to use double SHA256. One reason we might want to keep dSHA256 is a blind signature might be done by giving only the single SHA256 hash to the signer. At the same time, a non-Bitcoin signature scheme might use SHA512-SHA256. So a blind signer could distinguish the message type without learning the message.

sigversion is a response to Peter Todd’s comments on BIP143: https://petertodd.org/2016/segwit-consensus-critical-code-review#bip143-transaction-signature-verification <https://petertodd.org/2016/segwit-consensus-critical-code-review#bip143-transaction-signature-verification>

I make it a 0x01000000 at the end of the message because the last 4 bytes has been the nHashType in the legacy/BIP143 protocol. Since the maximum legacy nHashType is 0xff, no collision could ever occur.

Putting a 64-byte constant at the beginning should also work, since a collision means SHA256 is no longer preimage resistance. I don’t know much about SHA256 optimisation. How good it is as we put a 64-byte constant at the beginning, while we also make the message 64-byte longer?

For CHECKSIGFROMSTACK (CSFS), I think the question is whether we want to make it as a separate opcode, or combine that with CHECKSIG. If it is a separate opcode, I think it should be a separate BIP. If it is combined with CHECKSIG, we could do something like this: If the bit 10 of SIGHASH2 is set, CHECKSIG will pop one more item from stack, and serialize its content with the transaction digest. Any thought?



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  reply	other threads:[~2018-06-01 17:03 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-31 18:35 [bitcoin-dev] SIGHASH2 for version 1 witness programme Johnson Lau
2018-06-01 15:03 ` Russell O'Connor
2018-06-01 17:03   ` Johnson Lau [this message]
2018-06-01 18:15     ` Russell O'Connor
2018-06-01 18:45       ` Johnson Lau
2018-07-02 18:23 ` Gregory Maxwell
2018-08-30 20:38 ` Johnson Lau
2018-08-30 20:51   ` Christian Decker
2018-08-31  7:42     ` Johnson Lau
2018-09-03 13:53       ` Christian Decker

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