> On 1 Jun 2018, at 11:03 PM, Russell O'Connor <rocon...@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: 

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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