On Thu, May 31, 2018 at 2:35 PM, Johnson Lau via bitcoin-dev <
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
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