[bitcoin-dev] SIGHASH2 for version 1 witness programme

Russell O'Connor roconnor at blockstream.io
Fri Jun 1 18:15:32 UTC 2018

On Fri, Jun 1, 2018 at 1:03 PM, Johnson Lau <jl2012 at xbt.hk> wrote:

> On 1 Jun 2018, at 11:03 PM, Russell O'Connor <roconnor at blockstream.io>
> wrote:
> On Thu, May 31, 2018 at 2:35 PM, Johnson Lau via bitcoin-dev <
> bitcoin-dev at 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
> 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?

In theory, having a fixed 64 byte constant at the beginning results in zero
overhead for those 64 bytes.  An implementation would just start the usual
SHA-256 algorithm with a different pre-computed and fixed initial value
than SHA-256's standard initial value.  The SHA-256 padding counter would
also need to start at 64*8 bits rather than starting at 0 bits.  In
practice, assuming a OpenSSL-like implementation of SHA-256, it should be
easy to implement this optimization. One would replace SHA256_Init call
with a variant that initializes the SHA256_CTX to this pre-computed value
and sets SHA256_CTX's num counter to the appropriate value.  Non-optimized
implementations can still just add the 64 byte prefix and use any SHA-256

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?

I prefer a different opcode for CHECKSIGFROMSTACK because I dislike opcodes
that pop a non-static number of elements off the stack.  Popping a dynamic
number of stack elements makes it more difficult to validate that a Script
pubkey doesn't allow any funny business.
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