On Fri, Jun 1, 2018 at 1:03 PM, Johnson Lau <jl2...@xbt.hk> wrote: > 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 < > firstname.lastname@example.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 implementation. 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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