From: "Russell O'Connor" <roconnor@blockstream•io>
To: Johnson Lau <jl2012@xbt•hk>
Cc: bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] SIGHASH2 for version 1 witness programme
Date: Fri, 1 Jun 2018 14:15:32 -0400 [thread overview]
Message-ID: <CAMZUoKmqdT3fte0o-CSppMV125u9zmxheaP549=nqkeVGSryMA@mail.gmail.com> (raw)
In-Reply-To: <C3ED56D2-CB1F-4DE5-AB43-F826705806FB@xbt.hk>
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On Fri, Jun 1, 2018 at 1:03 PM, Johnson Lau <jl2012@xbt•hk> wrote:
> 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> 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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next prev parent reply other threads:[~2018-06-01 18:15 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-31 18:35 Johnson Lau
2018-06-01 15:03 ` Russell O'Connor
2018-06-01 17:03 ` Johnson Lau
2018-06-01 18:15 ` Russell O'Connor [this message]
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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