[bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

[Johnson Lau]

If we sign the txids of all inputs, we should also explicitly commit to their values. Only this could fully eliminate any possible way to lie about input value to hardware wallets

> Does it make sense to keep SIGHASH_NONE?
SIGHASH_NONE should be kept. ANYONECANPAY|NONE allows donation of dust UTXOs to miners

> I think NONE without NOFEE doesn't make much sense…….
We might refuse to sign weird combinations like NOFEE|ALLINPUT|ALLOUTPUT. But to keep the consensus logic simple, we should just validate it as usual.

> OP_MASK seems a bit complicated to me. …...
Yes, it looks complicated to me, and it improves security only in some avoidable edge cases in SIGHASH_NOINPUT:

The common case: the exact masked script or address is reused. OP_MASK can’t prevent signature replay since the masked script is the same.

The avoidable case: the same public key is reused in different script templates. OP_MASK may prevent signature replay is the masked script is not the same.

The latter case is totally avoidable since one could and should use a different public key for different script.

It could be made much simpler as NOINPUT with/without SCRIPT. This again is only helpful in the avoidable case above, but it doesn’t bring too much complexity.

> I don't have a reason why, but committing to the scriptCode feels to me like it reduces the "hackiness" of NOINPUT a lot.
OP_MASK is designed to preserve the hackiness, while provide some sort of replay protection (only in avoidable cases). However, I’m not sure who would actually need NOINPUT with KNOWNSCRIPT

> On 21 Nov 2018, at 4:29 AM, Anthony Towns via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:
> 
> On Mon, Nov 19, 2018 at 02:37:57PM -0800, Pieter Wuille via bitcoin-dev wrote:
>> Here is a combined proposal:
>> * Three new sighash flags are added: SIGHASH_NOINPUT, SIGHASH_NOFEE,
>> and SIGHASH_SCRIPTMASK.
>> * A new opcode OP_MASK is added, which acts as a NOP during execution.
>> * The sighash is computed like in BIP143, but:
>>  * If SIGHASH_SCRIPTMASK is present, for every OP_MASK in scriptCode
>> the subsequent opcode/push is removed.
>>  * The scriptPubKey being spent is added to the sighash, unless
>> SIGHASH_SCRIPTMASK is set.
>>  * The transaction fee is added to the sighash, unless SIGHASH_NOFEE is set.
>>  * hashPrevouts, hashSequence, and outpoint are set to null when
>> SIGHASH_NOINPUT is set (like BIP118, but not for scriptCode).
> 
> Current flags are {ALL, NONE, SINGLE} and ANYONECANPAY, and the BIP143
> tx digest consists of the hash of:
> 
>  1 nVersion
>  4 outpoint
>  5 input scriptCode
>  6 input's outpoint value
>  7 input's nSeq
>  9 nLocktime
> 10 sighash
> 
>  2 hashPrevOuts (commits to 4,5,6; unless ANYONECANPAY)
>  3 hashSequence (commits to 7; only if ALL and not ANYONECANPAY)
>  8 hashOutputs
>       - NONE: 0
>       - SINGLE: {value,scriptPubKey} for corresponding output
>       - otherwise: {value,scriptPubKey} for all outputs
> 
> The fee is committed to by hashPrevOuts and hashOutputs, which means
> NOFEE is only potentially useful if ANYONECANPAY or NONE or SINGLE is set.
> 
> For NOINPUT, (2),(3),(4) are cleared, and SCRIPTMASK (which munges (5))
> is only useful given NOINPUT, since (4) indirectly commits to (5). 
> 
> Given this implementation, NOINPUT effectively implies ANYONECANPAY,
> I think. (I think that is also true of BIP 118's NOINPUT spec)
> 
> Does it make sense to treat this as two classes of options, affecting
> the input and output side:
> 
>  output: (pick one, using bits 0,1)
>    * NONE -- don't care where the money goes
>    * SINGLE -- want this output
>    * ALL -- want exactly this set of outputs
> 
>  input: (pick one, using bits 4,5)
>    * PARTIALSCRIPT -- spending from some tx with roughly this script (and
>                       maybe others; SCRIPTMASK|NOINPUT|ANYONECANPAY)
>    * KNOWNSCRIPT -- spending from some tx with exactly this script (and
>                     maybe others; NOINPUT|ANYONECANPAY)
>    * KNOWNTX -- spending from this tx (and maybe others; ANYONECANPAY)
>    * ALL_INPUTS -- spending from exactly these txes
> 
>  combo: (flag, bit 6)
>    * NOFEE -- don't commit to the fee
> 
> I think NONE without NOFEE doesn't make much sense, and
> NOFEE|ALL|ALL_INPUTS would also be pretty weird. Might make sense to
> warn/error on signing when asking for those combinations, and maybe even
> to fail on validating them.
> 
> (Does it make sense to keep SIGHASH_NONE? I guess SIGHASH_NONE|ALL_INPUTS
> could be useful if you just use sigs on one of the other inputs to commit
> to a useful output)
> 
> FWIW, OP_MASK seems a bit complicated to me. How would you mask a script
> that looks like:
> 
>   OP_MASK IF <p> ENDIF <q> ...
> 
> or:
> 
>   IF OP_MASK ENDIF <p> ...
> 
> I guess if you make the rule be "for every OP_MASK in scriptCode the
> *immediately* subsequent opcode/push is removed (if present)" it would
> be fine though -- that would make OP_MASK in both the above not have
> any effect. (Maybe a more explicit name like "MASK_PUSH_FOR_SIGHASH"
> or something might be good?)
> 
> I don't have a reason why, but committing to the scriptCode feels to me
> like it reduces the "hackiness" of NOINPUT a lot.
> 
> Cheers,
> aj
> 
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