[Bitcoin-development] 75%/95% threshold for transaction versions

s7r s7r at sky-ip.org
Sun Apr 26 00:01:10 UTC 2015

Thank you all for your comments. The youtube video was indeed very
educative and nice to watch.

It's true that malleability is not the end of the world, but it is
annoying for contracts and micropayment channels, especially refunds
spending the fund tx before it is even in the blockchain, relying solely
on its txid.

BIP62 is good for preventing 3rd parties (non signers) to mutate txids,
but cannot do anything against 2nd parties (signers). I think we can
solve both by using NORMALIZEDTXID - wouldn't this be simpler and easier
to implement? Why are we talking about P3SH when we can just upgrade
P2SH to support additional OP codes? I saw that there have been talks
about a hard fork for increasing the block size, might as well take the
opportunity and fix this for good, by implementing BIP62, NORMALIZEDTXID
as well as BIP65. Couldn't all these be part of P2SH?

On 4/25/2015 6:40 PM, Stephen Morse wrote:
> Hi Gregory,
>     In particular not covering the ID allows for transaction replay which
>     can result in monetary losses far more severe than any possible
>     mishandling of malleability could result in. Byzantine attackers can
>     costlessly replay your old transactions any time anyone reuses an
>     address, even accidentally (which cannot be easily prevented since
>     they can race).
> With the SIGHASH_WITHOUT_PREV_VALUE flag, signatures have to explicitly
> specify that they are to be signed without the previous UTXO's
> value/amount. This means that, at worst, replay attacks can send the
> money to the same place it was sent before (which in many cases is
> likely not be a loss of funds), and only if the amount sent to the
> reused address is the exact same as it was before. I don't think this is
> worse than an attacker being able to mutate their transaction and extort
> a merchant who accepts zero-conf transactions. Anyway, not signing the
> input ID wouldn't exactly be the norm, there would be a defined set of
> flags for standard use cases. Not signing the input TXID would only be
> used in specialized cases, such as setting up micropayment channels. 
>     There are no free lunches;  the proposal linked to there is itself a
>     game of wack-a-mole with assorted masking flags; 
> I agree that it is also a bit of wac-a-mole, but the defined space of
> issues is possibly more limited here. There are only X number of things
> that can be signed/not signed in a transaction, and the 'Build your own
> nHashType' proposal enables you to fully specify which of those are
> being signed. If you don't want to get burned by not fully signing your
> transactions, then don't use the non-standard sighash flags.
>     many of which we have
>     no notion of if they're useful for any particular application(s); 
> A few of the flags, indeed, may not ever be useful. But we can't predict
> the future, and I think it's better to build in a more flexible solution
> now than to wish we had more flexible nHashTypes later.
> To the original point of this thread, hopefully the suggested proposal
> won't be necessary as wallets will upgrade to use version 3 transactions
> and the rules associated with them over time. 
> Best,
> Stephen
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