Re: [bitcoin-dev] BIP 174 thoughts

[Achow101 <achow101-lists@achow101.com>]

Hi,​

On June 26, 2018 11:09 PM, William Casarin <jb55@jb55.com> wrote:

> ​​
> 
> Hey Andrew,
> 
> If I'm reading the spec right: the way it is designed right now, you
> 
> could create hundreds of thousands of zero bytes in the input or output
> 
> key-value arrays. As far as I can tell this would be considered valid,
> 
> as it is simply a large array of empty dictionaries. Is this right? I'm
> 
> worried about buffer overflows in cases where someone sends a large blob
> 
> of zeros to an unsuspecting implementation.

No, that is incorrect. That whole paragraph is actually outdated, it was intended
for the possibility of adding output maps, which we have already done. I have 
removed it from the BIP.

However, it is possible for a PSBT to contain very large unknown key-value pairs 
which could potentially cause a problem. But I do not think that large PSBTs should 
really be a problem as they are really something that the user has to enter rather 
than something received remotely without user interaction.



On June 27, 2018 6:39 AM, Andrea via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

> ​​
> 
> Hi William, Andrew, list,
> 
> As noted by William there are some types missing in the global-types definition, because the number of each map for I/O must be known to the parser in order to use the correct definitions for the types. At the moment a parser reading a key-value record does not know whether it should read it as per-input type or per-output, a way to address this is to declare in advance the number of maps and ensure they are ordered (inputs first). If you haven't already worked out some types for that i propose using:
> 

Parsers actually do know because that information is present in the unsigned transaction 
at the beginning of each PSBT. Since each input and output must be accounted for,
a parser can just parse the unsigned transaction and from there it can know how
many inputs and outputs to expect. If it sees more or less, it should throw an error
as the transaction is invalid.

Of course this implies that implementations will need to parse the unsigned transaction,
but not all actually need to. Combiners do not need to, they just need to merge the
maps together and follow the key uniqueness rule. They don't really need to know
or care about the number of inputs and outputs, just that the PSBTs being merged
share the same unsigned transaction and have the same number of maps.

Other roles need to understand the unsigned transaction anyways, so they still need
to parse it thus this isn't really a problem for those roles.

>     
>     On another note I think we can set a hard limit on the size of the PSBT, currently is 'legal' to produce a very large PSBT with an excessive number of Inputs and Outputs. By excessive I mean that even in the best case scenario (using the smallest possible scriptPubKey as in P2WPKH) it is possible to create a PSBT that would certainly create an invalid transaction (because of its size) when finalized. I haven't found anything related to this in the previous discussions, please ignore this if it was already proposed/discussed.
>     

I don't think such a limitation is practical or useful. A transaction can currently have, at most,
~24000 inputs and ~111000 outputs (+/- a few hundred) which is well beyond any useful limit.
Additionally, such limits may not be as extensible for future work. It is hard to determine what
is a reasonable limit on transaction size, and I don't think it is useful to have a limit. At worst
we would simply create an invalid transaction if it were too large.


Andrew