Hello all, given some recent work and discussions around BIP 174 (Partially Signed Bitcoin Transaction Format) I'd like to bring up a few ideas.
First of all, it's unclear to me to what extent projects have already worked on implementations, and thus to what extent the specification is still subject to change. A response of "this is way too late" is perfectly fine. So here goes: * Key-value map model or set model. This was suggested in this thread: https://twitter.com/matejcik/status/1002618633472892929 The motivation behind using a key-value model rather than a simple list of records was that PSBTs can be duplicated (given to multiple people for signing, for example), and merged back together after signing. With a generic key-value model, any implementation can remove the duplication even if they don't understand fields that may have been added in future extensions. However, almost the same can be accomplished by using the simpler set model (the file consists of a set of records, with no duplication allowed). This would mean that it would technically be legal to have two partial signatures with the same key for the same input, if a non-deterministic signer is used. On the other hand, this means that certain data currently encoded inside keys can be dropped, reducing the PSBT size. This is particularly true for redeemscripts and witnessscripts, as they can just be computed by the client when deserializing. The two types could even be merged into just "scripts" records - as they don't need to be separated based on the way they're looked up (Hash160 for P2SH, SHA256 for P2WSH). The same could be done for the BIP32 derivation paths, though this may be expensive, as the client would need to derive all keys before being able to figure out which one(s) it needs. One exception is the "transaction" record, which needs to be unique. That can either be done by adding an exception ("there can only be one transaction record"), or by encoding it separately outside the normal records (that may also be useful to make it clear that it is always required). * Ability for Combiners to verify two PSBT are for the same transaction Clearly two PSBTs for incompatible transactions cannot be combined, and this should not be allowed. It may be easier to enforce this if the "transaction" record inside a PSBT was required to be in a canonical form, meaning with empty scriptSigs and witnesses. In order to do so, there could be per-input records for "finalized scriptSig" and "finalized witness". Actually placing those inside the transaction itself would only be allowed when all inputs are finalized. * Optional signing I think all operation for the Signer responsibility should be optional. This will inevitably lead to incompatibilities, but with the intent of being forward compatible with future developments, I don't think it is possible to require every implementation to support the same set of scripts or contracts. For example, some signers may only implement single-key P2PKH, or may only support signing SegWit inputs. It's the user's responsibility to find compatible signers (which is generally not an issue, as the different participants in a setup necessarily have this figured out before being able to create an address). This does mean that there can't be an unconditional test vector that specifies the produced signature in certain circumstances, but there could be "For implementations that choose to implement signing for P2PKH inputs using RFC6979, the expected output given input X and access to key Y is Z". On the other hand, the Combiner and Finalizer roles can probably be specified much more accurately than they are now. * Derivation from xpub or fingerprint For BIP32 derivation paths, the spec currently only encodes the 32-bit fingerprint of the parent or master xpub. When the Signer only has a single xprv from which everything is derived, this is obviously sufficient. When there are many xprv, or when they're not available indexed by fingerprint, this may be less convenient for the signer. Furthermore, it violates the "PSBT contains all information necessary for signing, excluding private keys" idea - at least if we don't treat the chaincode as part of the private key. For that reason I would suggest that the derivation paths include the full public key and chaincode of the parent or master things are derived from. This does mean that the Creator needs to know the full xpub which things are derived from, rather than just its fingerprint. * Generic key offset derivation Whenever a BIP32 derivation path does not include any hardened steps, the entirety of the derivation can be conveyed as "The private key for P is equal to the private key for Q plus x", with P and Q points and x a scalar. This representation is more flexible (it also supports pay-to-contract derivations), more efficient, and more compact. The downside is that it requires the Signer to support such derivation, which I don't believe any current hardware devices do. Would it make sense to add this as an additional derivation method? * Hex encoding? This is a very minor thing. But presumably there will be some standard way to store PSBTs as text for copy-pasting - either prescribed by the spec, or de facto. These structures may become pretty large, so perhaps it's worth choosing something more compact than hexadecimal - for example Base64 or even Z85 (https://rfc.zeromq.org/spec:32/Z85/). Cheers, -- Pieter _______________________________________________ bitcoin-dev mailing list firstname.lastname@example.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev