Hi everyone,

I would like to propose a standard format for unsigned and partially signed
transactions.

===Abstract===

This document proposes a binary transaction format which contains the
information
necessary for a signer to produce signatures for the transaction and holds
the
signatures for an input while the input does not have a complete set of
signatures.
The signer can be offline as all necessary information will be provided in
the
transaction.

===Motivation===

Creating unsigned or partially signed transactions to be passed around to
multiple
signers is currently implementation dependent, making it hard for people
who use
different wallet software from being able to easily do so. One of the goals
of this
document is to create a standard and extensible format that can be used
between clients to allow
people to pass around the same transaction to sign and combine their
signatures. The
format is also designed to be easily extended for future use which is
harder to do
with existing transaction formats.

Signing transactions also requires users to have access to the UTXOs being
spent. This transaction
format will allow offline signers such as air-gapped wallets and hardware
wallets
to be able to sign transactions without needing direct access to the UTXO
set and without
risk of being defrauded.

The full text can be found here:
https://github.com/achow101/bips/blob/bip-psbt/bip-psbt.mediawiki

Andrew Chow