{sign|verify}message replacement

Luke Dashjr and Jim Posen discuss the use of a sign message to prove the availability of funds without compromising fungibility or hot/cold wallet separation. The discussion revolves around using not necessarily specific UTXOs to validate present possession of funds. Jim Posen suggests including nLockTime and nSequence in the proof container, which defaults to 0 and 0xFFFF respectively. In response to a query on tweaking the default SIGHASH_ALL, a suggestion is made to append a byte at the end of the sig for encoding checks but keep it as 0x00 since it does not affect the outcome of the verification.