[nullc]

I believe this was most extensively discussed as part of a long chat that Peter Todd was a part of, so no surprise that you've seen him talk about it. Off-chain banks stuff has been a long term pet interest of his.

In that discussion we applied a merkel-sum tree data-structure— a pet datastructure that I'd previously proposed for making compact proofs of blockchain invalidity in Bitcoin (in order to make a future bitcoin world where no one runs full nodes safe from inflation and theft by miners)— to PT's bank fraud proofing application.

You may find the log interesting: https://people.xiph.org/~greg/bitcoin-wizards-fraud-proof.lo...

Search for "auditable off-chain transactions" and "Merkle-sum-tree"

(I left in a lot of unrelated stuff since it makes the meandering conversation make a bit more sense. Though a lot of this continues a long running dialog about cryptographic-wankery that has been going on for years)

Ultimately these schemes require the use of a jamming free broadcast network of some kind... otherwise they run into the same problems certificate transparency has where you can substitute the commitment on the fly. Fortunately, Bitcoin provides a global consensus mechanism which could be used to directly attach the commitment to the coins being spoken for.