Mediated transfer of funds between three parties Alice wants Bob to transfer 10btc to Carol, and is willing to pay Bob 11btc for the transfer.

They both entrust Trent to mediate the transaction, and Alice is willing to pay Trent 1btc as commission.

What is the correct and most secure protocol for performing this contract?



Edit: does the following protocol make sense?


Alice broadcasts a transaction (Tx1) with 2 outputs:

11btc with an output script specifying 2-of-3 multi-signature verification, specifying Alice, Bob and Trent as recipients
1btc to Trent (commission)

Bob verifies Alice's transaction:

Verified: Bob broadcasts a transaction (Tx2) sending Trent 10btc
Rollback - Bob is unsatisfied or fails to transfer the funds to Trent: Trent broadcasts a transaction (Tx3) sending Alice her 11btc back, specifying Tx1's first output as an input and signs the input. When Alice wants to spend Tx3, adding her signature to the input satisfies the 2-of-3 requirement (and the process stops here)

Trent broadcasts a transaction (Tx4) sending Bob the 11btc, specifying Tx1's first output as an input. When Bob wants to spend Tx4, adding his signature to the input satisfies the 2-of-3 requirement
Trent broadcasts a transaction (Tx5) sending Carol 10btc, specifying Tx2 as an input
Your protocol makes sense, but i would modify it as follows


the 1btc sent to trent should also be mutlisig with Alice's signature as the 2nd one, so that if Trent fails to do what he promised he gets no commission. ((the 1btc will stay in limbo, unless Alice and Trent can somehow work this out)
The Tx2 should also be multisig and Carol's signature should be added, that way Trent can't run with the money.