Algorithm 5 Peer-to-peer protocol.

B sets receiver ← ${\mathrm{T}}_{\mathrm{B}}$.cert if ${\mathrm{T}}_{\mathrm{B}}$.cert $\ne \perp$ and calls ${\mathrm{T}}_{\mathrm{B}}$.PrepareTxn in his trusted application T. Otherwise, receiver ← ${\mathrm{cert}}_{\mathrm{B}}$ and calls prepare transaction Algorithm 7 from his untrusted application UT. The output is forwarded to B and then forwarded to A with message including message ‘RequestPayment’ [RequestPayment, [timestamp, x], $\sigma$, receiver]. Upon receiving [RequestPayment, [timestamp, x], $\sigma$, receiver] from B, client A sends P to B where P ← TA.Pay (x, timestamp, receiver). Upon receiving P from A, client B performs the following steps: (a) Abort if any of the following conditions is true: PayVerify(P) ≠ 1 or P.receiver ≠ receiver or P ∈ B.inPaymentLog. (b) B adds P to B.inPaymentLog in his UT and sends [ReceivedPayment] to A. (c) If P.receiver.type = “T”, then B calls ${\mathrm{T}}_{\mathrm{B}}$.Collect (P). (d) Otherwise, B engages in the Claim protocol Algorithm 8 with BS as soon as B is online.