A network communication device includes a host interface, which is coupled to communicate with a host processor, having a host memory, so as to receive a work request to execute a transaction in which a plurality of data blocks are to be transferred over a packet network. Processing circuitry is configured to process multiple data packets so as to execute the transaction, each data packet in the transaction containing a portion of the data blocks, and the multiple data packets including at least first and last packets, which respectively contain the first and last data blocks of the transaction. The processing circuitry is configured to compute a transaction signature over the data blocks while processing the data packets so that at least the first data block passes out of the network communication device through one of the interfaces before computation of the transaction signature is completed.