Abstract: A computer-implemented method of mitigating the effect of a security threat present in a computer file of a known type. The method involves modifying the file to mitigate unsafe behaviour when the file is interpreted in accordance with the known type and/or in accordance with one or more different file types, and subsequently checking the modified file to ensure that it exhibits a known safe behaviour when interpreted according to the one or more different file types. The modified file may be checked to ensure it exhibits known safe behaviour when interpreted according to the one or more different file types. There may be two or more independent checks each according to one of the one or more different file types. The independent checks may be done in parallel. The step of modifying may include adding a damper to the file.

Abstract: A data structure is transmitted from a first computer to a second computer by transmitting a plurality of messages from the first computer to the second computer. Each message contains data of a node of the structure and one or more references to child pointer locations in previously transmitted nodes. The data of each transmitted node is stored in the second computer, and the location of each node is stored in a table of addresses and the or each reference includes an index into the table of addresses. The first computer may then use indexes into the table to identify which nodes need updating, and thus avoid the need for the first computer to know how the second computer is allocating nodes in memory. The or each reference may include an index into an array of child pointers belonging to a node, may include the sequence number of the parent node of the node being transmitted and may also include the index into the parent node's children to the pointer to the node being transmitted.

Abstract: Apparatus (104) for connecting two or more computer networks having two or more network interface machines (201, 202, 203) each arranged to be connected to a respective computer network with a bidirectional communications link (105, 106, 107) enabling the network interface machine to receive data from and transmit data to the computer network. The network interface machines are connected together with at least one content checker (210, 211) to enable data to be transmitted from one network interface machine to another, and arranged such that data transmitted from one network interface machine to another network interface machine must pass via a content checker. Each network interface machine is arranged to transmit flow control data. The network interface machines are connected to the content checkers only by unidirectional communications links.