Towards Secure Network Programming and Recovery in Wireless Sensor Networks

A number of multi-hop wireless reprogramming systems haveemerged for sensor network retasking but none of these sys-tems support a cryptographically-strong, public-key-basedsystem for program authentication or any form of recoveryfrom authenticated, but Byzantine, programs. The tradi-tional techniques for authenticating a program and recover-ing from Byzantine user programs, namely a digital signa-ture of the program hash and hardware-based memory pro-tection, respectively, are not suited to resource-contrainedsensor nodes. We present techniques that are consistentwith the limited resources of sensor networks, can be usedto secure existing wireless reprogramming systems, and al-low recovery from Byzantine programs. Our solution to thesecure reprogramming problem is based on authenticatedstreams. A program image consists of several code and datasegments that are mapped to a series of messages for trans-mission over the network. A hash of the first message inthis series is digitally signed and the hash and signature areprepended to the series. The signed hash authenticates thefirst message, which in turn contains a hash of the secondmessage. Similarly, the second message contains a hash ofthe third message, and so on, recursively binding each mes-sage to the one logically preceding it in the series through thehash chain. The solution to the recovery problem requiresboth on- and off-chip hardware support in the form of awrite-protected boot block and a grenade timer. Recoveryis enforced by periodically resetting the node which executesa trusted bootloader located in the boot block. We imple-mented the security and recovery primitives using TinyOSand demonstrated that the overhead incurred is small com-pared with the cost of network programming.