We initiate the investigation of {\\em gate}-tampering attacks againstcryptographic circuits. Our model is motivated by the plausibility of

tampering directly with circuit gates and by the increasing use of {\\em tamper

resilient gates} among the known constructions that are shown to be resilient

against {\\em wire-tampering} adversaries. We prove that gate-tampering is {\\em

strictly} stronger than wire-tampering. On the one hand, we show that there is

a gate-tampering strategy that perfectly simulates any given wire-tampering

strategy. On the other, we construct families of circuits over which it is

impossible for any wire-tampering attacker to simulate a certain gate-tampering

attack (that we explicitly construct). We also provide a tamper resilience

impossibility result that applies to both gate and wire tampering adversaries

and relates the amount of tampering to the depth of the circuit. Finally, we

show that defending against gate-tampering attacks is feasible by appropriately

abstracting and analyzing the circuit compiler of Ishai et al.

\\cite{Ishai:2006a} in a manner which may be of independent interest.

Specifically, we first introduce a class of compilers that, assuming certain

well defined tamper resilience characteristics against a specific class of

attackers, can be shown to produce tamper resilient circuits against that

same class of attackers. Then, we describe a compiler in this class for which

we prove that it possesses the necessary tamper-resilience characteristics

against gate-tampering attackers.