This paper investigates power allocation strategies
over a bank of independent parallel Gaussian wiretap channels
where a legitimate transmitter and a legitimate receiver communicate
in the presence of an eavesdropper and a friendly jammer.
We give algorithms to compute the optimal power allocation
strategy of the jammer in the degraded scenario. We also give an
algorithm to compute power allocations strategies of the jammer
in a general scenario, leading to significant performance gains in
relation to isotropic jamming. Additionally, we provide a set of
results that cast further insight into the problem. In our scenario,
which is applicable to current OFDM communications systems,
we demonstrate that the proposed jammer power allocation
strategy can lead to considerable secrecy gains.