  We propose a new adversarial attack on frequency-hopping based wireless
communication between two users, namely Alice and Bob. In this attack, the
adversary, referred to as Eve, instantaneously modifies the transmitted signal
by Alice before forwarding it to Bob within the symbol-period. We show that
this attack forces Bob to incorporate Eve's signal in the decoding process;
otherwise, treating it as noise would further degrade the performance akin to
jamming. Through this attack, we show that Eve can convert a slow-fading
channel between Alice and Bob to a rapid-fading one by modifying every
transmitted symbol independently. As a result, neither pilot-assisted coherent
detection techniques nor blind-detection methods are directly applicable as
countermeasures. As potential mitigation strategies, we explore the
applicability of frequency-hopping along with (i) On-Off keying (OOK) and (ii)
Binary Frequency-Shift-Keying (FSK) as modulation schemes. In the case of OOK,
the attacker attempts to introduce deep-fades on the tone carrying the
information bit, whereas in the case of BFSK, the attacker pours comparable
energy levels on the tones carrying bit-$0$ and bit-$1$, thereby degrading the
performance. Based on extensive analyses and experimental results, we show that
(i) when using OOK, Bob must be equipped with a large number of receive
antennas to reliably detect Alice's signal, and (ii) when using BFSK, Alice and
Bob must agree upon a secret-key to randomize the location of the tones
carrying the bits, in addition to randomizing the carrier-frequency of
communication.
