This paper investigates joint resource allocation problem involving subcarrier allocation, subcarrier pairing (SCP), and power allocation for maximizing sum secure rate in a decode-and-forward (DF) relay assisted OFDMA system with untrusted users. Optimal SCP is highlighted as a novel notion of channel gain tailoring rather than conventional channel gain matching. It is proven that the optimal SCP intends to pair subcarriers such that the variance between the effective channel gains is minimized. It is also shown that depending on the individual power budget constraints at source and the relay, SCP can take either a subordinate role for improving the energy efficiency or the main role of improving the spectral efficiency of the system. SCP as a channel gain tailoring strategy and its role in improving the spectral as well as energy efficiency of cooperative secure communication are highlighted through the simulation results.