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In this paper a physical layer security scheme is proposed for secret transmission of data between legitimate parties, Alice and Bob, in presence of an illegitimate receiver, Eve. It is assumed that there is no knowledge of Eve's channel state information, and Alice/Bob exploit common randomness of wireless environment to randomize data over independent subcarriers. Specifically, we consider concurrent transmission of secret/normal data over a set of independent subcarriers to meet the goal of achieving high uncertainty in information received by illegitimate party. It is first shown that this scheme provides perfect secrecy in information-theoretic sense. Next, a power allocation strategy is designed to maintain normal service with specified data rate requirement and minimum power. Alice and Bob then use the available power to opportunistically achieve the maximum possible secrecy rate. Satisfactory performance curves of this scheme is shown under different parameters (number of total subcarriers, data requirement of the normal service, and main channel average SNR level) using complementary cumulative distribution function of obtained secrecy rate. One goal in this work is to demonstrate how the price for secret communication is paid in terms of consumed power and occupied subcarriers when Alice/Bob has no knowledge of wiretap channel state information.