This paper proposes an efficient encryption technique based on Dynamic and Secure Substitution Box (DS2B) design suitable for IoT and resource-constrained platforms. The DS2B has the advantages of simple structure and good encryption performance. A different number of strong S-boxes could be generated with minor variations in the DS2B parameters. Performance analyses of the DS2B, including differential/linear cryptanalysis, bijective, nonlinearity, strict avalanche criterion (SAC), and bit independence criterion (BIC) have been presented where high nonlinearity, and low differential uniformity are achieved. Besides, a comparison with recent S-boxes is introduced which shows the robustness of the DS2B. To verify the DS2B, a speech encryption engine was realized on FPGA. Various security analyses were evaluated including the National Institute of Standards & Technology (NIST) statistical test suite, number of samples changes rate NSCR, mean square error (MSE), correlation, histogram, and spectrogram. In this work, the silence periods are encrypted in a way that makes them much harder to detect using the DS2B. The proposed encryption scheme achieved a throughput of 9.87 G bit/s, which is higher than previous work. A Xilinx Nexys 4 FPGA evaluation board was used for implementing and verifying the design.