<p>Modern video encoders produce a compressed bitstream that contains different syntax elements, where selective encryption algorithms encrypt some of those syntax elements to achieve a desired level of security. Existing selective encryption algorithms fail to provide the reasons for using specific syntax elements or discuss their individual impact on performance. Accordingly, this paper starts by analyzing different syntax elements such as Intra-Prediction Mode (IPM), Non-Zero Quantized Coefficients (NZ-QC), and Motion Vector Difference (MVD). Their contribution to the video quality is then investigated using 12 standard videos having 3450 frames. Different encryption algorithms that suit each of the chosen syntax elements are designed using the logistic map as an example of a pseudo-random bit generator. A proposed Selective Encryption System (SES) utilizes the designed algorithms to encrypt different combinations of syntax elements. The best encryption combinations are selected from 31 cases using a designed figure of merit with 106,950 encrypted frames. The maximum reported time increase of 0.39% demonstrates SES efficiency. SES robustness and the resulting video quality are evaluated using standard evaluation metrics, where the average Peak Signal to Noise Ratio (PSNR) and Structural Similarity Index Metric (SSIM) are 6.964 and 0.230, respectively. Furthermore, SES is tested against high-definition videos where results and comparisons demonstrate that SES is effective, efficient, and suitable for real-time applications.</p>