This paper investigates the encryption efficiency of RC6 block cipher application to digital
 images, providing a new mathematical measure for encryption efficiency, which we will call the
 encryption quality instead of visual inspection, The encryption quality of RC6 block cipher is investigated
 among its several design parameters such as word size, number of rounds, and secret key length and
 the optimal choices for the best values of such design parameters are given. Also, the security analysis
 of RC6 block cipher for digital images is investigated from strict cryptographic viewpoint. The security
 estimations of RC6 block cipher for digital images against brute-force, statistical, and differential attacks
 are explored. Experiments are made to test the security of RC6 block cipher for digital images against
 all aforementioned types of attacks. Experiments and results verify and prove that RC6 block cipher is
 highly secure for real-time image encryption from cryptographic viewpoint. Thorough experimental tests
 are carried out with detailed analysis, demonstrating the high security of RC6 block cipher algorithm. So,
 RC6 block cipher can be considered to be a real-time secure symmetric encryption for digital images.