A new secure image transmission technique is proposed, which transforms automatically a given large-volume secret image into a so-called secret-fragment-visible mosaic image of the same size. The mosaic image, which looks similar to an arbitrarily selected target image and may be used as a camouflage of the secret image, is yielded by dividing the secret image into fragments and transforming their color characteristics to be those of the corresponding blocks of the target image. Skillful techniques are designed to conduct the color transformation process so that the secret image may be recovered nearly lossless. Modern day computer security relies heavily on cryptography as a means to protect the data that we have become increasingly reliant on. The main research in computer security domain is how to enhance the speed of encryption algorithm. The computing capability of Graphic Processing Unit as a coprocessor of the CPU can leverage massive parallelism. This paper presents a novel algorithm for calculating modulo value that can process large power of numbers which otherwise are not supported by built-in data types. First the traditional algorithm is studied. Secondly, the parallelized encryption algorithm is designed. Thirdly, the designed algorithm is realized for small prime numbers and large prime number. As a result the main fundamental problem of encryption algorithm such as speed and use of poor or small prime numbers that has led to significant security holes, despite the encryption algorithm’s mathematical soundness can be alleviated by this algorithm. Public key encryption and decryption is computationally heavy because a lot of modular multiplications with very large numbers are needed to perform these tasks. Therefore public key algorithm is known to be much slower than symmetric key algorithms.