Hybrid Cryptosystem for Image Security by Using Hill Cipher 4x4 and ElGamal Elliptic Curve Algorithm

Cryptography used to send digital images to keep them safe from unauthorized parties who want to see or change the digital image. In cryptography, there is Hybrid Cryptosystem method, which is a method by combining the advantages in 2 types of cryptographic algorithms, symmetrical and asymmetrical. Hill Cipher 4x4, with a 4x4 square matrix, is used as a symmetrical algorithm, which has a relatively fast processing time. ElGamal Elliptic Curve is used as an asymmetric algorithm, where the key used for the encryption and decryption process is a different key, so no key distribution is needed. The Algorithms implemented in an Android-based system using the Java programming language. The conclusion obtained from system testing is that the elliptic curve parameter generation time process is directly proportional to the magnitude of the prime number, the time process of image encryption-decryption is directly proportional to the image pixel size, and the time process of key matrix encryption-decryption is directly proportional to the value of the private key.