With the growth of networks, the Internet of Things (IoT) and new cyber attacks pose threats to privacy and security, secure communication is therefore becoming one of the most crucial concerns. For this purpose, symmetric algorithms namel; The Rijndael algorithm, the Serpent algorithm, and the TWOFISH algorithm pay equal attention. In this paper, the TWOFISH algorithm’s mathematical complexity is improved by using substitution boxes (S-boxes) drawn from a multiplicative group of units of chain ring . As these S-boxes have the property of having copious generators, they, therefore, produce a rich algebraic complexity. Moreover, the time complexity of the proposed work is modified by processing the 64-bit block throughout the process and reducing the number of subkeys. To measure the strength of the proposed algorithm, various standard color digital images, with a size of , are encrypted and tested. The computation speed of the encryption is compared to the standard TWOFISH algorithm’s speed and found that the newly designed algorithm is quite fast. For security analysis and quality assessment, various statistical tests are performed on the standard encrypted images. The results recommend that the proposed algorithm is a strong candidate for digital image encryption.