Abstract

This paper presents the development and simulation of a communication system in a Local Area Network (LAN) environment based on the 8-bit ASCII character encoding standard combined with a monoalphabetic substitution cipher. The primary objective of the study is to provide a clear and systematic illustration of the fundamental principles of data encryption, transmission, and decryption in a computer network environment, while verifying the reliability of data exchange through the TCP/IP protocol.
The system is designed according to a client–server architecture and implemented in Python, utilizing the socket library to establish communication between two Windows-based computers within the same LAN. On the sender side (client), the input character string undergoes several processing stages: (1) conversion of each character into its 8-bit ASCII representation, (2) encryption using a predefined monoalphabetic substitution table, and (3) encapsulation of the encoded data into a byte stream for transmission via TCP. This procedure enables learners to clearly observe the relationship between textual data, binary representation, and the data encapsulation mechanism employed during network transmission.
On the receiver side (server), the system accepts the incoming byte stream through the established TCP connection and performs decryption using the corresponding inverse substitution table. The decrypted data are then converted from 8-bit ASCII codes back into the original character string, allowing comparison with the initial message to evaluate data integrity and transmission accuracy. The use of TCP ensures reliable communication with built-in error control and packet ordering, thereby highlighting the role of the transport layer in the network communication model.
Experimental results demonstrate that the system operates stably in a LAN environment, with transmitted data accurately recovered and no discrepancies observed under normal connection conditions. Although the proposed model employs a simple encryption technique, it provides significant pedagogical value for courses related to communication principles, data representation structures, computer networks, and fundamental cryptographic techniques. Furthermore, the system establishes a foundation for future extensions toward more advanced encryption schemes or the integration of error detection and correction mechanisms in subsequent studies.