Development of Dynamic Key Based on Pseudo-Random Algorithm in Vigenere Cipher for Hybrid Vigenere-ElGamal Encryption to Secure Documen Data

The security of digital documents has become increasingly critical in line with the growing threats to the confidentiality and integrity of information. This study aims to develop a hybrid cryptographic system that combines the Vigenère Cipher with dynamic keys generated through the Linear Congruential Generator (LCG) and the ElGamal algorithm. The application of dynamic keys in the Vigenère Cipher enhances randomness and reduces predictability, while the ElGamal algorithm strengthens protection through public-key cryptography. The system was designed and implemented as a desktop application using Visual Basic .NET Framework 4.0, supporting the encryption and decryption of PDF, DOCX, and XLSX files. Each data byte is processed according to the algorithm sequence stored in a hybrid file format, ensuring accurate decryption results. Experimental testing demonstrated that the system can successfully restore encrypted documents to their original form without data loss, thereby maintaining document integrity. However, the ciphertext size increased to nearly three times that of the original file due to the ElGamal mechanism, which produces two values for each byte. This study concludes that the hybrid Vigenère–ElGamal method with LCG-based dynamic keys can effectively enhance the security of digital document encryption, although it results in a larger ciphertext size.