Three-Pass Protocol Implementation in Vigenere Cipher Classic Cryptography Algorithm with Keystream Generator Modification

Vigenere Cipher is one of the classic cryptographic algorithms and included into symmetric key cryptography algorithm, where to encryption and decryption process use the same key. Vigenere Cipher has the disadvantage that if key length is not equal to the length of the plaintext, then the key will be repeated until equal to the plaintext length, it course allows cryptanalysts to make the process of cryptanalysis. And weaknesses of the symmetric key cryptographic algorithm is the safety of key distribution factor, if the key is known by others, then the function of cryptography itself become useless. Based on two such weaknesses, in this study, we modify the key on Vigenere Cipher, so when the key length smaller than the length of plaintext entered, the key will be generated by a process, so the next key character will be different from the previous key character. In This study also applied the technique of Three-pass protocol, a technique which message sender does not need to send the key, because each using its own key for the message encryption and decryption process, so the security of a message would be more difficult to solved.