  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.
