Abstract :
The chat application system generally only sends and receives data. Data
sent and received in the form of text messages and in the form of image files. Lots
of data text messages and image files that traffic on the internet network. Without
realizing it, the data can be reviewed by irresponsible parties. Therefore data
security is needed, so that the data is safe from negative matters. To secure the
data, the method used is cryptography. So the security system created in the chat
application uses a method of modifying the vigenere cipher algorithm. The
modification is by using the catalan number to generate a new key, and then
encrypted using the vigenere cipher algorithm. The results of the encryption are
then randomized using the double columnar transposition method. The results of
encryption and description testing can run with the labcar. In addition there are
several other tests, namely testing the maximum limit of sent or received
characters with a number of 4,116 characters, and the speed of data entering or
appearing in the chat menu, with the increasing number of characters sent, the
longer the message to enter or appear in the chat.