Improving the reliability of Ciphertext on Vigenere Cipher Algorithm Using Recurrence Random Key

The weakness of vigenere cipher lies in the short key and repeated periodically, allowing the key repeatedly with the same letter in the encryption process, it is used by cryptanalysis using kasiski method to guess the key length, after the length key is known guessing done plaintext by exhaustive key search so ciphertext of this algorithm can be solved. Modifications vigenere cipher is a solution to strengthen the ciphertext. Modifications made that useing looping random keys on algoritam vigenere cipher. The methodology used by quantitative method with a descriptive approach with nonparametric statistical test through a test mark. Testing was performed using six plaintext, three plaintext using Indonesian and English. The key for that used consists of five key in English and five keys in Indonesian. The test results showed that the method kasiski can not guess the key length of the algorithm viginere modified. At vigenere unmodified algorithm shows the difference in strength of the ciphertext, so the key length can be guessed by kasiski method. The test results showed signs sig is 0.00 and if Sig <0.05 or 0.00 <0.05, significant difference in the presence vigenere modification.