The weakness of the vigenere cipher lies in its short key and is repeated, so there is a key loop in encrypting messages, this is used by cryptanalysts using the Kasiski method to know the key length so it can solve this algorithm. The Kasiski method uses repetitive cryptograms found in the ciphertext to determine the key length. Modification of the vigenere cipher solves strengthen the cipher by using arranged keys to make it difficult to crack the keys against the Kasiski method attacks. This study analyzes the strength of the vigenere cipher ciphertext and the modification of the vigenere cipher from the encryption results composed of plaintexts that use different keys. Against the Kasiski method attack. The method used in this study is a quantitative method with a descriptive approach through nonparametric statistical tests on the sign test. The results showed a difference in the ciphertext's strength from the encryption process even though using the same key. The kaiseki method if can know not all key lengths the ciphertext in the compilation does not occur repetitive cryptograms. Modification of the vigenere cipher has a positive effect on the strength of the cipher and can influence the strength of the ciphertext that is built against the Kasiski method attacks.
