  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.
