" The fundamental weakness of the Vigenère cipher is its cyclical nature. If the keyword is five letters long, then every fifth letter of the plaintext is encrypted according to the same cipher alphabet. If the cryptanalyst can identify the length of the keyword, the ciphertext can be treated as a series of five monoalphabetic ciphers, and each one can be broken by frequency analysis. However, consider what happens as the keyword gets longer. Imagine a plaintext of 1,000 letters encrypted according to the Vigenère cipher, "

― Simon Singh , The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography