Original sentence: During World War I, inventors in several countries realized that a purely random key sequence, containing no repetitive pattern, would make a polyalphabetic substitution, in principle, unbreakable.Finding such weak points in Enigma encipherment, before and during World War II, led to sustained Allied decryption of German Enigma ciphers.For some three centuries, the Vigenère cipher was considered to be completely secure (le chiffre indéchiffrable—"the indecipherable cipher").In the 15th and 16th centuries, in Europe, the idea of a polyalphabetic substitution cipher was developed, among others by the French diplomat Blaise de Vigenère (1523-96).It was this repetition that allowed Babbage and Kasiski to achieve their breaks.Nevertheless, Charles Babbage (1791–1871) and later, independently, Friedrich Kasiski (1805–81) succeeded in breaking this cipher.The key's letters indicate which line of the Vigenère square is used to encipher each letter of the plaintext so as to produce the ciphertext.The presence of repetition or of guessable elements in either the key or the message are the weaknesses that allow cryptanalysts to seek patterns that can enable them to break a cipher
Edited sentence: A polyalphabetic substitution cipher is more complicated and for some three centuries the Vigenère cipher was considered to be completely secure (le chiffre indéchiffrable—"the indecipherable cipher").During World War I, inventors in several countries realized that a purely random key sequence, containing no repetitive pattern, would make a polyalphabetic substitution cipher, in principle, unbreakable