In 1920, Friedman was the first to discover this variant's weaknesses: the cryptanalyst has statistical information about the key (assuming that the block of text is in a known language) and that information will be reflected in the ciphertext.
In 1920, Friedman was the first to discover this variant's weaknesses. The problem with the running key Vigenère cipher is that the cryptanalyst has statistical information about the key (assuming that the block of text is in a known language) and that information will be reflected in the ciphertext.