I suppose the longer the keyphrase, the harder it will be to decypher, though I don't really know enough about cryptography to say.

this cipher is more commonly known as a Vigenere Cipher. how hard it is to crack depends on how long the message is relative to the keyphrase. if the keyphrase is longer than the message (and is only used once, ever), it is a one-time pad and is provably unbreakable. in the more realistic case where the message is longer than the keyphrase so that there are a few repeats, it can be broken very easily by calculating the index of coincidence.

anders pearson