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

If your keyphrase is as long as your message and you use a different keyphrase every-time, your code is impossible to analyze and hence cannot be broken.

A brute force approach (decrypting with all possible keyphrases) would of course yield the plain text message amongst all possible solutions, but it would not be possible to choose with certainty which message was the original one.

The method to use a keyphrase only once is known as the "one time keypad"-method (see Why Are One-Time Pads Perfectly Secure?).

An implementation of a one-time keypad in one line of PERL can be found here.

The weak point of course is the making and distribution of the keys. If this is done in a deterministic way (or even a pseudo-random way) the cypher is open to analysis and can be broken. A workable solution is suggested here.

CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law