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I meant the algorithm for converting a password into an address. The expensive bit of any brainwallet is going to be generating the keys from the wallet. If everyone using a brainwallet uses the same algorithm then a cracker who brute-forces possible passwords can check the generated public keys for each trial password against all existing public keys with significant funds in them very cheaply (eg. using a bloom filter). If the trial password matches anyone's brain wallet then he has a hit.