1. A computer implemented method of processing an online transaction, the method comprising: generating from an online transaction of a purchaser, a plurality of keys from key fields of the transaction that individually do not reliably identify the purchaser; for each key, retrieving a profile of historical transactions associated with the key; weighting each profile with a weight indicating a degree to which the profile is associated with current purchaser; and generating a fraud score indicative of the likelihood of fraud in the online transaction as a function of the weighted profiles and the current transaction.