The designers analyzed IDEA to measure its strength against differential cryptanalysis and concluded that it is unsusceptible only under certain assumptions. No successful linear or differential attacks have been reported. Some classes of weak keys have been found — E.g. (Daemen "et al.", 1994) — but these are of little importance, being so rare to be explicitly avoided. As of 2007, the best attack which applies to all keys can break IDEA if reduced to 6 rounds (the full IDEA cipher uses 8.5 rounds).