Simplify the following text for a Grade 11 native English-speaking person.
Original: Rijmen and Preneel warn that designing an algorithm to resist only known attacks is generally not a good design principle .
Simplified: Rijmen and Preneel tried attacking MacGuffin with different S-boxes , taken directly from DES . This version proved to be slightly stronger , but they warn that designing an algorithm to resist only known attacks is generally not a good design principle .