Simplify the following text for a Grade 10 native English-speaking person.
Original: In cryptography , a weak key is a key which when used with a specific cipher , makes the cipher behave in some undesirable way , and simplifies breaking ( cracking ) the ciphertext .
Simplified: In cryptography , a weak key is a key , which , used with a specific cipher , makes the cipher behave in some undesirable way .