Simplify the following text for a Grade 10 native English-speaking person.
Original: Many modern symmetric block ciphers uses Feistel networks , and the structure and properties of Feistel ciphers have been extensively explored by cryptographers .
Simplified: Many modern and also some old symmetric block ciphers are based on Feistel networks ( e.g. GOST 28147-89 block cipher ) , and the structure and properties of Feistel ciphers have been extensively explored by cryptographers .