webform asymmetric algorithm file encryption and decryption

Question

https://www.c-sharpcorner.com/article/implement-symmetric-and-asymmetric-cryptography-algorithms-with-c-sharp/

 

the above link is shown in windows form,i am trying to implement with webform it gives me error while decrypting

please help me to implement with webform asymmetric algorithm file encryption and decryption except part of symmetric algorithm...