We now show how an attack with chosen ciphertext can be used to break an RSA encryption.
1. Show that the multiplicative property holds for RSA, i.e., show that the product of two ciphertexts is equal to the encryption of the product of the two respective plaintexts.
2. This property can under certain circumstances lead to an attack. Assume that Bob first receives an encrypted message y1 from Alice which Oscar obtains by eavesdropping. At a later point in time, we assume that Oscar can send an innocent looking ciphertext y2 to Bob, and that Oscar can obtain the decryption of y2. In practice this could, for instance, happen if Oscar manages to hack into Bob's system such that he can get access to decrypted plaintext for a limited period of time.