In section 4.1 under the subsection

 1) In section 4.1 under the subsection on the motivations for the Feistel cipher structure. it was stated that for a block of n bits, the number of different reversible mappings for the ideal blockcipher is 2n!.  How is this justified?

or

2) Describe how with 128 chosen ciphertexts, an adversary can decrypt any ciphertext without knowledge of the secret key k.