$2.$ Consider a very simple symmetric block encryption algorithm, in which $64-$bits blocks of plaintext are encrypted using a $128-$bit key. Encryption is defined as

C$=($P$$K$0)$ K$1$

where $=$ ciphertext; $=$ secret key; $0=$ leftmost $64$ bits of $$; $1=$ rightmost

$64$ bits of K$,=$ bitwise Exclusive$-$Or; and $$ is addition mod $264.$ Show the decryption

equation. That is$,$ show the equation for $$ as a function of $,0$ and $1.$