RSA$-$OAEP

Consider RSA with padding $($OAEP$)$ used as a block cipher with block size $2048$

bits in ECB mode. For the purposes of this question we do not need the details of

how OAEP works, we just need to know:

OAEP is an algorithm that takes an $m-$bit block and mixes it with a $k-$bit

random string to return an $(m+k)-$bit padded block. To be concrete, here

we have $m=2048,$ and we will take $k=1536.$

OAEP is invertible: given the padded block $($and the details of the OAEP

algorithm$),$ it is possible to recover the original unpadded block without

any additional information.

$($a$)$ Should the padding be done to the plaintext block before encryption, or to the

ciphertext block after encryption, or does it not matter? Why?

$($b$)$ How does padding $($properly done$)$ affect the nominal unicity point?

$($It might be helpful to think about how the padding affects the entropy.$)$