code: def refresh(ek: EncryptionKey, ciphertext: Ciphertext) -> Ciphertext:
      c = ciphertext.raw
      s = gen_randomness(ek=ek, shape=c.shape).raw
      sn = tf_big.pow(s, ek.n, ek.nn)
      d = (c * sn) % ek.nn
      return Ciphertext(ek, d)
  
imports: from typing import Optional
  from typing import Tuple
  import tensorflow
  import tf_big
label_desc: Use image node_id 7 for calling a global function with example usage: refresh(ek, ciphertext) and returns: Ciphertext