code: def getDelta(original, update):
      
      # Compute both keys
      k = genKw(*original)
      kPrime = genKw(*update)
  
      # Compute delta,p'
      delta = (kPrime * inverse(k, orderGt())) % orderGt()
      pPrime = generatorGt()**kPrime
      return delta,pPrime
docs: Generates an update token delta_{k->k'}.
      @original: values for k: (w,msk,s)
      @update: values for kPrime: (w',msk',s')
      @return (delta, p'): @delta = k'/k, @p' is a new pubkey based on k'.