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'.

def getDelta(original, update):
    """
    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'.
    """
    # Compute both keys
    k = genKw(*original)
    kPrime = genKw(*update)

    # Compute delta,p'
    delta = (kPrime * inverse(k, orderGt())) % orderGt()
    pPrime = generatorGt()**kPrime
    return delta,pPrime