def getDelta(original, update):
<EOL>k = genKw(*original)<EOL>kPrime = genKw(*update)<EOL>delta = (kPrime * inverse(k, orderGt())) % orderGt()<EOL>pPrime = generatorGt()**kPrime<EOL>return delta,pPrime<EOL>
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'.