Calculate a^b mod n, used in cryptography

powmod.py

def powmod(a,b,n):
     '''
     calculates a^b mod n
     :a base number
     :b power
     :n A prime number
     '''
     c = 0
     f = 1
     k = b.bit_length()
     while k >= 0:
         c *= 2
         f = (f*f)%n
         if b & (1 << k):
             c += 1
             f = (f*a) % n
         k -= 1
     return f
if __name__='__main__':
    assert(powmod(2,5,17) == 15)
    assert(powmod(2,4,17) == 16)
    assert(powmod(3,3,17) == 10)
    assert(powmod(3,4,17) == 13)