def selectE(fn, halfkeyLength):
while True:
# e and fn are relatively prime
e = random.randint(0, 1 << halfkeyLength)
(x, y, r) = extendedGCD(e, fn)
if r == 1:
return e
def modularInverse(a, n):
assert (extendedGCD(a, n) == 1)
s = 0
t = 1
r = n
old_s = 1
old_t = 0
old_r = a
while r != 0:
quotient = old_r // r
old_r, r = r, old_r - quotient * r
old_s, s = s, old_s - quotient * s
old_t, t = t, old_t - quotient * t
if old_s < 0:
old_s = n + old_s
return old_s
def computeD(fn, e):
(x, y, r) = extendedGCD(fn, e)
# y maybe < 0, so convert it
if y < 0:
return fn + y
return y
