Rima

f = [int(x) for x in bin(int(FLAG.hex(), 16))[2:]]
f.insert(0, 0)
for i in range(len(f)-1): f[i] += f[i+1]

a = nextPrime(len(f))
b = nextPrime(a)

g, h = [[_ for i in range(x) for _ in f] for x in [a, b]]

c = nextPrime(len(f) >> 2)
for _ in [g, h]:
    for __ in range(c): _.insert(0, 0)
    for i in range(len(_) -  c): _[i] += _[i+c]

g, h = [int(''.join([str(_) for _ in __]), 5) for __ in [g, h]]

for _ in [g, h]:
    if _ == g:
        fname = 'g'
    else:
        fname = 'h'
    of = open(f'{fname}.enc', 'wb')
    of.write(long_to_bytes(_))
    of.close()

One important observation is that the flag length uniquely determine the length of g.enc and h.enc, so by binary searching the flag length by running the script, one finds that the flag has 32 characters. Once we get this,

a, b, c

are also uniquely determined and we can easily reverse the whole process step by step.