norman-challenge.py

#!/usr/bin/env python

def encrypt(x):
    return [((a+1)*ord(b)%256) for a,b in zip(range(len(x)), x)]

def decrypt(a):
    for i in range(len(a)):
        while not (a[i] % (i+1) == 0 and a[i]/(i+1) in range(48,123)):
            a[i] += 256
        a[i] /= (i+1)
    return ''.join([chr(_) for _ in a])

import sys
if len(sys.argv) == 7:
    print(decrypt([int(_) for _ in sys.argv[1:]]))
elif len(sys.argv) == 2:
    print(encrypt(sys.argv[1]))

$ ./foo.py 50 194 162 148 93 94
2a6eye

$ ./foo.py 2a6eye
[50, 194, 162, 148, 93, 94]