My Vigenere ciper for CSRF

gistfile1.txt

# If this were split out of the get_random_string def we
# we wouldn't need to repeat it here...
VALID_CHARS = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'

def mangle(token):
    mask = get_random_string(len(token))

    value = ''.join([
        VALID_CHARS[ (VALID_CHARS.index(x) + VALID_CHARS.index(y)) % len(VALID_CHARS) ]
        for x, y in zip(token, mask)
    ])
    return '-'.join([value, mask])

def demangle(value):
    value, mask = value.split('-')

    return ''.join([
        VALID_CHARS[ (VALID_CHARS.index(x) - VALID_CHARS.index(y)) % len(VALID_CHARS) ]
        for x, y in zip(token, mask)
    ])