Cracking a specific known cipher method (not shown) by reverse engineering.

prng_known_seed.py

import random, string

random.seed("random")

secret = "BNZQ:8o149b15764q471k2533971t6w78liec"
decoded = ""

# Each line will definitely remain lower/upper/digit.
# To find out why, pay attention to the origin code.

for i in secret:
    if i.islower():
        stage0 = (ord(i) - ord('a'))  # range should now be 0 to 25.
        stage1 = (stage0 - random.randrange(0, 26))%26
        stage2 = (stage1 + ord('a'))
        decoded += chr(stage2)
    elif i.isupper():
        stage0 = (ord(i) - ord('A'))  # range should now be 0 to 25.
        stage1 = (stage0 - random.randrange(0, 26))%26
        stage2 = (stage1 + ord('A'))
        decoded += chr(stage2)
    elif i.isdigit():
        stage0 = (ord(i) - ord('0'))  # range should now be 0 to 25.
        stage1 = (stage0 - random.randrange(0, 10))%10
        stage2 = (stage1 + ord('0'))
        decoded += chr(stage2)
    else:
        decoded += i

print "Decoded flag: " + decoded