IceCTF InterceptedConversationsPt.2 writeup

decode.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import base64

P = [27, 35, 50, 11, 8, 20, 44, 30, 6, 1, 5, 2, 33, 16, 36, 64, 3,
     61, 54, 25, 12, 21, 26, 10, 57, 53, 38, 56, 58, 37, 43, 17, 42,
     47, 4, 14, 7, 46, 34, 19, 23, 40, 63, 18, 45, 60, 13, 15, 22,
     9, 62, 51, 32, 55, 29, 24, 41, 39, 49, 52, 48, 28, 31, 59]

S = [68, 172, 225, 210, 148, 172, 72, 38, 208, 227, 0, 240, 193,
     67, 122, 108, 252, 57, 174, 197, 83, 236, 16, 226, 133, 94,
     104, 228, 135, 251, 150, 52, 85, 56, 174, 105, 215, 251, 111,
     77, 44, 116, 128, 196, 43, 210, 214, 203, 109, 65, 157, 222,
     93, 74, 209, 50, 11, 172, 247, 111, 80, 143, 70, 89]

cipher = base64.b64decode(raw_input().strip()).decode('utf8')
plain = [0 for i in range(len(cipher))]

for j in range(0, len(cipher), 64):
    for i in range(64):
        plain[j + i] = (ord(cipher[j + P[i] - 1]) - S[i]) % 256

print ''.join([chr(p) for p in plain])

encode.py

# Python bytecode 3.5 (3350) disassembled from Python 2.7
# Embedded file name: encode.py
# Compiled at: 2016-08-08 07:10:09
import random
import base64

P = [27, 35, 50, 11, 8, 20, 44, 30, 6, 1, 5, 2, 33, 16, 36, 64, 3,
     61, 54, 25, 12, 21, 26, 10, 57, 53, 38, 56, 58, 37, 43, 17, 42,
     47, 4, 14, 7, 46, 34, 19, 23, 40, 63, 18, 45, 60, 13, 15, 22,
     9, 62, 51, 32, 55, 29, 24, 41, 39, 49, 52, 48, 28, 31, 59]

S = [68, 172, 225, 210, 148, 172, 72, 38, 208, 227, 0, 240, 193,
     67, 122, 108, 252, 57, 174, 197, 83, 236, 16, 226, 133, 94,
     104, 228, 135, 251, 150, 52, 85, 56, 174, 105, 215, 251, 111,
     77, 44, 116, 128, 196, 43, 210, 214, 203, 109, 65, 157, 222,
     93, 74, 209, 50, 11, 172, 247, 111, 80, 143, 70, 89]

inp = input()
inp += ''.join((chr(random.randint(0, 47)) for _ in range(64 - len(inp) % 64)))
ans = ['' for i in range(len(inp))]
for j in range(0, len(inp), 64):
    for i in range(64):
        ans[j + P[i] - 1] = chr((ord(inp[j + i]) + S[i]) % 256)

ans = ''.join(ans)
print(base64.b64encode(ans.encode('utf8')).decode('utf8'))
# okay decompiling encode.pyc