XtraORdinary

xtraordinary/encrypt.py

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+#!/usr/bin/env python3
+
+from random import randint
+
+with open('flag.txt', 'rb') as f:
+    flag = f.read()
+
+with open('secret-key.txt', 'rb') as f:
+    key = f.read()
+
+def encrypt(ptxt, key):
+    ctxt = b''
+    for i in range(len(ptxt)):
+        a = ptxt[i]
+        b = key[i % len(key)]
+        ctxt += bytes([a ^ b])
+    return ctxt
+
+ctxt = encrypt(flag, key)
+
+random_strs = [
+    b'my encryption method',
+    b'is absolutely impenetrable',
+    b'and you will never',
+    b'ever',
+    b'ever',
+    b'ever',
+    b'ever',
+    b'ever',
+    b'ever',
+    b'break it'
+]
+
+for random_str in random_strs:
+    for i in range(randint(0, pow(2, 8))):
+        for j in range(randint(0, pow(2, 6))):
+            for k in range(randint(0, pow(2, 4))):
+                for l in range(randint(0, pow(2, 2))):
+                    for m in range(randint(0, pow(2, 0))):
+                        ctxt = encrypt(ctxt, random_str)
+
+with open('output.txt', 'w') as f:
+    f.write(ctxt.hex())

xtraordinary/output.txt

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+57657535570c1e1c612b3468106a18492140662d2f5967442a2960684d28017931617b1f3637

xtraordinary/sol.py

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+#!python3
+from pwn import *
+from itertools import chain, combinations
+
+choice = 6
+length = 7
+
+def powerset(iterable):
+    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
+    s = list(iterable)
+    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))
+
+log.info("reading file")
+with open("output.txt", 'r') as f:
+    c = int(f.read().strip(), 16)
+log.info("read in cyphertext: {:x}".format(c))
+
+c = pack(c, 'all', 'big', False)
+
+random_strs = [
+    b'my encryption method',
+    b'is absolutely impenetrable',
+    b'and you will never',
+    b'ever',
+    b'break it'
+]
+
+def decrypt(ptxt, key):
+    ctxt = b''
+    for i in range(len(ptxt)):
+        a = ptxt[i]
+        b = key[i % len(key)]
+        ctxt += bytes([a ^ b])
+    return ctxt
+
+log.info("generating key candidates...")
+i = 0
+candidates = []
+for ss in powerset(random_strs):
+    _c = c
+    for s in ss:
+        _c = decrypt(_c, s)
+    _c = decrypt(_c, b'picoCTF{')
+    _c = _c.decode()
+    if _c[0:8].isprintable():
+        log.indented("{:2}: {}".format(i, _c[0:8]))
+        candidates.append((_c[0:8], ss))
+        i += 1
+
+key = candidates[choice][0][0:length].encode()
+decrypt_set = candidates[choice][1]
+log.success(key.decode())
+for word in decrypt_set:
+    c = decrypt(c, word)
+c = decrypt(c, key)
+log.success("flag found: {}".format(c))
+