Compress and Attack

compress_and_attack/compress_and_attack.py

@@ -0,0 +1,35 @@
+#!python3
+
+import zlib
+from random import randint
+import os
+from Crypto.Cipher import Salsa20
+
+flag = open("./flag").read()
+
+
+def compress(text):
+    return zlib.compress(bytes(text.encode("utf-8")))
+
+def encrypt(plaintext):
+    secret = os.urandom(32)
+    cipher = Salsa20.new(key=secret)
+    return cipher.nonce + cipher.encrypt(plaintext)
+
+def main():
+    while True:
+        usr_input = input("Enter your text to be encrypted: ")
+        compressed_text = compress(flag + usr_input)
+        encrypted = encrypt(compressed_text)
+        
+        nonce = encrypted[:8]
+        encrypted_text =  encrypted[8:]
+        print(nonce)
+        print(encrypted_text)
+        print(len(encrypted_text))
+
+if __name__ == '__main__':
+    main() 
+
+
+

compress_and_attack/flag

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+picoCTF{flagFLAG}

compress_and_attack/sol.py

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+#!python3
+from pwn import *
+import string
+import ast
+
+#conn = process(["python3", "compress_and_attack.py"])
+conn = remote("mercury.picoctf.net", 2431)
+conn.recvuntil(b": ")
+
+def encrypt(text):
+    conn.sendline(text)
+    nonce = ast.literal_eval(conn.recvline().decode())
+    encrypted = ast.literal_eval(conn.recvline().decode())
+    length = int(conn.recvline())
+    conn.recvuntil(b": ")
+    
+    return nonce, encrypted, length
+
+CHARACTERSET = string.ascii_lowercase + string.digits + " }_"
+
+prefix = "picoCTF{"
+pre_known = "sheriff_you_solved_"
+
+max_chars = 20
+sol_char = prefix[-1]
+sol_string = prefix+pre_known
+while sol_char != '}':
+    P = log.progress("trying characters... ")
+    min_char = ""
+    min_size = 1000000000000000000000000000000000
+    for c in CHARACTERSET:
+        P.status(sol_string+c)
+        s = 0
+        n = 1
+        for _ in range(n):
+            _, encrypted, _ = encrypt((sol_string+c).encode())
+            s += len(encrypted)
+        if s < min_size:
+            min_char = c
+            min_size = s
+    
+    sol_char = min_char
+    sol_string += min_char
+    P.success(sol_string)
+