kittycrypt: first writeup

2025/IrisCTF_2025/kittycrypt/README.md

@@ -1,13 +1,70 @@
 # KittyCrypt
 
-`Description`
+There is example input, output and flag_output with encrypt code. Since we know pair of input and output, we can know what is the key and find the flag with that key.
 
-`Solution`
 
 ```python
-foo = "bar"
+import binascii
+
+# Go 코드 CharSet의 역매핑 (이모지 → 16진수 한 글자)
+CAT_TO_HEX = {
+    "🐱": "0", "🐈": "1", "😸": "2", "😹": "3",
+    "😺": "4", "😻": "5", "😼": "6", "😽": "7",
+    "😾": "8", "😿": "9", "🙀": "A", "🐱‍👤": "B",
+    "🐱‍🏍": "C", "🐱‍💻": "D", "🐱‍👓": "E", "🐱‍🚀": "F",
+}
+
+def cat_to_hex_str(cat_cipher: str) -> str:
+    i = 0
+    hex_chars = []
+    sorted_keys = sorted(CAT_TO_HEX.keys(), key=len, reverse=True)
+
+    while i < len(cat_cipher):
+        for k in sorted_keys:
+            if cat_cipher.startswith(k, i):
+                hex_chars.append(CAT_TO_HEX[k])
+                i += len(k)
+                break
+        else:
+            raise ValueError(f"Unknown cat emoji at pos={i} -> {cat_cipher[i:i+5]}")
+    return "".join(hex_chars)
+
+def hex_str_to_keyed_text(hex_str: str) -> str:
+    raw = binascii.unhexlify(hex_str)
+    return raw.decode('utf-8', errors='replace')
+
+def recover_key(plaintext: str, keyed_text: str) -> list:
+    return [ord(k) - ord(p) for p, k in zip(plaintext, keyed_text)]
+
+def decrypt_cat_cipher(cat_cipher: str, keys: list) -> str:
+    hex_str = cat_to_hex_str(cat_cipher)
+    keyed_text = hex_str_to_keyed_text(hex_str)
+
+    decrypted = []
+    for i, ch in enumerate(keyed_text):
+        decrypted.append(chr(ord(ch) - keys[i]))
+    return "".join(decrypted)
+
+def main():
+    with open("example_input.txt", "r", encoding="utf-8") as f:
+        example_plain = f.read()
+    with open("example_output.txt", "r", encoding="utf-8") as f:
+        example_cipher = f.read()
+
+    ex_hex_str = cat_to_hex_str(example_cipher)
+    ex_keyed_text = hex_str_to_keyed_text(ex_hex_str)
+    keys = recover_key(example_plain, ex_keyed_text)
+
+    with open("flag_output.txt", "r", encoding="utf-8") as f:
+        flag_cipher = f.read()
+
+    flag_plain = decrypt_cat_cipher(flag_cipher, keys)
+    print("[+] Flag:")
+    print(flag_plain)
+
+if __name__ == "__main__":
+    main()
 ```
 
-![img](flag.png)
 
-flag is: **flag{foo_bar}**
+flag is: **irisctf{s0m371m3s_bY735_4r3n7_wh47_y0u_3xp3c7}**