> For the complete documentation index, see [llms.txt](https://docs.cooku222.kr/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.cooku222.kr/security/crypto/dreamhack/dreamhack-bytecaesar.md). # \[DreamHack] ByteCaesar #### 문제 링크 {% embed url="" %} #### 문제

#### Writeup ``` import random class Caesar: def __init__(self, key): assert isinstance(key, int) and 1 <= key <= 255 self._key = key def encrypt(self, msg): msg_enc = b"" for b in msg: msg_enc = msg_enc + bytes([(b + self._key) % 256]) return msg_enc def decrypt(self, msg): msg_dec = b"" for b in msg: msg_dec = msg_dec + bytes([(b - self._key) % 256]) return msg_dec def main(): key = random.randint(1, 255) with open("secret", "rb") as f: secret = f.read() cipher = Caesar(key) secret_enc = cipher.encrypt(secret) print("I believe Caesar cipher is greatest encryption of all time.") print("No one can leak my secret sentence!") print(f"my encrypted sentence > {secret_enc.hex()}") if __name__ == "__main__": main() ``` 이게 문제 파일 ``` I believe Caesar cipher is greatest encryption of all time. No one can leak my secret sentence! my encrypted sentence > 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 ``` 이게 encrypted -> 1 - 255로 난수 생성 -> 브루트포스 방식으로 암호 뚫기(어차피 패턴이 없음) ``` en_num = bytes.fromhex("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") encrypt = en_num for key in range(1, 255): msg_dec=b"" for b in encrypt: msg_dec = msg_dec + bytes([(b - key) % 256]) try: decoded = msg_dec.decode()#바이트를문자열로 if "DH{" in decoded:#DH가 보이면 프린트 print(f"{key}->{decoded}") except: continue ```

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