> 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-rot128.md).

# \[DreamHack] ROT128

#### 문제 링크

{% embed url="<https://dreamhack.io/wargame/challenges/852>" %}

&#x20;

#### 문제

<figure><img src="https://blog.kakaocdn.net/dna/tb2aQ/btsNhmahUk4/AAAAAAAAAAAAAAAAAAAAALPrD1FMs4s_0J4wEP7CC4o4qgCOG1qfi2OgWSEGkFVG/img.png?credential=yqXZFxpELC7KVnFOS48ylbz2pIh7yKj8&#x26;expires=1782831599&#x26;allow_ip=&#x26;allow_referer=&#x26;signature=%2B28xW2MjYNh20HraV%2FRcAa8ujPc%3D" alt="" height="385" width="1102"><figcaption></figcaption></figure>

&#x20;

#### WriteUp

```
#!/usr/bin/env python3

hex_list = [(hex(i)[2:].zfill(2).upper()) for i in range(256)]

with open('flag.png', 'rb') as f:
    plain_s = f.read()

plain_list = [hex(i)[2:].zfill(2).upper() for i in plain_s]

enc_list = list(range(len(plain_list)))

for i in range(len(plain_list)):
    hex_b = plain_list[i]
    index = hex_list.index(hex_b)
    enc_list[i] = hex_list[(index + 128) % len(hex_list)]

enc_list = ''.join(enc_list)

with open('encfile', 'w', encoding='utf-8') as f:
    f.write(enc_list)
```

-> rot128.py

&#x20;

```
# 0부터 255까지의 숫자를 16진수로 표현한 문자열 리스트를 생성합니다.
hex_list = [hex(i)[2:].zfill(2).upper() for i in range(256)]
print(hex_list)

# 'encfile' 파일을 UTF-8 인코딩으로 읽기 모드로 열고 내용을 'enc_s'에 저장합니다.
with open('encfile', 'r', encoding='utf-8') as f:
    enc_s = f.read()

# 'enc_s'의 내용을 두 글자씩 끊어 리스트로 만듭니다.
enc_list = [enc_s[i:i+2] for i in range(0, len(enc_s), 2)]

# 디코딩된 인덱스를 저장할 리스트 'dec_list'를 생성합니다.
dec_list = list(range(len(enc_list)))

# 'enc_list'의 각 요소에 대해 반복합니다.
for i in range(len(enc_list)):
    # 현재 두 글자 문자열의 인덱스를 'hex_list'에서 찾습니다.
    index = hex_list.index(enc_list[i])

    # 인덱스를 128을 빼고 리스트 길이로 나눠 디코딩합니다.
    dec_list[i] = hex_list[(index - 128) % len(hex_list)]

# 디코딩된 인덱스 리스트를 16진수로 변환하여 바이트 객체로 만듭니다.
dec_s = bytes(int(dec_list[i], 16) for i in range(len(dec_list)))

# 바이너리 쓰기 모드로 'decfile.png' 파일을 열고 바이트 객체를 씁니다.
with open('decfile.png', 'wb') as f:
    f.write(dec_s)
```

-> 복호화 코드

<figure><img src="https://blog.kakaocdn.net/dna/dZf9fv/btsNf9C29yC/AAAAAAAAAAAAAAAAAAAAABCfEXDUOaBMc0KZYthLY8JyU9ZVTsr8brPlcDiBzIed/img.png?credential=yqXZFxpELC7KVnFOS48ylbz2pIh7yKj8&#x26;expires=1782831599&#x26;allow_ip=&#x26;allow_referer=&#x26;signature=vsuDG0z%2BFzpb2T0RJH4dg29I%2FKw%3D" alt="" height="473" width="1066"><figcaption></figcaption></figure>


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