CodeShell.kr - Relay

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Challenge

一封正文空掉的邮件，题面说消息没走完。难度在于载荷在 Received 头的 id 字段里，而且顺序要按投递方向而不是文件顺序。

An empty message

一条空消息。

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https://codeshell.kr/challenges/misc-relay/

Solution

附件是一封正文只有一句 "The body did not make the trip." 的邮件。

Step 1：正文没有载荷，信息在头部：四个 Received 头各带一个 2 字母的 with SMTP id。

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Received: ... with SMTP id ACK   (最新)
Received: ... with SMTP id TR
Received: ... with SMTP id CK
Received: ... with SMTP id BA    (最旧)

Step 2：Received 头在文件里由新到旧排列，而投递顺序是由旧到新，所以倒序读：

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BA CK TR ACK -> BACKTRACK

Script

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#!/usr/bin/env python3
"""CodeShell.kr — Relay (Misc, 50p) solver.

The .eml body is empty; the payload is the 2-letter `with SMTP id` of each
Received header. Received headers are listed newest-first, so reading them
oldest-first spells the answer.

Usage: uv run python solvers/relay.py
"""

import email
import re
from pathlib import Path

EML = "assets/challenge-files/misc-relay.eml"


def main():
    msg = email.message_from_bytes(Path(EML).read_bytes())
    ids = [re.search(r"with SMTP id ([A-Za-z0-9]+)", v).group(1) for v in msg.get_all("Received", [])]
    print("received top-down", ids)
    print("oldest-first     ", "".join(reversed(ids)))


if __name__ == "__main__":
    main()
CodeShell{BACKTRACK}