Shift Change.

Challenge summary#

• Files: bundle.zip, dispatch.txt
• Ciphertext: etmpm{nlpdlc_nn51p682qnnm68834n53}
• Hint: “An exhausted night guard shifted every letter in a message. Punctuation survived.”
• Flag format: tibeb{...}

The hint and title “Shift Change” point to a Caesar shift, but only letters are shifted. Digits, braces, and underscores remain unchanged.

Step 1 — Inspect the files#

pwd && ls -la
file bundle.zip dispatch.txt
unzip -l bundle.zip && echo '--- dispatch.txt ---' && cat dispatch.txt

Output shows:

--- dispatch.txt ---
etmpm{nlpdlc_nn51p682qnnm68834n53}

bundle.zip just contains another copy of dispatch.txt, so the real challenge is the ciphertext itself.

Step 2 — Identify the cipher and shift#

The flag format is tibeb{...}.
Compare the start of the ciphertext with the expected plaintext:

cipher: etmpm{
plain : tibeb{

Take the first letter:

• e = 4
• t = 19

The encryption shift is:

e = t + k mod 26
4 = 19 + k mod 26
k = 11

So the message was encrypted by shifting each letter forward by 11.
To decrypt, shift letters backward by 11, which is the same as shifting forward by 15.

Step 3 — Decrypt the message#

Use this Python script:

ct = "etmpm{nlpdlc_nn51p682qnnm68834n53}"
pt = []

for ch in ct:
    if ch.isalpha():
        base = ord('a') if ch.islower() else ord('A')
        pt.append(chr((ord(ch) - base - 11) % 26 + base))
    else:
        pt.append(ch)

print(''.join(pt))

Output:

tibeb{caesar_cc51e682fccb68834c53}

Flag#

tibeb{caesar_cc51e682fccb68834c53}

Tricky parts to note#

1. Only letters are shifted.
   Digits like 51, 682, 68834, 53 and punctuation {, }, _ stay exactly the same.

2. The encryption shift is +11, so decryption is -11 or +15.
   If you try shifting by +11 while decrypting, you get gibberish. You need the opposite direction.

3. The inner word caesar confirms the cipher.
   After decrypting the first block nlpdlc, you get caesar, which is a strong hint that the Caesar cipher was used.

4. bundle.zip is a decoy/duplicate.
   It contains the same dispatch.txt, so no hidden files are needed.