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Tracks / Crypto Failures / Three-Letter Secret

Three-Letter Secret

Medium 30 pts

A message was 'encrypted' by XORing it with a short repeating key. You are told the key is CTF (three bytes, repeated). XOR the ciphertext against the repeating key and the plaintext — flag included — falls straight out.

Hints

Open them in order. They nudge, they do not solve.

Hint 1

The ciphertext is hex. Turn it into bytes first.

Hint 2

XOR byte i of the ciphertext with byte (i mod 3) of the key "CTF".

Hint 3

CyberChef: From Hex → XOR with key CTF (UTF-8). The flag is in the decrypted text.

The artifact

ciphertext (hex), key = "CTF" repeating:

373c2363322a223366253b3463202e2a27662f3524633d35633a2b382c29310b
2f300b282c2019263a25312d36373d292d296628312333742f3774352637342620

Concatenate both lines, decode hex to bytes, XOR with the repeating key.

Submit the flag

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