Rune Decryptor — UIUCTF 2026

Description

No separate organizer description was preserved in the task artifacts. The interactive service supplied the rules summarized below.

The TLS service ran 20 rounds. In each round it encrypted a multilingual literary excerpt using a fresh monoalphabetic mapping from plaintext letters to Unicode runes while retaining spaces and punctuation. It allowed five full-plaintext guesses, and an incorrect guess disclosed only the number of correctly decrypted symbols. Solving more than 70% of the rounds was required.

The server was reached at rune-decryptor.chal.uiuc.tf:1337 and required a kCTF proof of work before the first round.

Analysis

Why the oracle was not enough

The word “decryptor” initially suggested an RSA or padding oracle, but the live protocol immediately disproved that interpretation: this was a direct monoalphabetic-substitution problem. The score returned after a failed guess was a single weighted Hamming total over the whole paragraph. Five such scalar observations cannot recover an arbitrary fresh key containing roughly two dozen independent mappings. At best, that feedback could distinguish a few nearly complete candidates.

Direct substitution cryptanalysis was also unattractive because every round used another language and a new key:

• An exact search in the NLTK UDHR corpus found no matching excerpt.
• Generic multilingual quadgram hillclimbing converged to plausible-looking pseudo-language rather than valid prose.
• An exhaustive beam search across ten language dictionaries was slow, and mapped-symbol coverage did not rank languages reliably.

The decisive observation was outside the ciphertext. Under every paragraph, the service printed a redacted attribution derived from Title -- Authors. Alphanumeric characters became block glyphs, but spaces, punctuation, character counts, and the truncation point remained visible. This was a strong structural fingerprint.

...