CASE 017 · Sentence-Length Parity

Counted Testimony

A four-letter signal is carried by sentence length. Apply the recovered counting rule to every numbered sentence.

Advanced 425 pts Hint penalty: 60 pts

Artifact

Inspect this carefully

Field notes: thirty-two sentences were copied exactly from the recovered notebook.

  1. Morning mist covered the quiet service road.
  2. Western guards watched the silent loading gate today.
  3. A lantern flickered beside the storage shed.
  4. Fresh tracks crossed the gravel service lane.
  5. The old radio emitted a static burst.
  6. Patrol officers checked the silent lower corridor carefully.
  7. A folded map rested beneath wooden benches.
  8. The archive clerk noticed a missing folder yesterday.
  9. Rain softened dust around the loading ramp.
  10. One camera recorded movement behind eastern fences later.
  11. The basement clock stopped shortly after midnight.
  12. A narrow trail continued beyond broken fencing.
  13. Three footprints ended beside the utility door.
  14. River patrol heard engines moving steadily upstream.
  15. A silver key appeared under reception desks yesterday.
  16. Night supervisors logged an unusual power fluctuation overnight.
  17. Cold air entered through the stairwell window.
  18. A courier left the station before sunrise today.
  19. One notebook contained several erased pencil marks.
  20. The north elevator paused between two floors.
  21. A chemical odor lingered near the workshop briefly.
  22. Security lights dimmed across the central courtyard.
  23. Supply records showed one extra handheld flashlight.
  24. A maintenance badge lay beside the generator.
  25. Morning staff discovered sand across polished tiles.
  26. The rooftop antenna shifted during the storm nearby.
  27. A small envelope waited inside the locker.
  28. Footsteps echoed twice along the underground passage.
  29. The service van returned with muddy tires again.
  30. One witness recalled a red scarf nearby yesterday.
  31. The eastern alarm reset before scheduled inspection today.
  32. A final note mentioned the bridge route again.
Recovered counting rule

Each numbered list item is one carrier sentence. Inside that sentence, count only contiguous ASCII alphabetic runs matching [A-Za-z]+. Digits do not count. Punctuation is ignored. Apostrophes and hyphens break runs, so each alphabetic run is counted separately. Odd word count represents 0; even word count represents 1. Read all 32 carrier sentences in order and group the resulting bits into four 8-bit ASCII bytes.

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    About the technique

    Sentence-Length Parity

    Sentence-length parity steganography maps a deterministic property of whole carrier sentences, such as an odd or even alphabetic word count, to binary states. It is easy to verify when boundaries and counting rules are explicit, but fragile under editing, sentence splitting or merging, paraphrasing, and inconsistent tokenization.

    Read the related field guide →

    Questions

    Frequently asked

    What is sentence-length parity steganography?

    It is a linguistic hidden-information method that maps the measured length of each predefined carrier sentence, often odd versus even word count, to symbols such as binary 0 and 1.

    Why does this challenge define words as [A-Za-z]+ runs?

    Different parsers disagree about contractions, hyphens, numbers, Unicode letters, and punctuation. An explicit regular-language rule gives the encoder, player, and verifier the same deterministic units.

    Why is sentence-length encoding fragile?

    Adding or removing one counted word flips a parity bit, while splitting, merging, deleting, or reordering sentences can also desynchronize the remainder of the payload.