Method · cryptanalysis

Crack attempt on Herbal A under the fitted period-cipher setting

The period-cipher family's settings are taken from the T39 fit; the key and the sign boundaries are not. A planted Latin control laid into Herbal A's pages must be read on held-out pages before the same pipeline is run on the manuscript. Rung A: true sign boundaries. Rung B: boundaries learned.

CryptanalysisVersion 1.0Licence MIT3 runsUsed on 1 mystery

Questions it answers

Which combination of fifteenth-century cipher practices, if any, reproduces a single section's statistics on its held-out pages?

Code Python

Download module (.py)crack_period.pyView source

Runs and results 3 runs

Run	Mystery	Measured	Result	Against
RUN0DCBBPFSJ189	Voynich	Held-out key accuracy by use / character similarity to the true plaintext / dictionary coverage (true plaintext's)	300k iterations: 0.627 / 0.634 / 0.294 (0.697); 1.5M iterations: 0.706 / 0.728 / 0.336 (0.697)	Protocol bar: 70 % of the plaintext's dictionary coverage (0.49) or 50 % word accuracy
RUN0DCBBTB9HD1M	Voynich	Held-out key accuracy by use / character similarity / dictionary coverage (true plaintext's)	V 450, signs up to 6 glyphs: 0.114 / 0.062 / 0.232 (0.697); V 200, signs up to 3 glyphs: 0.228 / 0.082 / 0.271 (0.697)	Rung A (true boundaries): 0.706 / 0.728 / 0.336
RUN0DCBGWKQ4KGM	Voynich	Held-out element-trigram score (nats per element; Latin's own -2.418) / dictionary coverage of the decoded held-out text	Herbal A -2.563 / 0.296; shuffled -2.824 / 0.216	Planted control under the same rung-B pipeline: -2.767 / 0.271 (true plaintext 0.697)

Applied to

Voynich Manuscript

3 datasetsLatin genre texts, Naibbe cipher tables and example plaintexts (greshko/naibbe-cipher, commit 719f2f4c), ZL3b transcription

References

Lasry, G. 2024. Deciphering Historical Syllabic Ciphers. Proceedings of the 7th International Conference on Historical Cryptology (HistoCrypt 2024).