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The DECRYPT Project
Decryption of Secret Historical Manuscripts
  • Collection
  • Transcription
  • Decipherment
Go to the ciphers
in the DECODE database

NEWS: To learn more about the decipherment of Mary Stuart’s lost letters go to our Publications!
About Us

Cracking ciphers in a cross-disciplinary team

Thousands of enciphered historical manuscripts are buried in libraries and archives. Examples of such material are diplomatic correspondence and intelligence reports, private letters and diaries as well as manuscripts related to secret societies. The bulk of these historical manuscripts will remain undeciphered unless we can automate the processes involved in decoding them. Our aim is to develop resources and computer-aided tools for decoding of historical source material by using AI and cross-disciplinary research involving computational linguistics, computer vision, cryptology, history, linguistics and philology.

Within the DECRYPT project, we release resources and tools with open access to facilitate research in historical cryptology, allowing collection, analysis and decipherment of historical ciphertexts. Resources are collections of encrypted sources, and historical texts with language models. The tools facilitate the processing of the encrypted sources from transcription to decipherment. We list our resources and tools below, which are described in our scientific publications.

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Resources and Tools

The DECODE database contains a collection of digitized images of ciphertexts and encryption keys along with metadata information about their provenance, location, transcription, and possible cryptanalysis or commentary. The database enables search and all records in the database are open to the public. HistCorp is a collection of historical corpora and other useful resources and tools for researchers working with historical text. 

We provide tools for transcription and decipherment of historical ciphers using advanced machine learning algorithms. Historical cipher images can be transcribed, i.e. transformed into a computer readable text format with the help of the TranscriptTool. The transcribed ciphertext can be corrected and used as input to CrypTool which assists you in breaking a wide range of historical ciphertexts.  

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The DECRYPT Portal

PI

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Beáta
Megyesi

Project leader

Stockholm University

Sweden

Core Team

Participants

Further contributors

The DECRYPT Portal

Our resources and tools are open source and free

We provide a collection of encrypted historical sources, and tools for the automatic analysis and decryption using AI.

Resources
  • DECODE DATABASE

    a collection of thousands of historical ciphertexts and keys

  • HISTCORP

    a collection of historical texts and language models for 16 European languages

Transcription of Ciphers
  • CrypTool Transcriber and Solver (CTTS)

    a desktop tool for manual transcription

  • TranscriptTool

    a web-based tool for semi-automatic transcription using AI

Cryptanalysis
  • CrypTool 2 (CT2)

    a desktop tool for breaking historical and modern ciphers

  • CrypTool-Online (CTO)

    an online tool for breaking (simpler) historical ciphers

Terms of use

The source code of the platform and tools are being released as open source under the Apache license v.2.0 with the exception of the DECODE database with its special terms and conditions.

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DECRYPT Publications

2025

  • Bruton, M. and Megyesi, B. (2025) From Statistics to Neural Networks: Enhancing Ciphertext-Plaintext Alignment in Historical Substitution Ciphers for Automatic Key Extraction. In Proceedings of the 8th International Conference on Historical Cryptology (HistoCrypt 2025).
  • Palma, C. and Megyesi, B. (2025) DECODE2LOD: Connecting the DECODE Database with the Linked Open Data Cloud. In Proceedings of the 8th International Conference on Historical Cryptology (HistoCrypt 2025).

2024

2023

  • Láng, B. (2023) Transfer of knowledge in the field of universal language schemes 18th-19th centuries. In Krász, Lilla (ed.): Sciences between Tradition and Innovation – Historical Perspectives/Wissenschaften zwischen Tradition und Innovation – historische Perspektiven. Wien: Praesens Verlag, 2023, 37-55

  • Láng, B. (2023) Valódi és ál-kódfejtések a titkosírások történetében (Real and fake codebreaking in the history of cryptology) In: Bárdos, Dániel; Tuboly, Ádám Tamás (eds.) Emberarcú tudomány : Áltudományok ésösszeesküvés-elméletek szorításában. Budapest, Magyarország : Typotex Kiadó.

  • Lasry, G. (2023) Armand de Bourbon Poly-Homophonic Cipher - 1649. Histocrypt 2023. Published by Linköping Electronic Press. [https://doi.org/10.3384/ecp195699]

  • Lasry, G., Biermann, N. & Tomokiyo, S. (2023) Deciphering Mary Stuart’s lost letters from 1578-1584, Cryptologia, [https://doi.org/10.1080/01611194.2022.2160677]

  • Megyesi, B., Sikora, J., Fornmark, F., Waldispühl, M., Kopal, N., and Mikhalev, V. (2023) Historical Language Models in Cryptanalysis: Case Studies on English and German. In the Proceedings of the 6th International Conference on Historical Cryptology. HistoCrypt 2023. pp. 120-129. [https://doi.org/10.3384/ecp195701]

  • Mikhalev, V., Kopal, N., Esslinger, B., Waldispühl, M. Láng,B., and Megyesi B. (2023) What is the Code for the Code? – Historical Cryptology Terminology. In the Proceedings of the 6th International Conference on Historical Cryptology. HistoCrypt 2023. pp. 130-138. [https://doi.org/10.3384/ecp195702]

  • Souibgui, M. A., Biswas, S., Mafla, A., Biten, A. F., Fornés, A., Kessentini, Y., Lladós, J., Gomez, L., & Karatzas, D. (2023). Text-DIAE: A Self-Supervised Degradation Invariant Autoencoder for Text Recognition and Document Enhancement. Proceedings of the AAAI Conference on Artificial Intelligence, 37(2), 2330-2338. [https://doi.org/10.1609/aaai.v37i2.25328]

  • Souibgui, M. A., Torras, P., Chen, J.,  and Fornes. A. (2023) An Evaluation of Handwritten Text Recognition Methods for Historical Ciphered Manuscripts. Proceedings of 7th International Workshop on Historical Document Imaging and Processing (HIP). [https://doi.org/10.1145/3604951.3605509]

  • Torras, P.,  Souibgui, M. A., Chen, J., Biswas, S., and Fornes. A. (2023) Segmentation-Free Alignment of Arbitrary Symbol Transcripts to Images. Proceedings of 15th IAPR International Workshop on Graphics Recognition (GREC). [https://doi.org/10.1007/978-3-031-41498-5_6]

  • Waldispühl, M. Variation and Change. (2023) In Condorelli, M. & Rutkowska, H., The Cambridge Handbook of Historical Orthography. Cambridge: University Press. [book.pdf]

2022

  • Dahlke, C. and Megyesi, B. (eds) (2022) Proceedings of the 5th International Conference on Historical Cryptology. NEALT Proceedings Series 49. HistoCrypt 2022. Published by Linköping Electronic Press. [https://doi.org/10.3384/ecp188]
  • Fürthauer, N., Mikhalev, V., Kopal, N., Esslinger, B., Lampesberger, H., and Hermann, E. (2022) Evaluating Deep Learning Techniques for Known-Plaintext Attacks on the Complete Columnar Transposition Cipher. In the Proceedings of the 5th International Conference on Historical Cryptology. HistoCrypt 2022. [https://doi.org/10.3384/ecp188394] 
  • Gambardella, M.E., Megyesi, B., and Pettersson, E. (2022) Identifying Cleartext in Historical Ciphers. In Proceedings of the Workshop on Language Technologies for Historical and Ancient Languages. LT4HALA 2022. [https://aclanthology.org/2022.lt4hala-1.1]
  • Héder, M. and Megyesi, B. (2022) The DECODE Database of Historical Ciphers and Keys: Version 2. In Proceedings of the 5th International Conference on Historical Cryptology. HistoCrypt 2022. [https://doi.org/10.3384/ecp188397]
  • Jemni, S. K., Souibgui, M. A., Kessentini, Y., & Fornés, A. (2022). Enhance to Read Better: A Multi-Task Adversarial Network for Handwritten Document Image Enhancement. Pattern Recognition, vol. 123. [https://doi.org/10.1016/j.patcog.2021.108370] 
  • Kopal, N. and Megyesi, B. (2022) Die Kryptografen des Papstes: Entschlüsselt: Geheime Nachrichten aus dem Vatikan. Published in c't 3/22 auf Seite 134. [https://www.heise.de/select/ct/2022/3/2118013133459359622]
  • Kopal N. and Waldispühl M. (2022) Deciphering Three Diplomatic Letters Sent by Maximilian II in 1575. Cryptologia 46 (2), 103-127. [https://doi.org/10.1080/01611194.2020.1858370]
  • Lasry, G. (2022) Cracking SIGABA in Less than 24 Hours on a Consumer PC. Cryptologia. [https://www.tandfonline.com/doi/full/10.1080/01611194.2021.1989522] 
  • Lasry, G. (2022) Analysis of a 19th Century French Cipher Created by Major Josse. Cryptologia. [https://www.tandfonline.com/doi/full/10.1080/01611194.2021.1996484] 
  • Láng, B. (2022) Colonnele Frank's Indecipherable Chiffre. In the Proceedings of the 5th International Conference on Historical Cryptology. HistoCrypt 2022. [https://doi.org/10.3384/ecp188400]
  • Lehofer, A. (2022) Applying Hierarchical Clustering to Homophonic Substitution Ciphers using Historical Corpora, Cryptologia, 46:5, 422-438. [https://doi.org/10.1080/01611194.2021.1918801]
  • Magnifico, G., Megyesi, B., Souibgui, M.A., Chen, J., and Fornés, A. (2022) Lost in Transcription of Graphic Signs in Ciphers. In Proceedings of the 5th International Conference on Historical Cryptology. HistoCrypt 2022. [https://doi.org/10.3384/ecp188403]
  • Megyesi, B., Tudor, C., Kopal, N., Láng, B., Lehofer, A., and Waldispühl, M. (2022) What Was Encoded in Historical Cipher Keys in the Early Modern Era?. In Proceedings of the 5th International Conference on Historical Cryptology. HistoCrypt 2022. [https://doi.org/10.3384/ecp188404]
  • Megyesi, B., Tudor, C., Láng, B., Lehofer, A., Kopal, N., de Leeuw, K., and Waldispühl, M. (2022) Keys with Nomenclatures in the Early Modern Europe. Journal of Cryptologia UCRY 2113185. Taylor and Francis. [https://doi.org/10.1080/01611194.2022.2113185]
  • Souibgui, M. A., Bensalah, A., Chen, J., Fornés, A. & Waldispühl, M. 2022. A User Perspective on HTR Methods for the Automatic Transcription of Rare Scripts. The case of Codex Runicus. ACM Journal on Computing and Cultural Heritage. [https://doi.org/10.1145/3519306]
  • Souibgui, M. A., Biswas, S., Jemni, S. K., Kessentini, Y., Fornés, A., Lladós, J., & Pal, U. (2022). Docentr : An End-to-End Document Image Enhancement Transformer. In the Proceedings of the 26th International Conference on Pattern Recognition (ICPR2022). [https://doi.org/10.1109/ICPR56361.2022.9956101]
  • Souibgui, M. A., Biten, A. F., Dey, S., Fornés, A., Kessentini, Y., Gomez, L., ... & Lladós, J. (2022). One-shot Compositional Data Generation for Low Resource Handwritten Text Recognition. In Winter Conference on Applications of Computer Vision (WACV). [https://doi.org/10.1109/WACV51458.2022.00262] 
  • Souibgui, M.A., Fornés, A., Kessentini, Y., and Megyesi, B. (2022) Few Shots Are All You Need: A Progressive Learning Approach for Low Resource Handwritten Text Recognition. Journal of Pattern Recognition Letters, vol. 160, pp. 43-49. Elsevier. [https://doi.org/10.1016/j.patrec.2022.06.003]
  • Waldispühl, M. Variation and Change. In Condorelli, M. & Rutkowska, H. (2022) The Cambridge Handbook of Historical Orthography. Cambridge: University Press. [https://doi.org/10.1017/9781108766463]

2021

  • Chen, J., Souibgui, M.A., Fornés, A., and Megyesi, B. (2021). Unsupervised Alphabet Matching in Historical Encrypted Manuscript Images. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183154]
  • Dinnissen J. and Kopal N. (2021). Island Ramanacoil a Bridge too Far. A Dutch Ciphertext from 1674. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183156]
  • Kopal, N. and Waldispühl, M. (2021). Two Encrypted Diplomatic Letters Sent by Jan Chodkiewicz to Emperor Maximilian II in 1574-1575. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183160]
  • Láng, B. (2021). Transfer of knowledge in the field of universal language schemes 18th-19th centuries. In Lilla Krász, ed. Science between tradition and innovation, 2021. [https://doi.org/10.47074/HSCE.2022-2.15]
  • Láng, B. (2021). The Rohonc code: tracing a historical riddle. University Park: Penn State University Press, 2021. ISBN: 978-0-271-09020-7 [https://www.psupress.org/books/titles/978-0-271-09020-7.html]
  • Lasry, G. (2021). Modern Cryptanalysis of Schlüsselgerät 41. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183163]
  • Lasry, G. (2021). Deciphering a Letter from the French Ambassador in Holland to Louis XIV. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183162]
  • Leierzopf, E., Kopal, N., Esslinger, B., Lampesberger, H. and Hermann, E. A Massive Machine-Learning Approach For Classical Cipher Type Detection Using Feature Engineering. (2021). In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183164]
  • Megyesi, B. and Láng, B. (2021) Revealing Secrets from the Past: Studying Historical Ciphers. Abstract to Digitial History in Sweden, Dec. 9-10, 2021, Umeå University, Sweden 
  • Megyesi, B. and Tudor, C. (2021). Transcription of Historical Ciphers and Keys. Guidelines, version 2.0. Dept. of Linguistics and Philology, Uppsala University, Sweden. [guidelines.pdf]
  • Megyesi, B., Tudor, C., Láng, B, and Lehofer, A. (2021). Key Design in the Early Modern Era in Europe. In Proceedings of the 4th International Conference on Historical Cryptology. HistoCrypt 2021. [https://doi.org/10.3384/ecp183165]
  • Pau Torras, Mohamed Ali Souibgui, Jialuo Chen, Alicia Fornés. (2021). A Transcription Is All You Need: Learning to Align through Attention. In Proceedings of the 14th IAPR International Workshop on Graphics Recognition (GREC), 2021. [https://doi.org/10.1007/978-3-030-86198-8_11]

2020

  • Bean, R., Lasry, G., and Weierud, F. (2020) Eavesdropping on the Biafra-Lisbon link – breaking historical ciphers from the Biafran war. Cryptologia [https://doi.org/10.1080/01611194.2020.1762261]
  • Bean, R., Lasry, G., and Weierud, F. (2020) We decrypted messages from the Biafran war that have remained secret for 50 years, The Conversation, July 2020 [article]
  • Chen, J., Souibgui, M. A., Fornés, A., and Megyesi, B. (2020) A Web-based Interactive Transcription Tool for Encrypted Manuscripts. In Proceedings of the 3rd International Conference on Historical Cryptology. HistoCrypt 2020. pp. 52-59. Linköping Electronic Press. [https://doi.org/10.3384/ecp2020171008] 
  • Kopal N. (2020) Of Ciphers and Neurons – Detecting the Type of Ciphers Using Artificial Neural Networks. In Proceedings of the 3rd International Conference on Historical Cryptology. HistoCrypt 2020. pp. 77-86. Linköping Electronic Press. [https://doi.org/10.3384/ecp2020171011]

  • Láng, B. (2020) “Was it a Sudden Shift in Professionalization? Austrian Cryptology and a Description of the Staatskanzlei Key Collection in the Haus-, Hof- und Staatsarchiv of Vienna" In: Beata, Megyesi (ed.) In Proceedings of the 3rd International Conference on Historical Cryptology HistoCrypt 2020. Linköping University Electronic Press, Linköpings universitet, (2020) p. 87. [https://doi.org/10.3384/ecp2020171012]

  • Lasry, G. (2020) Solving a Tunny Challenge with Computerized Testery Methods. In Proceedings of the 3rd International Conference on Historical Cryptology. HistoCrypt 2020. [https://doi.org/10.3384/ecp2020171013]

  • Lasry, G., Megyesi, B., and Kopal, N. (2020) Deciphering Papal Ciphers from the 16th to the 18th Century. Cryptologia. [https://doi.org/10.1080/01611194.2020.1755915]
  • Lasry, G., Niebel, I., and Andersson, T. (2020) Deciphering German Diplomatic and Naval Attaché Messages from 1900-1915. Cryptologia. [https://doi.org/10.1080/01611194.2020.1755914]
  • Lehofer, A. (2020) Decrypting Historical Ciphers - A Way of Mathematical Competence Development. Opus et Educatio. [https://doi.org/10.3311/ope.381]
  • Megyesi, B. (2020a) Transcription of Ciphers and Keys. In Proceedings of the 3rd International Conference on Historical Cryptology. HistoCrypt 2020. pp. 106-115. Linköping Electronic Press. [https://doi.org/10.3384/ecp2020171014]
  • Megyesi, B. (2020b) Transcription of Ciphers and Keys: Guidelines. Uppsala University. Sweden. [guidelines.pdf]
  • Megyesi, B., Esslinger, B., Fornés, A., Kopal, N., Láng, B., Lasry, G., de Leeuw, K., Pettersson, E., Wacker, A., and Waldispühl, M. (2020) Decryption of historical manuscripts: the DECRYPT project. Cryptologia. [https://doi.org/10.1080/01611194.2020.1716410] 
  • Souibgui, M. A. and Kessentini Y. (2020) DE-GAN: A Conditional Generative Adversarial Network for Document Enhancement. IEEE Transactions on Pattern Analysis and Machine Intelligence (TPAMI) [https://doi.org/10.1109/TPAMI.2020.3022406] 
  • Souibgui, M. A., Kessentini, Y., and Fornés, A. (2020) A conditional GAN based approach for distorted camera captured documents recovery. In Proceedings of the 4th Mediterranean Conference on Pattern Recognition and Artificial Intelligence (MedPRAI2020) [https://doi.org/10.1007/978-3-030-71804-6_16].
  • Souibgui, M. A., Fornés, A., Kessentini, Y., and Tudor, C. (2020) A Few-shot Learning Approach for Historical Ciphered Manuscript Recognition. In Proceedings of the 25th International Conference on Pattern Recognition (ICPR2020) (accepted) [https://doi.org/10.1109/ICPR48806.2021.9413255].
  • Tudor, C, Megyesi, B, and Láng, B. (2020) Automatic Key Structure Extraction. In Proceedings of the 3rd International Conference on Historical Cryptology. HistoCrypt 2020. pp. 146-152. Linköping Electronic Press. [https://doi.org/10.3384/ecp2020171018]

2019

  • Baró A., Chen, J., Fornés, A., and Megyesi, B. (2019) Towards a Generic Unsupervised Method for Transcription of Encoded Manuscripts. In Proceedings of the 3rd International Conference on Digital Access to Textual Cultural Heritage (DATeCH2019), May 2019 Brussels, Belgium. [https://doi.org/10.1145/3322905.3322920]
  • Kopal, N. (2019) Cryptanalysis of Homophonic Substitution Ciphers Using Simulated Annealing with Fixed Temperature. In Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-25, 2019, Mons, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [http://www.ep.liu.se/ecp/158/012/ecp19158012.pdf] 
  • Láng, B. (2019) Dead ends in breaking an unknown cipher: experiences in the historiography of the Rohonc Codex Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [https://ep.liu.se/ecp/158/006/ecp19158006.pdf]
  • Lasry, G. (2019) A Practical Meet-in-the-Middle Attack on SIGABA, Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [https://ep.liu.se/ecp/158/005/ecp19158005.pdf]
  • Lasry, G. (2019) Solving a 40-Letter Playfair Challenge with CrypTool 2, Proceedings of the 2nd International Conference on Historical Cryptology, HistoCrypt 2019, June 23-26, 2019, Mons, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [https://ep.liu.se/ecp/158/010/ecp19158010.pdf]
  • Lasry, G., Nils Kopal, and Arno Wacker. (2019) Cryptanalysis of Enigma double indicators with hill climbing. Cryptologia 43.4: 267-292. [https://doi.org/10.1080/01611194.2018.1551253]

  • Megyesi, B., Blomqvist, N., and Pettersson, E. (2019) The DECODE Database: Collection of Historical Ciphers and Keys. In Proceedings of the 2nd International Conference on Historical Cryptology. HistoCrypt 2019, June 23-25, 2019, Mons, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [https://ep.liu.se/ecp/158/008/ecp19158008.pdf]

  • Pettersson, E. and Megyesi, B. (2019) Matching Keys and Encrypted Manuscripts. In Proceedings of 22nd Nordic Conference on Computational Linguistics, Nodalida 2019. [https://aclanthology.org/W19-6126/]

PhD Theses

  • Anna Lehofer (2024) The Applicability of Hierarchical Clustering to Historical Ciphers. PhD Thesis. Eötvös Loránd University, Budapest, Hungary. [https://doi.org/10.15476/ELTE.2023.248]
  • Mohamed Ali Souibgui (2022) Document Image Enhancement and Recognition in Low Resource Scenarios: Application to Ciphers and Handwritten Text. PhD Thesis. December 1, 2022. Computer Vision Center. Universitat Autònoma de Barcelona. Spain. [http://hdl.handle.net/10803/688317]

Bachelor's and Master's Theses

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