The Copiale cipher is an encrypted manuscript consisting of 75,000 handwritten characters filling 105 pages in a bound volume. Originally thought to date between 1760 and 1780, it was later found to date from the 1730s. It was first examined at the German Academy of Sciences at Berlin in the 1970s but did not come to public attention until 2011 when an international team announced that they had deciphered it. In April 2011, it was decoded with the help of modern computer techniques by Kevin Knight of the University of Southern California, along with Beáta Megyesi and Christiane Schaefer of Uppsala University in Sweden. They found it to be a German text encrypted by a homophonic cipher, a complex substitution code.
The manuscript includes abstract symbols, as well as letters from Greek and most of the Roman alphabet. The only plain text in the book is "Copiales 3" at the end and "Philipp 1866" on the flyleaf. Philipp is thought to have been an owner of the manuscript. The plain-text letters of the message were found to be encoded by accented Roman letters, Greek letters and symbols, with unaccented Roman letters serving only to represent spaces.
The researchers found that the initial portion of 16 pages describes an initiation ceremony for a secret society, namely the "high enlightened (Hocherleuchtete) oculist order" of Wolfenbüttel, or Oculists. A parallel manuscript is kept at the Staatsarchiv Wolfenbüttel. The document describes, among other things, an initiation ritual in which the candidate is asked to read a blank piece of paper and, on confessing inability to do so, is given eyeglasses and asked to try again, and then again after washing the eyes with a cloth, followed by an "operation" in which a single eyebrow hair is plucked.
The Copiale cipher is a substitution cipher. It is not a 1-for-1 substitution but rather a homophonic cipher: each ciphertext character stands for a particular plaintext character, but several ciphertext characters may encode the same plaintext character. For example, all the unaccented Roman characters encode a space. Seven ciphertext characters encode the single letter "e". In addition, some ciphertext characters stand for several characters or even a word. One ciphertext character ("†") encodes "sch", and another encodes the secret society's name.
The Oculists were a group of ophthalmologists led by Count Friedrich August von Veltheim, who died in April 1775. The Philipp 1866 Copiales 3 document, however, appears to suggest that the Oculists, or at least Count Veltheim, were a group of Freemasons who created the Oculist society in order to pass along the Masonic rites which had recently been banned by Pope Clement XII.
- "Computer Scientist Cracks Mysterious 'Copiale Cipher'". American Association for the Advancement of Science. October 25, 2011.
- Knight, Kevin, Megyesi, Beáta and Schaefer, Christiane (2011). "The Copiale Cipher". Uppsala Universitet, Institutionen för lingvistik och filologi website. Retrieved 2011-10-25. Includes images of the full text, as well as full translations in German and English.
- USC Scientist Cracks Mysterious "Copiale Cipher" on YouTube, on the official USC channel.
- Forscher knacken deutschen Geheim-Code (at Bild.de)
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-  (the complete proceedings) or  (the relevant presentation): Knight, Kevin, Megyesi, Beáta and Schaefer, Christiane "The Copiale Cipher," Proceedings of the 4th Workshop on building and using comparable corpora, pages 2-9, 49th Annual Meeting of the Association for Comparable Linguistics, 24 June 2011. Retrieved October 25, 2011
- Henning, Aloys "Eine frühe Loge des 18. Jahrhunderts: 'Die Hocherleuchtete Oculisten-Gesellschaft' in Wolfenbüttel", in: Europa in der frühen Neuzeit, Festschrift für Günter Mühlpfordt 5, Aufklärung in Europa, hg. Erich Donnert, Köln/Weimar/Wien 1999, S. 65-82.
- Shactman, Noah (16 November 2012). "They Cracked This 250-Year-Old Code, and Found a Secret Society Inside". wired.com. Wired (magazine). Retrieved 2 November 2013.
- Waugh. Rob "How translation software helped crack 'unbreakable' code in 1866 secret society manuscript Daily Mail, October 25, 2011, Retrieved October 25, 2011