From the English Wikipedia page [1] - there is more information on the German Wikipedia page [2]

The Copiale cipher is an encrypted manuscript consisting of 75,000 handwritten characters filling 105 pages in a bound volume. It is thought to date to between 1760 and 1780. 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 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. 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.

A parallel manuscript is kept at the Staatsarchiv Wolfenbüttel.

An article in The Guardian on the manuscript's deciphering is here [3]; another article here [4].