The Moustier altar inscriptions

The Moustier altar inscriptions are two encrypted texts displayed on side altars in the Church of St. Martin in Moustier, Belgium. Although they have been visible to the public for nearly two centuries, they remain unsolved.

The inscriptions first attracted the attention of the cryptologic community in 2013, when British researcher Nick Pelling discovered an article about them in a declassified issue of the NSA’s Cryptolog magazine. Since then, they have become one of the best-known unsolved cryptograms in Europe.

The two inscriptions are carved into the fronts of a Marian altar and a St. Martin altar. They consist mainly of letters, with a few additional symbols. At first glance, the texts resemble a simple substitution cipher, but numerous attempts to solve them have failed. Statistical analyses have not produced any convincing plaintext.

The following transcript of Marian altar inscription was made by Nick Pelling:

L F E G K R V Q
Y P Z H N R L B D
M F ^ N V D [
N ^ P V J H M ^
L F N ^ [ B K P
—–
N C L X B P D W
R N [ C H Z R P
M D X R ^ P L N
H F ^ L D N X W
E N L V N D ^ P N

Here is Nick’s transcript of the inscription on the St. Martin altar:

J N L K B F P R
V M G H W H[
Q L S B N F HP
M G [ K H V R
^ L R N F S X V
—–
P F V B L P M R
R A [ G K T D
B N D F J V R W
L U B F P N I D
C [ T R ^ Q M

Little is known about the origin of the inscriptions. The altars date from the nineteenth century, and it is generally assumed that the encrypted texts were created around the same time. Since they appear in a church, one obvious possibility is that they conceal religious texts, prayers, dedications, or Biblical quotations. However, no evidence supporting any of these hypotheses has been found.

One of the main challenges is that the encryption method is unknown. While a simple monoalphabetic substitution cannot be ruled out completely, many researchers believe that a more complex system was used. Proposed explanations include homophonic substitution, codebook systems, and other cipher methods popular in the eighteenth and nineteenth centuries.

In 2022, a new hypothesis was proposed by cryptologist George Lasry. He suggested that the inscriptions might be related to the Ave Maria cipher developed by Johannes Trithemius. According to this theory, the symbols could represent words from a religious code system rather than individual letters. Although the idea is intriguing, it has not yet led to a solution.

Several important questions remain unanswered:

• What do the inscriptions say?
• Who created them?
• Why were they placed on the altars?
• What encryption method was used?
• Are the two texts related to each other?

The Moustier altar inscriptions demonstrate that unsolved cryptograms are not always hidden in archives or private collections. Sometimes they can be found in plain sight, waiting for a codebreaker to uncover their secret. More than 180 years after their creation, the mystery remains unsolved.