Claude Fable 5.1 Decrypts 370-Year-Old Royalist Cipher

Anthropic's Claude Fable 5.1 has successfully decrypted the "Cyphral Distich," a complex puzzle that remained unsolved for 370 years. The breakthrough demonstrates the ability of frontier AI to resolve historical mysteries by synthesizing obscure clues across vast datasets.

Tasked by Vals AI, the model spent 44 minutes and approximately 176,000 tokens to crack the code. The Cyphral Distich, created in 1653 by Scottish writer and Royalist Sir Thomas Urquhart, consists of two lines of 32 numbers each located at the end of Urquhart's treatise, "Logopandecteision." The decrypted text reveals a Royalist prayer: "O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND."

The Mechanism of the Cipher

For centuries, human cryptanalysts failed to solve the puzzle because they focused on traditional frequency analysis and substitution ciphers. The actual solution relied on an internal book-key mechanism. Claude Fable 5.1 identified that the key was embedded within Urquhart's own text. Specifically, the model determined that the i-th number in the cipher served as a word index for the i-th "Proquiritation" (paragraph) in the text; by taking the first letter of that specific word, the message was revealed.

Following this success, the model applied a similar page-based word index method to decrypt the "Cyphral Octastich," another encrypted message found in Urquhart's 1652 book, "The Jewel."

Historical Context

Sir Thomas Urquhart was a committed Royalist during a period of intense political upheaval in Britain. The Cyphral Distich had long been regarded as a significant open problem in the field of historical cryptography. It appeared as an unsolved challenge in "Notes and Queries" as far back as 1899 and was listed by researcher Klaus Schmeh among the top 50 unsolved encrypted messages in history.

Why It Matters

This event signals a shift in how archival research and cryptanalysis are conducted. The resolution of the Distich suggests that the "human attention bottleneck"—the immense time required for a researcher to manually test hundreds of unpromising leads in obscure texts—is being removed.

According to Vals AI, the answer was simple in hindsight, noting that the model's success came from the fact that it "just kept looking until it found it." This suggests that AI's primary advantage in these cases is not necessarily advanced mathematical cryptanalysis, but a level of persistence and pattern recognition that exceeds human capacity.

What's Next

The success of Claude Fable 5.1 opens the door for the systematic application of LLMs to other "unsolvable" historical ciphers. Researchers will likely look toward other 17th-century texts and obscure manuscripts to see if similar internal keys were used. The primary question remaining is whether this persistence can be scaled to ciphers that lack a clear internal key or those that utilize more sophisticated mathematical transformations.