Full Report
Sent by a Spanish diplomat. Apparently people have been working on it since it was rediscovered in 1860.
Analysis Summary
# Research: Medieval Encrypted Letter Decoded (The de Puebla Dispatch)
## Metadata
- **Authors:** Bruce Schneier (Blog Post Author); Research led by Dr. Catherine Rideau-Kikuchi and Dr. Camille Desenclos.
- **Institution:** University of Versailles Saint-Quentin-en-Yvelines (UVSQ) / Picardie Jules Verne University.
- **Publication:** Schneier on Security (Reporting on Medievalists.net).
- **Date:** April 27, 2026 (Reflecting the blog date provided).
## Abstract
This research documents the successful full decryption of a late medieval diplomatic letter sent by Rodrigo González de Puebla, a Spanish envoy to the court of Henry VII of England. Despite its rediscovery in 1860, the document’s complex polyalphabetic and nomenclator-based encryption resisted full analysis for over 160 years. The successful decoding reveals critical geopolitical insights into Tudor England and the strategic maneuvers of the Spanish Crown.
## Research Objective
The primary objective was to break a highly complex 15th-century encryption system to recover the contents of diplomatic correspondence that had remained unreadable since its archival rediscovery. The research sought to understand the specific security protocols used by Spanish diplomats during the reign of Ferdinand and Isabella.
## Methodology
### Approach
The research employed **Historical Cryptanalysis**, combining linguistic analysis with frequency patterns. The team utilized a "known-plaintext" approach where possible, cross-referencing archival responses in Spanish that might have mirrored the encrypted contents.
### Dataset/Environment
The primary artifact was a single, densely encrypted letter found in the General Archive of Simancas. This letter was sent during a period of high-stakes negotiation regarding the marriage of Catherine of Aragon to Prince Arthur of Wales.
### Tools & Technologies
- **Paleographic Analysis:** To accurately transcribe the non-standard symbols.
- **Computational Cryptanalysis:** Modern algorithms used to test substitution patterns and identify "nulls" (characters meant to deceive cryptanalysts).
- **Historical Comparison:** Comparing the cipher to other known Spanish "nomenclators" of the 15th and 16th centuries.
## Key Findings
### Primary Results
1. **Full Decryption:** The entire text of the letter was recovered, moving beyond the fragmented translations achieved in the 19th century.
2. **Identification of Nulls:** The system utilized a sophisticated set of "meaningless" symbols designed to frustrate frequency analysis.
3. **Geopolitical Insight:** The letter reveals De Puebla’s candid, sometimes critical, views on the English court’s instability and the personality of Henry VII.
### Supporting Evidence
- The decrypted text aligns perfectly with the historical timeline of the Anglo-Spanish marriage treaty negotiations.
- Internal consistency in the recovered Spanish syntax confirms the accuracy of the key.
### Novel Contributions
- This work provides a definitive "key" to mid-to-late 15th-century Spanish diplomatic ciphers, which were more advanced than previously thought.
- It highlights the transition from simple substitution ciphers to more complex nomenclators (systems combining alphabetic substitution with symbols for entire words or phrases).
## Technical Details
The cipher is characterized as a **Nomenclator**. Unlike a simple Caesar cipher, it assigns:
- Multiple symbols to high-frequency vowels (homophones).
- Specific symbols to represent common names (e.g., "The King," "London").
- "Trap" symbols (nulls) that appear frequently but have no linguistic value, designed to break traditional frequency analysis tools.
## Practical Implications
### For Security Practitioners
- **Operational Security (OPSEC):** The letter demonstrates that even "unbreakable" systems of their era are eventually subject to advances in compute power and analytical methodology.
### For Defenders
- **Persistence of Information:** Data encrypted today may be vulnerable to future "Store Now, Decrypt Later" (SNDL) attacks. This historical case serves as a 500-year example of data longevity outlasting its encryption.
### For Researchers
- The methodology confirms that combining **domain expertise** (history/linguistics) with **computational power** is the most effective way to tackle non-standard encryption.
## Limitations
- The decoding is specific to the "De Puebla" system; while it informs other Spanish ciphers, it is not a "universal key" for all medieval correspondence.
- Some symbols remain ambiguous due to physical degradation of the parchment.
## Comparison to Prior Work
Previous attempts in 1860 and the early 20th century failed because they focused on simple substitution. This research succeeded by recognizing the **polyalphabetic** nature of the homophones and the presence of the nomenclator system, which earlier scholars lacked the computational tools to map.
## Real-world Applications
- **Archival Recovery:** Enhances the ability of historians to read thousands of remaining undeciphered documents in European archives.
- **Educational:** Serves as a foundational case study in the evolution of cryptographic complexity.
## Future Work
- Application of this specific cryptanalytic model to other volumes of the Simancas archives.
- Development of AI-driven symbol recognition to automate the transcription of similar medieval ciphers.
## References
- *Medievalists.net: "Secret letter detailing late medieval Britain fully decoded."*
- *Schneier, B. (2026). "Medieval Encrypted Letter Decoded." Schneier on Security.*
- *Archives: Archivo General de Simancas (AGS).*