LUCI cipher (1937): Replication files and datasets

LUCI_ZENODO/├── README.md├── data/│ ├── tabla_luci.csv│ └── mensajes_cifrados.csv├── code/│ └── decrypt_luci.py└── docs/ └── methodology.pdf Cryptography and Secret Warfare in 1937 Spain: Technical Reconstruction of the LUCI Cipher Abstract This article presents the first comprehensive technical reconstruction of the LUCI cipher, an encryption system employed by Francoist forces in 1937 and successfully decrypted by the Republican Cipher Bureau. Based on direct analysis of unpublished primary archival documentation preserved at the Spanish National Historical Archive (AHN, FC-1564, file 20), the study demonstrates that LUCI functioned as a numerical homophonic substitution cipher incorporating syllabic and lexical units, embedded within the tradition of European diplomatic nomenclators of the interwar period. Through classical cryptanalysis and computational replication, the internal structure of the cipher is reconstructed, decrypted messages are validated, and its resistance to frequency-based attacks is quantitatively assessed. The results show that LUCI occupied an intermediate technological position between classical manual systems and emerging mechanical encryption devices, revealing a higher level of sophistication than previously attributed to non-mechanized Francoist communications. Simultaneously, the study highlights the professional competence of Republican cryptanalytic services, documenting their capacity for systematic interception, structural reconstruction, and forensic validation of encrypted material within judicial-military circuits. The study also provides fully replicable computational resources (scripts and datasets) to support transparency and future comparative research, contributing to current debates on open science and digital history. Keywords Spanish Civil War; cryptography; intelligence history; homophonic cipher; archival research; digital history; LUCI cipher; military communications