Half a century of naval communications security: the Perea code and its operational legacy in the Spanish Navy

This article presents a historical and technical analysis of the Perea Signal Code, employed by the Spanish Navy from the late nineteenth century until the mid-twentieth century. It examines its structural design, operational deployment under combat conditions, subsequent revisions, and its coexistence with mechanical cipher systems such as Enigma. By situating the Perea Code within broader developments in naval cryptography, the study evaluates its longevity, operational resilience, and structural vulnerabilities. The case challenges deterministic narratives of technological displacement in cryptography and demonstrates how codebook systems, when supported by strict physical security, disciplined radio procedure, and periodic superencipherment, could remain viable well into the era of mechanical encryption. At the same time, its partial reconstruction during World War II reveals the structural vulnerabilities inherent in prolonged codebook retention.