Contrary to the belief in the nineteenth century, ciphers were used in Spain before the accession of King Ferdinand (the Catholic), as noted in Tomokiyo (2018), "Spanish Ciphers before Accession of King Ferdinand: 1470-1479" (Academia.edu) and, much before that, Galende Diaz (1993-1994), "La escritura cifrada durante el reinado de l os Reyes Catolicos y Carlos V" (pdf, footnotes 3, 5).
I reconstructed a cipher used between Eleanor of Navarre [Princess Leonor of Navarre] and her father John II from a letter with interlinear decipherment (DECODE R9484). The letter (Olite, 1 January 1476) is preserved in Biblioteca de la Real Academia de la Historia (BRAH) and is already mentioned in Galende Diaz (1993-1994), footnote 5.
Eleanor (Wikipedia), half-sister of Ferdinand the Catholic, was a daughter of John II, King of Aragon, and his first wife, Blanche I of Navarre. She was regent of Navarre during her father's absence.
Héder, M ; Megyesi, B. The DECODE Database of Historical Ciphers and Keys: Version 2. In: Dahlke, C; Megyesi, B (eds.) Proceedings of the 5th International Conference on Historical Cryptology HistoCrypt 2022. Linkoping, Sweden : LiU E-Press (2022) pp. 111-114. , 4 p. [pdf]
Megyesi Beáta, Esslinger Bernhard, Fornés Alicia, Kopal Nils, Láng Benedek, Lasry George, Leeuw Karl de, Pettersson Eva, Wacker Arno, Waldispühl Michelle. Decryption of historical manuscripts: the DECRYPT project. CRYPTOLOGIA 44 : 6 pp. 545-559. , 15 p. (2020) [link]
Megyesi, B., Blomqvist, N., and Pettersson, E. (2019) The DECODE Database: Collection of Historical Ciphers and Keys. In Proceedings of the 2nd International Conference on Historical Cryptology. HistoCrypt 2019, June 23-25, 2019, Mons, Belgium. NEALT Proceedings Series 37, Linköping Electronic Press. [pdf]