At a glanceNot solved · text not obtained
- The letter
- Pandolfo Malatesta, archbishop of Patras and brother of Gianfrancesco’s wife Paola, to Gianfrancesco Gonzaga, lord of Mantua, Rome, 22 April 1428. Clear Latin with three cipher runs.
- Where
- Archivio di Stato di Mantova, Archivio Gonzaga. DECODE R1854, image 1; the record was catalogued as one letter of unknown sender.
- The cipher
- 262 letter-like signs of 29 kinds, with separators strewn between them. Not a simple or homophonic substitution in Latin or Italian.
- How it was tried
- Annealing under a dozen hypotheses and in seven languages, with controls; the Gonzaga register’s Malatesta keys and two other key records.
- Why it resists
- No key material: the likely source, a ciphered letter of Pandolfo’s of 1438 with its contemporary translation, is not online.
- Still open
- All three runs.
This letter is image 1 of DECODE R1854, which holds four letters to Gianfrancesco Gonzaga in four ciphers; the other three are deciphered and have their own pages: a Gonzaga agent in Rome, 9 August 1428, an informant at Forlì, 6 November 1428 and Francesco de’ Cattanei, February 1430, in which Pandolfo, as archbishop of Patras, appears offering his see’s lordship to Venice.
01 The letter
The letter opens in clear with the news, just arrived in Rome, of the peace between the duke of Milan and the league (the peace of Ferrara, 19 April 1428), “licet nondum publicatam”. Pandolfo writes that he has conceived plans “valde utilia” for his brother-in-law, and the secret parts follow in three runs: after “decrevi ne” (58 signs), after “quo modo valeat” (70) and after “nolit rem ipsam negligere” (134). The signs are letter-like (ſſ, η, ʒ, H, cc, ∩, I, ƀʒ, v, ꭏ), with separators strewn between them at random. The images are not in the public domain, so none is reproduced here.
02 What was tried
The index of coincidence is 0.056 with 29 sign types and about 44% vowel-like signs, which points to a near-monoalphabetic system. Yet annealing in Latin and in Italian (fifteenth- and sixteenth-century and modern models), with and without word spaces, with costed nulls, with vowel and consonant classes, reversed by line, by word and globally, and with digraph signs split or merged, gave nothing. The same solver solves a synthetic 266-letter Latin text in a monoalphabetic cipher even with 12% of its signs corrupted. A second, sign-by-sign transcription corrected three misreadings and made no difference.
Tests for a digraphic system (sign-pair index 0.005), a periodic one (flat column index for periods 2–8) and a transposition (vowel alternation above 200 shuffles: substituted text in its own order) were negative. Annealing in Spanish, Catalan, French, German and Portuguese and with order-3 models over 150 restarts, a syllable-aware anneal (each sign a letter, one of 27 syllables or word signs, or a null), and, on 5 October 2026, the separators kept as five extra cipher signs, all gave no words.
The Gonzaga chancery register on DECODE (R1853, about 1401–1420) has keys “cum Malatesta d(omi)ni Pandulfi”, “nova cum fratribus de Malatestis” and “cum Karolo de Malatestis”. They share several shapes with these signs (ſt, H, Λ, v, ꭏ, ƀʒ) but not their values. The Este keys of R1855 (including one for Malatesta Novello, 1441) and the Florentine keys of R1852 (1424–32) have different sign sets.
03 What could open it
The best lead is Pandolfo’s letter of 1 March 1438, nearly all in cipher (ASMn, AG, b. 1081, c. 159–160), whose contemporary Italian translation survives (b. 840, c. 95) and is printed by Anna Falcioni. If its cipher is the same family, that pair would give the key. Its images are not online, and the print gives the plaintext only.
04 Sources
- DECODE R1854 (5 images) and R1853 (the Gonzaga cipher register), de-crypt.org/decrypt-web/RecordsView/1854. Images © Archivio di Stato di Mantova; not reproduced here.
- Anna Falcioni, “Pandolfo Malatesti e il concilio ecumenico di Ferrara (1438)”, with an appendix printing the 1438 letter from ASMn, AG, b. 840 c. 95 and b. 1081 c. 159–160; and A. Falcioni, “La crittografia e le cifre diplomatiche della cancelleria di Sigismondo Pandolfo Malatesti”, Quaderni dell’Accademia Fanestre 3 (2004), 187–190 (not seen).
- “Malatesta, Pandolfo”, Dizionario Biografico degli Italiani (Treccani), which cites his partly ciphered autograph letters in ASMn, AG, E.XXVII.2, b. 1081.
The transcriptions, solver runs and controls are in targets/malatesta1428/; the solvers in targets/r1854/.