Here’s a French letter from the 17th century, encrypted probably in a homophonic system or a nomenclator. Can a reader decipher it?

A representative of Le Manuscrit Français has contacted me. Le Manuscrit Français is a French trading company specializing in the sale of autograph letters and manuscripts of historical figures.

The representative has come across an encrypted letter from the 17th century he would like to have deciphered. Here it is:

Source: 17th century letter

The letter contains a few cleartext passages written in French. The plaintext is most likely French, too.

Le Manuscrit Français has asked me not to publish any details about the background of this letter for the moment. I will, however, provide more information next week.

As can be seen, the author of the letter used an alphabet consisting of numbers (one-digit and two-digit) and symbols. In my view, there are two likely explanations for this:

  • Each number and each symbol stands for a letter. Frequent letters (such as the E) have several representations. An encryption system of this kind is referred to as “homophonic substitution”.
  • Each number stands for a letter, each symbol stands for a word. If this is the case, we are dealing with a nomenclator.

In addition, some numbers or symbols might have no meaning at all (nulls) or even make the letter before or after meaningless (nullifyer).

As frequent readers of this blog will know, breaking a homophonic cipher or a nomenclator can be difficult, or even not impossible, if no additional information, such as a crib or the substitution table, is available.

However, many encryption systems of this kind are flawed. For example, many nomenclators make an easy-to-detect distinction between letters and words. This might be the case here, too. If the system used by the letter author is a nomenclator, I’m pretty sure that the numbers and the symbols are used differently – most likely, numbers stand for letters, while symbols stand for words. In general, the homophonic substitutions and nomenclators used in the 17th century were often weak compared to the systems of later centuries.

All in all, I think there’s a good chance that this cryptogram can be broken. If a reader is successful, please let me know.


Further reading: An unsolved encrypted letter from the 17th century

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Kommentare (7)

  1. #1 Thomas
    13. Juni 2020

    Oh, a letter by Cardinal Richelieu from Nov. 1641!
    Hence the cipher presumably was devised by his cryptologist Antoine Rossignol, https://en.m.wikipedia.org/wiki/Rossignols. This site provides similar nomenclators: http://cryptiana.web.fc2.com/code/fronde.htm

  2. #2 Norbert
    Berlin
    13. Juni 2020

    The good news is, the vast majority of symbols and numbers are homophones for single letters. Here is a first shot (still missing some words at the end):

    (cleartext: J’ay receu vostre lettre du 28e du passé)
    apres avoir veu jusques ou voz [soldats?] peuvent vous mener. Celle-cy
    est pour vous dire qu’on n’oubliera rien de ce qui se pourra
    pour tacher de vous, l’armee estant pour cest effect
    logee aux environs du Mont Hulin. Si vous n’etes secouru
    dans le temps auquel je vous restera encore assez
    [soldats?] pour faire vostre capitulation.
    [Le Roy?] vous permet de la faire desirant conserver tant
    de braves gents qui sont avec vous si vous pouvez passer ce
    mois en l’estat que vous estez et qu’ il vous reste assez de
    [soldats?] pour faire par aprez vostre capitulation vous sauverez
    sans doubte vostre plaoe(plane?) ou vous ruinerez tres asseurement
    l’armee des ennemis. C’est a vous a bien prendre voz
    mezures et faire vostre capitulation si claire et si seure
    qu’on ne puisse vous y manquer. N’oubliez pas de stipuler
    qu’on vous conduize seurement droit a Hesdin ou a Doullan
    sans vous faire prendre aucun detour.

    (cleartext: A Paris ce 6e novembre 1641. Si par vostre bonne conduitte vous avez)
    quelque rezerve que vous n’avez pas voulu(?)
    mander qui vous donne moyen d’aller quelques [45] dans le
    mois de decembre (…)
    et de nostre coste n’oublierons rien a cette fin.

    So, can I get the letter for reward? 😉

  3. #3 Thomas
    13. Juni 2020

    @Norbert

    Outstanding, congratulations!

    3.000 – 4.000 Euros as reward, would that be sufficient for you? Maybe Klaus will buy it at the auction and reward you: https://www.debaecque.fr/lot/104536/12654928?npp=20&sort=1& 😉

  4. #4 Norbert
    Berlin
    13. Juni 2020

    “plaoe” is most likely misspelled for “place” (not “plane”). The same military expression, “sauver la place”, occurs in the last paragraph, after “decembre”:

    [50] tenons la place sauvee et de nostre coste n’oublierons rien a cette fin.

    @Thomas: deal!
    @Klaus: just kidding – if you buy it you can keep the letter 🙂

  5. #5 Norbert
    Berlin
    14. Juni 2020

    Up to five homophones per letter (or even six, depending on how you interpret the symbols)!
    Most numbers show a striking regularity:

    9 = a
    8 = b? (does not occur)
    7 = c
    6 = d
    5 = e
    4 = f

    10 = i

    19 = l
    18 = m
    17 = n
    16 = o
    15 = p
    14 = q
    13 = r
    12 = s

    20 = t
    21 = u
    22 = x? (does not occur)
    23 = y
    24 = z

  6. #6 Norbert
    Berlin
    14. Juni 2020

    I made some corrections and arranged the lines according to the original letter. Also, I added some accents for better readability. The rest of the orthography, apart from a few obvious mistakes, has been left unchanged. It should be said, however, that accents at that time usually were ignored in the process of encrypting, and that they were used anyway differently and less often than today, and, for the sake of completeness, that my distinction between u and v is also only for readability – cryptologically, they count as one character. The same goes for i and j. Cleartext in bold, decrypted text in italics.

    1: J’ay receu vostre lettre du 28e du passé. Après avoir veu jusques où voz [44 = soldats ?] peuven-
    2: t vous mener, celle-cy est pour vous dire qu’on n’oubliera rien
    3: de ce qui se pourra pour tâcher de vous [39 = secourir ?], l’armée estant
    4: pour cest effect logée aux environs du Mont Hulin. Si vous
    5: n’estez secouru dans le temps auquel il vous restera encor-
    6: es assez de [44 = soldats ?] pour faire vostre capitulation, [Delta-like symbol = Le Roy ?] vous
    7: permet de la faire désirant conserver tant de braves
    8: gentz qui sont avec vous. Si vous pouvez passer ce mois
    9: en l’estat que vous estez et qu’il vous reste assez de
    10: [44 = soldats ?] pour faire par aprez vostre capitulation, vous sauve-
    11: rez sans doubte vostre place ou vous ruinerez très a-
    12: sseurement l’armée des ennemis. C’est à vous à bien prendre
    13: voz mezures et faire vostre capitulation si claire
    14: et si seure qu’on ne puisse vous y manquer. N’oubliez
    15: pas de stipuler qu’on vous conduize seurement droit
    16: à Hesdin ou à Doullan sans vous faire prendre aucun de-
    17: stour. A Paris ce 6e novembre 1641. Si par vostre bonne conduitte vous avez quelque
    18: rézerve que vous n’avez pas voul-
    19: u(?) mander qui vous donn-
    20: e moyen d’aller quelqu-
    21: es [45] dans le mois de dé-
    22: cembre, [50] tenons la place
    23: sauvée et de nostre costé
    24: n’oublierons rien a cet-
    25: te fin. Je suis entierement content
    26: de vous.

  7. #7 Marc
    15. Juni 2020

    @Norbert,
    who else? Great job! 😉