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Les Codes Secrets Décryptés

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Les Codes Secrets Décryptés Les codes secrets décryptés 3ème édition corrigée et augmentée Didier Müller Nymphomath Éditions Contact : [email protected] © Nymphomath Éditions, 2018-2020 version 3.0 : 9 septembre 2018 version 3.2 : 13 décembre 2020 ISBN 978-2-8399-2485-6 À Pierre Baud « It may be roundly asserted that human ingenuity cannot concoct a cipher which human ingenuity cannot resolve... » « On peut affirmer que l'ingéniosité humaine ne peut pas élaborer un chiffre que l'ingéniosité humaine ne peut résoudre... » Edgar Allan Poe A Few Words on Secret Writing, 1841. Avant-propos AVANT-PROPOS La cryptologie, plus communément appelée la science des codes secrets, est à la fois une science et un art. C'est une science, car elle fait appel aux mathématiques et à l'informatique, que ce soit pour chiffrer des messages ou pour les décrypter. La cryptologie est aussi un art, car elle fait appel aux talents d'intuition, d'imagination et d'invention du décrypteur, ces facultés étant elles-mêmes secondées par des connaissances linguistiques approfondies. Je voulais écrire un livre qui s'inscrirait dans la ligne de ceux des grands cryptologues du 20 ème siècle : Baudouin, Friedman, Givierge, Langie, Sacco, Fouché Gaines, Sinkov, et quelques autres. Leurs livres ne se contentaient pas de présenter des systèmes de chiffrement, ils montraient aussi leurs faiblesses et comment les décrypter. Je vous invite donc à un voyage dans le temps, depuis l'Antiquité jusqu'à nos jours, où vous découvrirez les systèmes de chiffrement qui ont marqué leur époque. Nous regarderons plus d'une centaine de chiffres, la plupart ne demandant que du papier et un crayon. Plus nous avancerons dans ce livre, et plus les systèmes de chiffrement deviendront complexes. Au chapitre 11, je ne ferai qu'effleurer la cryptographie moderne, car elle fait appel à des notions mathématiques et informatiques très complexes, que j'essaierai de simplifier au maximum. On peut faire l'impasse sur ce chapitre ou se référer à la bibliographie si on veut en savoir plus. Les codes secrets ne sont pas l'apanage des militaires et des diplomates. Des auteurs célèbres, férus de cryptographie - Edgar Allan Poe, Jules Verne, François Rabelais - ont écrit des romans où le décryptement d'un message secret jouait un rôle central. En les étudiant de près, nous verrons à quel point ces textes sont réalistes. Des sculpteurs, peintres, dessinateurs, politiques s'amusent aussi avec les codes secrets. Nous verrons quelques exemples amusants. Par rapport aux livres de mes glorieux prédécesseurs, la grande originalité de celui-ci est d'être couplé avec un site web dont l'adresse est : www. apprendre-en-ligne.net /crypto J'ai commencé ce site en 2001 déjà, avec l'intention d'en faire un cours en ligne pour mes élèves de lycée. Le sujet est tellement passionnant que ce site a grandi, grandi... et trouvé son aboutissement dans cet e-book, après deux publications sur papier. La deuxième édition a paru en 2011. Depuis, j'ai appris moult nouvelles choses et découvert d'autres passionnés sur le web, notamment le cryptologue et écrivain allemand Klaus Schmeh qui tient un blog fantastique sur le sujet : « Klausis Krypto Kolumne ». Tout cela m'a donné envie de reprendre la plume (ou plutôt le clavier) et d'améliorer la deuxième édition. De plus, je voulais écrire un e-book, qui permet de faire des mises à jour fréquentes, d'améliorer sans frais l'iconographie, et de se passer d'intermédiaires. De plus, d'un simple clic, on peut consulter d'autres documents intéressants qui complètent le propos. i Avant-propos Cette troisième édition est en effet un « e-book augmenté ». Vous trouverez au fil des pages des « QR codes » ressemblant à ceci : Vous pourrez cliquer sur ce QR code si vous lisez ce livre sur un support numérique. Vous pourrez ainsi mettre les « mains dans le cambouis » et tester les codes secrets que vous trouverez dans ce livre. Si vous avez préféré imprimer cet e-book, il vous faudra installer sur votre smartphone une application qui lit les QR codes. Vous pointerez ensuite un QR code avec la caméra de votre appareil pour accéder à la page web qui vous permettra d'essayer le système de chiffrement présenté. Dans cette troisième édition, je voulais rendre hommage à Pierre Baud, mon ami cryptologue qui nous a quittés en 2014. Quelques passages de cet e-book ont été écrits par lui. Conventions Dans la plupart des livres, les lettres d'un message codé sont en majuscules, celle d'un message clair en minuscules. J'ai aussi adopté cette convention. Sauf exceptions, les messages clairs ne contiennent pas de lettres accentuées, ni d'espaces, ni de signes de ponctuation. On ne chiffrera donc que les lettres de a à z. Traditionnellement, on regroupe les lettres d'un cryptogramme par groupes de cinq, car ainsi le nombre de lettres se compte plus rapidement. D'autre part, on complique la tâche du casseur de code en ne respectant pas les espaces entre les mots du clair. J'utiliserai des abréviations du style [ABCD12] pour les références à un livre de la bibliographie, où « ABCD » sont, en principe, les quatre premières lettres de l'auteur et « 12 » les deux derniers chiffres de l'année de parution du livre. La numération des pages vous étonnera peut-être. J'écrirai d'abord le numéro du chapitre, suivi du numéro de la page dans ce chapitre. Par conséquent, la première page de chaque chapitre C aura le numéro C-1. Je pourrai ainsi facilement modifier un chapitre sans me préoccuper des autres. En effet, cet e-book va évoluer avec le temps ! La version que vous avez sous les yeux est d'ailleurs la première mise à jour complète (version 3.1). ii Table des matières Table des matières 1. Introduction..........................................................................................................................................1-1 1.1. Définitions de termes courants........................................................................................................1-1 1.2. Alice et Bob....................................................................................................................................1-4 1.3. Classification des systèmes de chiffrement....................................................................................1-5 1.4. Repères historiques.........................................................................................................................1-5 1.5. Références.....................................................................................................................................1-14 2. Stéganographie.....................................................................................................................................2-1 2.1. Encres invisibles.............................................................................................................................2-3 2.2. Les Ave Maria de Trithème............................................................................................................2-5 2.3. Grille de Cardan..............................................................................................................................2-6 2.4. L'alphabet bilitère de Francis Bacon...............................................................................................2-7 2.5. Sémagrammes...............................................................................................................................2-15 2.6. Micropoint....................................................................................................................................2-17 2.7. Acrostiches....................................................................................................................................2-21 2.8. Code de Trevanion........................................................................................................................2-25 2.9. Lettres à double entente................................................................................................................2-25 2.10. Stéganographie oulipienne..........................................................................................................2-26 2.11. La méthode de Grandpré.............................................................................................................2-27 2.12. Le Barn code du SOE.................................................................................................................2-28 2.13. Message caché dans une image numérique................................................................................2-30 2.14. Cryptographie visuelle................................................................................................................2-34 2.15. La stéganographie dans la littérature et l'art...............................................................................2-37 2.16. Références...................................................................................................................................2-47 3. Codes et répertoires.............................................................................................................................3-1 3.1. Le code de Mary Stuart...................................................................................................................3-4 3.2. Code de Popham.............................................................................................................................3-5 3.3. Les codes commerciaux..................................................................................................................3-6
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