Ford est attablé au Milliways, le dernier restaurant avant la fin du monde. Il souhaite partager son émerveillement du spectacle (qu'il a déjà vécu, mais dans des conditions telles qu'il n'avait pu en profiter, si l'on peut utiliser le passé dans ce cas puisqu'en fait c'était dans le présent. Ou le futur ? Bref). Il décide d'envoyer un message à Arthur lui proposant un rendez-vous, mais souhaite protéger sa correspondance.
Ainsi, il chiffre le message caractère par caractère. L'alphabet utilisé
contient 64 caractères :
abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 .
La clé est une séquence de nombres compris entre 0 et 63.
Pour chiffrer : JeCode avec la clé 12, 45, 23, il transforme le message
en séquence de nombres, selon la place des caractères dans l'alphabet de
chiffrement :
J e C o d e
35 04 28 14 03 04
Puis il réalise un ou exclusif entre les nombres du message et la clé, en répétant la clé si nécessaire :
35 04 28 14 03 04
XOR 12 45 23 12 45 23
-----------------
= 47 41 11 02 46 19
Enfin, il retransforme les nombres obtenus en lettres de l'alphabet :
47 41 11 02 46 19
V P l c U t
Une fois chiffré, le message JeCode devient donc VPlcUt, ce qui est
indéchiffrable pour qui ne connaît pas la clef. Heureusement, il a déjà
convenu d'une clef avec Arthur.
Afin de pouvoir retrouver la clé, sans toutefois la laisser trop en évidence, une idée leur est venue lors d'une soirée bien arrosée au Gargle Blaster Pan-Galactique : noyer la clé utile au milieu de séquences de nombres sans signification.
Précisément, les valeurs à utiliser comme clé sont les seules valeurs, prises dans l'ordre des séquences, qui n'apparaissent qu'une seule fois (une équipe de chercheurs a tenté de déterminer comment le Gargle Blaster Pan-Galactique pouvait faire naître des idées aussi brillantes dans des cerveaux finalement si ordinaires, pour finalement conclure, après plusieurs années, qu'ils auraient eu une vie plus intéressante dans un autre domaine d'activité avant de tous démissionner pour devenir comptables).
Par exemple, si les séquences étaient :
12 03 15 15 24
13 16 22 24 22
03 13 45 16 23
Alors la clé serait constituée des trois nombres 12, 45, 23.
Les séquences réelles convenues entre Ford et Arthur sont plus longues et plus nombreuses. Il y a 100 séquences de 20 nombres :
53,51,51,04,61,10,19,42,39,25,52,32,09,47,15,26,39,18,57,33
30,22,25,04,58,03,03,45,04,61,51,20,11,26,57,14,23,45,22,04
13,29,10,42,49,29,40,29,43,62,40,37,12,11,23,45,19,46,32,12
06,50,08,45,51,47,03,23,61,43,52,49,22,36,11,05,39,36,46,47
53,58,52,63,32,62,46,26,57,14,36,02,32,58,33,03,49,11,11,33
03,33,61,37,45,51,21,02,18,42,46,45,53,34,12,46,23,14,05,62
22,05,42,42,26,53,45,34,46,46,34,21,36,29,02,34,11,49,46,33
42,04,62,04,23,11,26,57,59,51,25,02,53,42,52,37,47,40,23,37
29,23,09,56,35,32,14,33,42,62,53,42,57,22,52,36,21,23,32,49
04,05,09,33,22,61,09,26,49,04,15,03,62,61,20,52,11,56,10,25
04,22,36,45,26,12,14,25,37,51,22,36,58,20,59,61,56,36,14,05
04,42,05,51,62,42,19,61,25,05,52,56,63,03,39,35,02,15,34,45
53,33,34,25,26,47,19,36,63,05,46,45,22,53,02,62,03,29,43,39
01,49,25,20,53,45,32,21,32,56,61,57,10,37,58,14,56,51,53,19
23,47,34,14,54,10,57,39,25,37,47,63,33,57,02,18,34,54,47,22
29,21,47,03,26,26,51,62,46,37,39,39,56,02,19,35,14,15,43,49
49,32,25,51,04,33,61,46,43,02,57,11,10,12,19,12,46,03,57,29
04,47,56,14,61,14,05,22,56,51,14,34,53,56,23,63,23,02,35,36
03,09,15,14,02,46,40,23,51,36,15,59,09,33,33,53,35,43,14,21
47,32,61,11,51,14,11,57,43,14,15,36,45,09,10,62,57,05,14,15
04,29,56,63,42,49,45,39,36,23,52,05,52,33,59,62,34,21,39,14
61,29,18,12,34,32,25,43,05,59,49,20,40,53,03,54,61,25,36,14
22,62,21,39,02,54,21,39,56,09,05,49,59,62,54,20,04,47,05,49
53,58,33,47,12,10,57,21,09,03,54,35,04,05,14,22,05,33,11,43
62,34,04,53,26,42,63,52,02,14,51,49,57,49,54,18,09,11,09,33
54,37,32,15,49,51,56,40,18,22,54,52,29,12,19,26,61,57,47,14
53,49,11,33,19,14,37,21,53,11,14,46,49,25,32,04,61,32,32,40
31,32,25,03,56,12,09,47,53,05,59,26,61,19,15,03,12,33,61,22
42,51,10,19,25,63,14,19,62,47,35,09,40,02,19,62,34,33,33,45
27,47,35,61,10,58,39,18,11,52,49,20,19,04,21,25,43,32,43,37
04,63,22,10,39,18,47,46,59,19,23,62,42,25,56,22,43,05,25,09
04,32,56,10,03,26,57,42,61,42,25,32,22,56,52,09,09,34,39,14
10,18,25,23,42,15,54,09,15,09,18,11,14,12,03,35,11,25,14,18
62,34,14,57,02,19,22,15,43,59,05,11,19,40,26,36,36,23,49,19
00,45,23,18,21,21,09,39,23,53,59,09,51,63,26,26,40,59,57,15
02,45,36,61,02,14,43,25,40,22,42,53,54,39,39,18,34,20,36,19
21,21,56,47,14,52,42,32,20,54,37,53,11,52,12,34,32,33,33,45
61,62,34,26,04,52,03,49,12,02,52,36,15,62,15,10,52,39,54,21
62,09,56,21,21,26,05,61,63,04,02,12,32,59,52,22,35,52,45,34
03,05,09,37,21,26,43,43,39,46,54,03,03,19,49,54,53,43,12,03
60,61,57,39,09,33,15,22,58,46,36,52,32,32,21,61,57,57,33,25
45,20,18,51,22,25,37,26,33,05,42,05,32,09,57,10,18,37,36,11
58,23,58,43,53,26,46,11,46,63,53,42,10,58,53,54,58,37,39,46
14,25,14,61,52,39,43,54,57,46,58,46,59,34,02,43,26,29,52,35
35,58,37,43,02,56,11,42,62,34,04,62,36,52,36,12,14,26,58,05
35,62,29,05,18,63,53,05,32,04,46,33,25,15,36,54,49,43,42,11
19,05,26,51,43,45,43,49,61,11,39,39,33,63,02,22,09,25,12,45
63,25,49,32,49,51,18,36,03,19,47,14,11,37,20,57,33,34,21,47
28,11,09,58,11,54,26,12,43,54,25,52,15,42,46,58,34,04,57,05
02,03,62,35,21,56,46,02,43,32,10,61,15,42,19,19,39,23,56,37
49,18,56,03,45,26,37,22,14,34,47,52,46,05,39,10,19,21,36,14
22,33,39,03,52,10,39,58,36,47,14,57,46,59,45,23,53,21,21,09
49,33,10,22,57,57,11,09,29,52,23,40,25,46,04,12,33,53,34,34
29,39,35,34,18,56,47,52,36,29,53,25,46,49,10,56,52,32,49,43
07,49,53,63,14,15,37,56,52,21,47,56,02,42,37,43,39,26,18,05
63,11,37,21,32,19,26,33,23,52,04,57,33,54,32,32,29,46,21,15
22,10,52,10,18,51,12,56,04,14,59,15,20,12,46,33,36,59,03,36
44,10,51,12,57,10,09,25,14,32,12,34,20,58,04,59,54,32,29,23
22,21,57,04,37,25,26,39,05,18,39,56,26,32,63,34,33,39,18,57
49,32,29,14,39,26,11,19,53,34,14,23,52,35,11,03,47,56,05,58
03,47,52,47,49,23,23,32,56,61,47,09,02,11,56,14,05,37,11,43
04,05,25,47,19,56,43,52,19,57,61,15,26,05,29,20,43,05,10,42
36,49,10,14,25,51,49,58,26,37,21,62,29,35,36,52,54,54,21,21
29,20,49,05,63,09,63,39,34,03,52,33,61,54,15,45,22,47,37,43
49,61,43,34,11,33,32,49,42,51,14,05,54,46,20,39,56,63,34,53
22,58,33,18,12,35,45,26,45,34,19,39,12,40,29,11,57,26,57,62
18,49,57,53,56,37,52,20,57,43,21,04,04,03,53,63,62,03,15,45
17,18,02,04,21,45,29,26,58,26,03,49,43,02,63,61,62,20,49,29
02,35,47,18,54,42,52,57,04,40,04,57,53,57,43,59,03,56,46,37
41,20,02,63,19,34,18,15,18,02,40,62,05,61,33,59,26,42,09,36
47,19,20,54,10,11,09,52,09,42,32,29,05,61,56,57,15,37,10,33
34,37,45,39,51,40,36,49,39,22,61,33,39,46,56,57,21,20,58,46
18,21,20,22,26,21,03,22,62,05,46,39,52,04,34,23,49,51,49,53
35,39,11,09,10,40,21,49,63,14,33,51,09,61,15,10,37,11,53,36
56,53,47,58,51,26,37,56,37,56,57,19,53,52,14,18,59,59,23,49
55,29,49,53,63,58,56,34,23,03,14,49,15,12,37,19,46,34,32,32
45,34,58,45,04,22,47,12,23,12,09,25,09,21,10,36,57,61,43,46
53,49,23,49,12,09,26,15,42,19,39,19,20,10,04,22,09,09,14,37
32,58,33,39,34,14,22,51,26,39,42,15,03,42,63,58,14,04,04,18
25,33,14,43,39,40,33,40,40,62,54,15,29,61,59,61,04,54,21,32
19,22,33,25,03,29,40,09,57,40,22,03,59,23,20,32,15,26,09,54
22,03,43,39,02,23,15,10,03,61,53,05,18,36,23,49,15,53,35,36
53,47,61,49,12,36,51,09,61,52,25,52,04,12,11,49,23,39,04,54
24,51,10,33,23,14,04,39,09,62,23,20,14,54,25,53,19,23,54,33
10,45,18,33,12,35,51,23,43,63,52,29,54,02,46,45,33,12,18,53
39,58,05,43,04,12,03,12,53,26,29,62,02,52,22,09,61,54,43,43
35,04,39,21,33,51,43,53,05,11,10,35,34,61,04,09,34,63,39,33
48,35,09,43,47,05,62,25,40,21,15,25,29,22,15,15,29,37,54,26
22,52,53,05,51,59,49,43,42,34,23,09,37,26,53,58,14,54,09,29
45,18,62,32,56,09,42,56,25,46,14,59,62,57,18,11,22,56,49,49
32,43,18,32,49,47,19,62,26,11,63,42,46,49,04,05,40,39,36,63
34,61,25,05,22,39,12,61,58,21,43,09,63,47,47,34,49,56,62,54
38,16,19,34,04,23,52,21,63,57,40,51,58,51,39,57,10,61,58,34
36,53,09,18,56,59,05,33,40,22,22,53,39,62,32,29,49,54,02,43
63,02,42,18,26,57,61,58,43,33,45,02,29,61,62,19,14,12,12,26
29,53,09,14,26,54,20,36,25,22,11,33,61,52,29,12,39,09,43,45
02,10,29,03,43,58,29,11,51,14,47,11,05,33,03,42,22,37,34,33
22,22,21,63,63,42,39,14,19,61,37,58,25,10,56,57,49,58,49,52
22,47,53,18,12,52,46,12,35,34,21,53,54,42,23,21,23,56,14,05
49,18,23,11,62,23,14,15,09,14,26,33,46,46,49,05,39,56,53,11
Le message chiffré qu'envoie Ford est :
1jlXmyHooOo5Sfxrq7TyinEG2bHU67R.sv 3AJICAavmdbHjeWCpHvx0CHRyACcn
Aidez Arthur à trouver l'heure du rendez-vous (donnez la réponse sous la forme HH:MM).
