Svenska
1586 publicerade Vigenère boken "Traicté des Chiffres ou Secrètes Manières d'Escrire", där han bland annat beskrev två typer av autokey-chiffer, det ena använder den krypterade texten som nyckel, den andra den dekrypterade texten. För att starta krypteringen användes en på förhand överenskommen bokstav. I kombination med tio hemliga alfabet var antagligen chiffret någorlunda säkert för sin tid. Ifall de alfabet som används är kända finns det bara 26 möjliga nycklar (för de flesta europeiska språk, svenskan har 29) och då är kryptosystemet lika svagt som ett shift-chiffer av caesartyp med bara några tiotal möjliga nycklar.
Uppvärmning
Den moderna versionen av Vigenères autokey använder en tabula recta baserad på standardalfabetet (a-z, eller på svenska a-ö, i bokstavsordning) och ett kodord på mer än ett tecken. I och med att kodtabellen är helt känd, är kryptot enkelt att knäcka om man kan gissa korrekt klartext och längden på nyckelordet. För mer detaljer se Wikipedia.
Modifieringar
För att förstärka kryptosystemet så kan man använda andra alfabet, något som Vigenère tydligen gjorde, men jag hittade inte hur han gjort det, så jag gjorde en variant på det moderna autokey-kryptot där kodalfabetet inte är standard.
Detaljerna för hur det fungerar (och kan knäckas) finns i "Excercises" nedan.
English
In 1586 Vigenère published the book "Traicté des Chiffres ou Secrètes Manières d'Escrire", where he, amongst other items, describes two types of autokey ciphers. One of the ciphers use the encrypted text as a key, the other one the decrypted message. To get the encryption and decryption started, he proposed using a single letter. In kombination with ten secret alphabets this was probably resonably safe for its time. However, if the alphabets are known there would only be 26 possible keys in e.g. English which makes the whole scheme as weak as a simple caesar type shift cipher.
Warmup
The current version of Vigenères autokey uses a tabula recta based on a standard alphabet (a-z in order) with a message key longer than one character (possibly much longer). Since the code table is fully known, the crypto would be easily broken if guessing a plausible piece of clear text and the length of the message key correctly. For more details see Wikipedia.
Modifications
To strengthen the cipher system you can use non standard alphabets, as Vigenère did himself. As I could not find any reliable information on how he used his secret alphabets I created a variation of the modern autokey cipher using a none standard code alphabet.
The details on how to use and break this cipher follows below.
Exercises
The exercices are only available in English as there would be a lot of text to translate.
Encryption
How to encrypt a number using the digits as the alhabet:
- Base alphabet: 0123456789
- Coding alphabet: 2718459036
- Message key: 314
- Message: 473893812220887
Code table:
. 0123456789
0 2718459036
1 7184590362
2 1845903627
3 8459036271
4 4590362718
5 5903627184
6 9036271845
7 0362718459
8 3627184590
9 6271845903
The top row is the clear text, the left column is the key and the character inside the table is the encryption. Notice that there is a symmetry in that code and clear text are interchangeable.
Ecnrypting the text:
473833812220887 message
314473893812220 key (message key followed by message)
030119925281220 encrypted text
Breaking the cipher
To break this cipher you will need to know:
- The encrypted message.
- The basic alphabet.
- A fairly long crib (known piece of plain text).
The crib must be long enough to ecnrypt itself at least partially, i.e. longer than the message key. The crib may have unknown letters inside itself, as long as at least some letters overlap.
The message we are going to break is
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
and we know it encodes a geocache coordinate in the vincinity of Ryd, Linköping.
Through some incredible luck, or after trying many other options, this is a promosing step in cracking the message, with a message key lenght of three:
northfiveeight............................................................................................
...northfiveeight.........................................................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
From this we can reconstruct part of the decoding table and fill in the diagonals:
. abcdefghijklmnopqrstuvwxyz abcdefghijklmnopqrstuvwxyz
a .......................... .fgi..q..wxys........eb...
b .......................... fgi..q..wxys........eb....
c .......................... gi..q..wxys........eb....f
d .......................... i..q..wxys........eb....fg
e ......xy.................. ..q..wxys........eb....fgi
f ....w..................... .q..wxys........eb....fgi.
g .......................... q..wxys........eb....fgi..
h .....................g.... ..wxys........eb....fgi..q
i ....s..............f...... .wxys........eb....fgi..q.
j .......................... wxys........eb....fgi..q..
k .......................... xys........eb....fgi..q..w
l .......................... ys........eb....fgi..q..wx
m .......................... s........eb....fgi..q..wxy
n ...................q...... ........eb....fgi..q..wxys
o .......e.................. .......eb....fgi..q..wxys.
p .......................... ......eb....fgi..q..wxys..
q .......................... .....eb....fgi..q..wxys...
r .....b.................... ....eb....fgi..q..wxys....
s .......................... ...eb....fgi..q..wxys.....
t ........f................. ..eb....fgi..q..wxys......
u .......................... .eb....fgi..q..wxys.......
v ........i................. eb....fgi..q..wxys........
w .......................... b....fgi..q..wxys........e
x .......................... ....fgi..q..wxys........eb
y .......................... ...fgi..q..wxys........eb.
z .......................... ..fgi..q..wxys........eb..
With the partial table above we can decode a few more characters, remember that the key is the decrypted text shifted three steps. New characters are highlighted as capitals:
northfiveeightDE..........................................................................................
...northfiveeightDE.......................................................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
Just two new characters, DE, but a probable guess here would be DEGREES, so fill in that:
northfiveeightDEGREES.....................................................................................
...northfiveeightDEGREES..................................................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
Use these new characters to fill in more of the coding table:
. abcdefghijklmnopqrstuvwxyz
a .fgi..q.Vwxys.......Zeb..N
b fgi..q.Vwxys.......Zeb..N.
c gi..q.Vwxys.......Zeb..N.f
d i..q.Vwxys.......Zeb..N.fg
e ..q.Vwxys.......Zeb..N.fgi
f .q.Vwxys.......Zeb..N.fgi.
g q.Vwxys.......Zeb..N.fgi..
h .Vwxys.......Zeb..N.fgi..q
i Vwxys.......Zeb..N.fgi..q.
j wxys.......Zeb..N.fgi..q.V
k xys.......Zeb..N.fgi..q.Vw
l ys.......Zeb..N.fgi..q.Vwx
m s.......Zeb..N.fgi..q.Vwxy
n .......Zeb..N.fgi..q.Vwxys
o ......Zeb..N.fgi..q.Vwxys.
p .....Zeb..N.fgi..q.Vwxys..
q ....Zeb..N.fgi..q.Vwxys...
r ...Zeb..N.fgi..q.Vwxys....
s ..Zeb..N.fgi..q.Vwxys.....
t .Zeb..N.fgi..q.Vwxys......
u Zeb..N.fgi..q.Vwxys.......
v eb..N.fgi..q.Vwxys.......Z
w b..N.fgi..q.Vwxys.......Ze
x ..N.fgi..q.Vwxys.......Zeb
y .N.fgi..q.Vwxys.......Zeb.
z N.fgi..q.Vwxys.......Zeb..
Use this table to decode some more characters in the message, notice how we get a "runaway" effect deoding a few extra characters:
northfiveeightdegrees..O..U..O............................................................................
...northfiveeightdegrees..O..U..O.........................................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
The cache is probably at N 58 24 or N 58 25, so we fill in TWOFOUR, fill up more of the decoding table and the new message is:
northfiveeightdegreestwofourpoin.th.ee.i..e..h............................................................
...northfiveeightdegreestwofourpoin.th.ee.i..e..h.........................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
At this point you may see more or less of a pattern, let's just add a "t" for "point" and see where it gets us:
northfiveeightdegreestwofourpoinTthRee.i..e..h............................................................
...northfiveeightdegreestwofourpoinTthRee.i..e..h.........................................................
tcmqebfgwsixyfwynzvxwrdqaxvbivngufzxryjshhsexyflcqqfhsrjxroprmenwynxzvxwrywryxyeryqlsgufzxrykvetljldehqfhs
Continue like this, guessing or revealing more and more of the text, the new guess is in capitals:
northfiveeightdegreestwofourpointthreeNiNEeIGhT..n..e..a..................................................
northfiveeightdegreestwofourpointthreenineeightMInUTeSeast..e..v..e..e..t..e..h..e..i..t..e..e..o..m..u..s
northfiveeightdegreestwofourpointthreenineeightminuteseastONeFIvEdegreesthreethreepointthree.er.on.mi.ut.s
northfiveeightdegreestwofourpointthreenineeightminuteseastonefivedegreesthreethreepointthree.eroonemiNutEs
Towards the end the decoding table will be full enough to decode most of the message.
"north five eight degrees two four point three nine eight minutes east one five degrees three three point three zero one minutes" also known as "N 58 24.398 E 15 33.301"
Uppgiften / Your task
För att hitta cachen behövde du knäcka det krypterade meddelandet nedan.
To find the cache, you need to break the encrypted message below.
Intercepted message / Uppsnappat meddelande
qeydv ehqbu qesvu sbudd daekj ecbsa tauga lpklo gzmll wqqtp dveuz
wlsyq uyqvc llpmy vspxq clxbv nbifb xcjxp ekikl tdlnt igzmi hjxiz
xibqv tjpsy spjmy pgfkv euwoc xqclk je
Ledtrådar / Clues
Två ord i klartexten är kända / Two words of the clear text are known:
Geochecker
Due to mistakes, the message will not reveal the correct coordinates, so you need to use the checker to obtain the correct coordinates:
I may at a later date change the message to have the correct coordinates, but as this is an autokey message it is not a trivial task to fix without giving away information.
After decoding the hint, you can get further clues using certitude.
Ignore the coordinates listed on certitude, this is only about the decrypted hint.