Le chiffre indéchiffrable is a French phrase meaning
“the indecipherable cipher”. The phrase was applied to
a cryptographic method developed in the 19th century by Blaise
Vigenere and is now referred to simply as the Vigenere cipher. The
cipher was considered unbreakable without knowledge of the keyword.
Prior to the Vigenere cipher, the most common ciphers were the
monoalphabetic ciphers such as the Caesar cipher. In these ciphers,
a letter is substituted for by another letter or symbol. Thus an
“C” might be represented by an “E”.
Monoalphabetic ciphers are relatively easy to break by letter
frequency analysis. In the English language, all letters are not
used with the same frequency. In sufficiently long English
passages, “E”, “T”, and “A”
appear as 12.7%, 9.1% and 8.2% of all the letters. The less common
letters “X”, “Q”, and “Z” only
appear 0.15%, 0.10% and 0.07% of the time. In the 1500s Mary Queen
of Scots helped plot the overthrow of Queen Elizabeth I and
communicated with Lord Babington using a mononalphabetic cipher.
Unfortunately, the Queen’s cryptanalyst used letter frequency
and was able to decipher the strange looking symbols. Mary Queen of
Scots ended up being beheaded.
The Vigenere cipher is known as a polyalphabetic cipher. It is a
process of encrypting letters using a series using different Caesar
ciphers based on letters of a keyword. If the keyword has five
different letters then five rotating Caesar ciphers are used. Now
it is possible that the letter “C”,when encrypted, is
represented by five different letters making frequency analysis
very difficult or indéchiffrable.
The location of the cache is given below. It is encrypted using
the Vigenere cipher. If you can come up with the keyword, the
solution will easily come to you. If you cannot come up with the
keyword, will it be le chiffre indéchiffrable ?
ahz pfz ps dr r ltagp mbaahme uvtopv. moe xetal in lzwkei me t
arzi. kal cvgyx ps gsttaey ek gvrol kpv ndrv wlgmivl hny jfny tcvvx
wodrk morzi qxyo aslk tiiykxz. tci ttjhz mj evcvxvw ht rijm uiii
wbce yixklen eew mopv whbr kszga trs kayez xykle hmenaen. xyx jaxlv
bz ljgrmld vffna oii ynudmiu yleo swy ahz xitpl. d ed tmrvmu ba
mdkym ie rik bm io vrbus v pfm.