To find this cache you must solve this difficult cipher. But
there's a twist. This cipher is intended to be solved by community
effort right here on the page. I invite everyone to post his or her
ideas or progress in a note. Such things as useful web links,
methods of solution, elimination of certain approaches, and so on
should all be shared. However, please don't post:
1) any portion of the true coordinates
2) any portion of the plaintext of the message (although I might
post as a hint later)
3) the primer or any of its digits (or enough information about the
other digits to give it away)
4) any information about the final cache location that would allow
someone to find it without solving the cipher
I will have to delete spoilers of this sort. I will add hints to
the cache page every few days until someone either solves it or
shows enough progress that I think it is time to let them finish
and claim FTF. The hints will be on the page, not in the logs, so
you won't be notified just by being on the watchlist. You'll have
to check the page from time to time.
A
GroMARK cipher is a hobbyist cipher. The name comes from
Gronsfeld with Mixed Alphabet and Running Key. It uses what is
called a pseudorandom key generated by mod 10 chain addition. That
sounds complicated, but it really isn't. You start with a key
number, called a primer. Let's use 4835 as an example. You keep
generating more numbers at the end by adding together the digits in
pairs, saving only the final digit if the sum is 10 or more. In
this case since 4+8=12, you discard the 1 and append 2 to the end,
so the key is now 48352. Since 8+3=11 then next digit is a 1, and
you have 483521. You continue this until you have one digit for
every letter of the message you are encrypting. Next you create a
mixed alphabet and place it directly below a regular alphabet. For
each letter to be enciphered you find it in the regular (plaintext)
alphabet, count to the right the number of characters shown by the
key number, and take the letter in the mixed (ciphertext) alphabet
below. That's the next ciphertext letter. To decrypt a message you
create the same cipher alphabet and pseudorandom key (because you
know the primer) and just reverse it. You find the ciphertext
letter in the mixed alphabet, count the right number of letters to
the left and the letter above is the plaintext letter. That's all
there is to it. Here's a simple example.
ABCDEFGHIJKLMNOPQRSTUVWXYZ -
Plain alphabet
ZQEKRCAIWPXVBHODLTSNGFMJUY - Mixed (Ciphertext) alphabet Primer
4835
MEETMEATNOONTOMORROW - plaintext message
48352187395024526978
LBIUOCWZLJNHFSTLJZFR - ciphertext
The ciphertext is
typically written in 5-letter blocks:
LBIUO CWZLJ NHFST
LJZFR
Notice that when encrypting the second R you can't go 9 spaces to
the right, so you keep counting past the Z back to the beginning.
In this case, 9 spaces to the right of R is A, and under the A is a
Z, so that is the letter used to substitute. Similarly, 8 spaces to
the right of the W is E, and under the E is R.
So how do you break one of these ciphers? When you are given a
portion of plaintext (a crib) and the primer, but not the mixed
alphabet, you recreate the numerical key string by simple addition
and try to place the crib spot by spot. If the crib is not in the
right place, you will usually get a conflict, such as two different
letters in the same place in the mixed alphabet. Whenever you don't
get a conflict you try a test decryption using the letters you
have. If t looks good, you keep it and try to fill in more letters
in the mixed alphabet. It is difficult but pretty straightforward.
Here is the secret message:
QEHHK FYKPZ FYJFV MDKXH DIQTE
NAXAF HHRYN CIFSO HHRNU XHVPT NGYBF
TFZTG CAKGB UKTHS OZXJO TCQDT OZEDJ TPSVG RAFTD DHBMV KEODY
SOYQK
GXDXI RAYDZ IKOHI ZRGOS WWJRX LIZZH ARYDH XBYLQ UJZHC SXLSW NBZNQ
W
In order to log a find you must not only find the cache but
also e-mail me the key to the mixed alphabet.
The way keys are mixed is explained on the ACA page linked above.
See the example there using the word ENIGMA, although the columns
may be taken off either in alphabetical order as shown, or simply
left to right.