Guinea Road GroMARK Cipher Cache Mystery Cache
Guinea Road GroMARK Cipher Cache
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Difficulty:
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Terrain:
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Size: 
(regular)
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As the name implies, this cache is located near Guinea Road in Berwick, Maine. The cache is not at the posted coordinates, but is approximately a half mile from that point. The coordinates are the recommended parking area for the cache. As an alternate, when school is not in session, park at 43 17.042N 070 49.146 W for a smoother and more water/ice free trail to the cache.
A GroMARK cipher (See: (visit link) 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 it 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:
Primer 31416
AWXTO XIIRZ GMYVM WAMTF MWFFG DVYEI LHXLG VLJND IKMVX FDUJB
OCLXM DRLBW KZOGZ SBFTC LHCGM SKMPO LDFHB DMELO ILAXB NOGKT
TTATC MJRMB XDKUW MQIZF ISHXX
Last key number 2
Note that I have provided you with both the five digit key primer (31416) and the key digit (2) for the final letter as a check of your arithmetic.
Additional Notes: The initial finders of this cache have told me that the cipher coordinates are off by +50 feet. Once you have solved the cipher, add 0.006 to the North coordinate and 0.014 to the West coordinate for a more accurate cache position.
Feel free to send me E-mails with questions concerning this cipher type. For those who want it, I have a tutorial on decryption of a GroMARK in both MS Word and MS Powerpoint format. I encourage you all to work in teams to solve the cipher.
Finally, in order to log a find you must not only find the cache but also E-mail me proof that you have actually solved the cipher (i.e., Finding the cache by an exhaustive search in the area of the parking coordinates will not count). Log your find and then E-mail me either the key to the mixed alphabet or a piece of the plain text of the decrypted message. Logs, which are not followed by an E-mail message, will be deleted after 1 week.
I would like to thank fellow geocacher and ACA member ‘The Rat’ for generously allowing me to use his excellent write up on the Gromark cipher as part of this cache log.
Additional Hints
(Decrypt)
Sbe gur pelcgbtenz: Cebonoyr jbeq ‘pbbeqvangrf’
Sbe gur pnpur: Uvag rapvcurerq va pelcgbtenz
Treasures
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