Mystery Cache

Solving Puzzles IV - Codes & Ciphers (Advanced)

A cache by Bob & Brenda - Reno Send Message to Owner Message this owner
Hidden : 5/26/2013
In Nevada, United States
3 out of 5
1.5 out of 5

Size: Size: micro (micro)

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Geocache Description:

Cache is not located at the posted coordinates.

This is the fourth in a series of caches that we hope will help cachers learn some of the tricks to solving puzzle caches. Although experienced puzzle solvers can jump in at any point in the series, each successive lesson is meant to build on concepts that were demonstrated in the previous caches. The first cache in the series contains some background information on puzzle caches as well as a link to the tools we use when solving puzzle caches.

This series is not meant to be an "end all" on how to solve every puzzle cache that exists. It is only a starting point on what to look for when you see a puzzle cache. If you go through this series, you should be able to solve most puzzle caches that have a difficult of 3 stars or less. If we gave away all our secrets, then we wouldn't have anything to do but put out lamp post caches.

This series of caches contains the following caches

Cache Techniques
GC4AFC5 Hidden Information
GC4AJ0W Lists
GC4ANJW Codes & Ciphers (Basic)
GC4ANKB Codes & Ciphers (Advanced)
GC4AZHP Images & Steganography
GC4AZHW Wordplay
GC4B2YM Mathematics
GC4B2YP Logic
GC4B2ZK Final Exam


Alert: There are downloadable files in our toolbox and printable copies (PDF) of the puzzles for the caches in this series. These files are not required to solve the puzzles, although they may be useful to you for both this cache series and other puzzle caches you solve. As the cache owner, I represent that these files are safe to download although they have not been checked by Groundspeak or by the reviewer for possible malicious content. Download these files at your own risk.


The goal of decyphering puzzle is to reveal the hidden message/coordinates. The cache title, description, hints, or hidden information may give clues as to what the cipher and key are. Quickly scan the encypted message and look for things that stand out such as strange fonts or groupings of numbers. These may give you an eduated guess as to what cipher is being used. Don't be afraid to experiment with different ideas. Not every attempt at decryption will yield a plain text message.

Remember that the end goal will be a set of coordinates. Look at what is encrypted. Is there any way to decrypt the beginning word to north or three, which would be common for most caches in our area? For example, if the beginning of some encrypted text would be FGJLZ LZJWW, we could assume that some sort of substitution cipher was being used. Since we see the second encrypted word ends in ww, we could make our first hypothesis that W = E, and the second word is THREE. Since the first word ends in JLZ, we can decypher that as fgRTH--probably a good indication that the first word is NORTH.

Having said that, of course, puzzle creators will omit punctuation and spaces, and possibly add extraneous text to make it harder for you to solve.

If the information is just a single list of numbers and they don't match any of the numbering systems described below is the total number of chacters evenly divisible by 15? If so, break down the list with whatever the quotient is and each of those numbers should yield a portion of the coordinates.

As we discussed in our previous lesson, ciphers can be broken down into two basic types: substitution and transposition. We covered basic monoalphabetic ciphers where each letter of a ciphertext alphabet corresponds to one (and only one) letter in the plaintext alphabet. This lesson will cover more advanced techniques such as the polyalphabetic ciphers (where a single ciphertext letter may correspond to multiple plaintext values), polygraphic ciphers (which use groups of letters instead of single letters), and transposition ciphers.

Polyalphabetic Ciphers

Simple monoalphabetic ciphers like the ones shown in the previous lesson used a simple code wheel that could be rotated to a set position agreed upon by the sender and receiver. Once that was done, all they had to do to encrypt the message was to read the corresponding ciphertext and plaintext characters. Polyalphabetic ciphers use what is known as a tabula recta--you could think of it as multiple code wheels fitting together.

A FRENCH cryptographer Blaise de Vigenère reinvented a 16th century Italian cryptographers code that now bears his name. The Vigenère cipher is a popular cipher used by geocachers to encrypt messages, clues, and coordinates for their caches. To encode a message using the tabula recta, the first thing you need is a key--this can be a single word or phrase. For our example, we'll use the word GROUNDSPEAK as a key to encrypt the phrase "TAKE ME GEOCACHING".

Start by writing your message (omitting spaces and puctuation): TAKEMEGEOCACHING

Then use your key and repeat it until it matches the length of the message: GROUNDSPEAKGROUN

Going through your message, look at the ROW that matches the letter from the plaintext and the COLUMN that matches the key word. The intersection will be the encrypted letter. To start our sample, we look at row T and column G which gives us Z. Then we look at row A and column R, which gives us R. Repeat this through the entire message and we end up with an encypted text of ZRYYZHYTSCKIYWHT. As you can see, the letter Z decrypts to multiple letters based on what position it is in the message.

Tabula Recta

To decrypt the message, the first thing you need is the key. Hopefully, you found this using methods we've discussed in previous lessons (or in some that we'll cover later). Once you have the key, use the ROW of the key letter and the COLUMN of the encrypted text. The intersection of these will give you the decrypted letter.

Up until 1945, the US Army used a system similar to this (known as the M-94) except the letters on each line/wheel were scrambled. ic_nevadamike created four caches using a modified version of the M-94. If you'd like to try these caches they are (in ascending order of difficulty) Crypto 2, Crypto 3, Crypto 4, and Crypto. The actual message of Crypto is easy to decipher but it requires other puzzle solving skills as well to find the final cache.

Polygraphic Ciphers

Where the monoalphabetic and polyalphabetic substitution ciphers encrypted a single letter at a time, a polygraphic ciphers encrypt groups of letters together at the same time. With a simple substitution cipher, there are only 26 possibilities on how each letter can be encrypted. Using a polygraphic cipher, this number increases to 26 x 26 = 676 possibilities.

Most polygraphic ciphers use a 5 x 5 arrangement of letters known as a Polybius square. As you can see to the right, the letters are simply placed in order in 5 rows of 5. The letters I and J are treated as the same letter. If we wanted to encrypt our message "take me geocaching", we would look up each letter in our message and use the corresponding row and column. So our encypted message would be:

44 11 25 15 32 15 22 15 34 13 11 13 23 24 33 22

However, this becomes just a simple substitution cipher like we discussed in our last lesson.

1 2 3 4 5
1 a b c d e
2 f g h i/j k
3 l m n o p
4 q r s t u
5 v w x y z
The Playfair cipher is a way to overcome that and make it a true polygraphic cipher. It also uses a Polybius square along with a key word or phrase. To lay out your square for a Playfair cipher, take your key and eliminate any duplicate letters. Place those letters beginning in the top left corner of the square. Once you have used all the letters in the key, place the remaining letters of the alphabet (in order) in the rest of the squares. For example, if we used the key "Bob and Brenda", our square would look like the one on the right.

Here is our "take me geocaching" message encrypted using our table to the right: snic ig rcaeckikdf

There are four simple rules for encrypting and decrypting a message created with the Playfair cipher. This Wikipedia article does a much better job graphically showing how the rules work than what we could do here in the space provided. There are many online tools available for quickly solving a Playfair cipher once you have the key.

b o a n d
r e c f g
h i/j k l m
p q s t u
v w x y z

Transposition Ciphers

Transposition ciphers takes the letters in the plaintext message and change their position to create the ciphertext. A letters you see when view a transposition cipher are the actual letters in the message. Since you probably don't know the pattern, a bit of brute force is required if the hider didn't provide any additional information.

One of the more common transposition methods is to write your message in a square. Once again, we'll work with our "take me geocaching" plaintext. Since the phrase has 16 characters, we can easily break that up into 4 rows of 4. If the text is not evenly divisible, add random characters to the end of the plaintext until you can form a square. So in this case our square would look like the one to the right.


There are many ways we could now encrypt this message:




Any pattern can be used as long as the sender and receiever both agree to what it should be. This will be your key to solving these puzzles. If you suspect that the hider is using a transposition cipher and there aren't any clues as to the pattern, start off with a the simple column format.

  1. Count the number of characters in the message
  2. Create tables of each of the dimensions that could make up that number. For example, if there were 48 characters, you could have the following dimensions: 2 X 24, 3 X 16, 4 X 12, 6 X 8, 8 X 6, 12 X 4, 16 X 3, 24 X 2. We skip the any that give us only 1 row or column because they don't rearrange the message.
  3. Fill in each of the rows with the letters from the encrypted message
  4. Read down the columns to see if they make up a word.

Another type of transposition cipher is the rail fence. This cipher can be traced back to the ancient Greeks who used a device known as a scytale (rhymes with Italy) to encrypt messages. It consisted of a strip of parchment wrapped around a cylinder. The letters of the plaintext message would be rearranged when the ribbon was unwound. The key was the diameter of the cylinder. In a rail fence encryption, the number of rails is the key.

To encrypt a message using rail fence, first determine the number of "rails" you will use. For our example, we will use a 4 rail fence and our standard "take me geocaching" phrase. We write down the letters in a zig-zag pattern, one letter on each "rail" leaving any spaces (shown with * here). So our fence would look like this:

T     E     A
 A   M *   C C   G
  K *   G O   H N
   E     E     I

To complete the encyption, we simple read each row to arrive at the following ciphertext:


There are plenty of online sites that will allow you to quickly decode a rail fence cipher if you know the number of rails. You could also use these to brute force solve a fence rail by simply changing the number of rails on each run until you have a readable message.

Our toolbox has some Excel spreadsheets that can help you solve these transposition ciphers.

Book Ciphers

The final type of cipher we're going to discuss is the book cipher. This wasn't originally on our list, but there seems to be a few caches of this type floating around. These ciphers use either a specified book such as a popular novel, a famous document (the US Constitution) or any other written work that is relatively stable.

Text is encrypted by referencing that letter at some position within the key document. Some things to look for:

  • A single number could reference a chapter or page. Look for the first letter or word on that page/chapter.
  • Two digits could refer to a page and line or page and paragraph
  • Three digits often refers to the page, line, and word in that line


These last two lessons are by no means going to allow you go out and solve a difficulty 5 puzzle. However, they will give you the tools and some insights to at least allow you to give them a shot.

Here are a few links that you may find helpful when solving encyption puzzle caches or to practice different types of encryption:

This Cache


Additional Hints (Decrypt)

1. ynfg yrffba
2. abger cebsrffvba - guvf hfrq gb or ninvynoyr va gur cebsvyr cntr. trbpnpuvat.pbz erzbirq vg. Gur nafjre vf "argjbex ratvarref"
3. fhojnl cbjre?

Decryption Key


(letter above equals below, and vice versa)



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