Scary Blue Question Mark: Codes and Ciphers Mystery Cache

You'll be looking for a regular sized lock-n-lock hidden in under a fallen tree.

Codes & Cryptography

Introduction

Cryptology is a complex and fascinating subject matter.  So much has been written, that it's tough to write about the subject matter here in enought detail to do it justice.  In regards to Ciphers and Geocaching, please visit I'd Rather Be Caching, Solving Puzzle Caches: Ciphers and ePeterso2's Puzzle Solving 101 - Lesson 7: Cryptography. (most of the text on within this cache description is taken from that cache - thanks ePeterso2!)

What Is a Cipher?

A cipher is a pair of methods for encrypting and decrypting messages into and from readable text to and from gibberish.

Some encrypted messages, in an attempt to make them more difficult to crack, they use a key to further hide the method of encrypting the message.

Types of Ciphers

A substitution cipher is very simple – replace every letter of the alphabet with some other letter or symbol. The key to this cipher is the mapping of one set of letters to another.

Caesar

The Caesar cipher is a cipher with an offset (key value). To encrypt a message the alphabet is shifted a set value. For example with a shift of three, the letter A would become D, B would become E, and so on when you get to Z, you loop around to A.

The Caesar cipher is often called “ROT” (short for “rotate”), and “ROT” is often followed by the offset amount. The hint in a cache description page is encrypted using a Caesar cipher with an offset of 13 (aka, ROT13). This value is convenient because the encryption and decryption methods are exactly the same – A encrypts to N, and N encrypts to A.

Cryptogram

A cryptogram is a puzzle consisting of a short quotation encrypted using a simple substitution cipher. The mapping from plaintext to ciphertext letters is random – there is no ordering to the ciphertext letters, like there is in the Caesar and Atbash ciphers. The puzzle is to figure out the mapping and reveal the quotation.

A substitution cipher doesn’t have to use just letters. Any set of symbols can be used, including:

• numbers, (A = 1, B = 2, etc. is very common),
• font symbols (other than letters and numbers),
• computer codes (such as ASCII or EBCDIC),
• pictures (like Geocoin Icons),
• barcodes (UPC symbols found on the backs of packages or code pustal and curior envelopes),
• Morse code (or -- --- .-. ... .   -.-. --- -.. .),
• semaphore flags (or clock faces),
• presses on the telephone keypad (2 is A, 22 is B, 222 is C, 3 is D, etc.),

The fundamental problem with all simple substitution ciphers is that they can be attacked using a cryptanalysis method called frequency analysis. This is just a fancy way of saying “count the number of times each letter or symbol appears in the ciphertext”. The letter that appears the most is probably E, followed closely by T, A, O, I, and N.

Complex substitution ciphers were developed to foil attempts to break the code via frequency analysis. The goal of these methods is to try to get all symbols in the ciphertext to appear with roughly the same frequency.

A polyalphabetic cipher is one in which a single ciphertext letter does not correspond to a single plaintext letter. The letter A at one point in the ciphertext may decode to a completely different letter than an A at a different point.

Tabula Recta

If the cipher wheel is one of the primary tools used in substitution ciphers, then the tabula recta is one of the primary tools used in polyalphabetic ciphers. A tabula recta looks like this:

Vigenère

The Vigenère cipher is one of the most common ones which uses a tabula recta. The cipher requires the sender and receiver to agree upon a word to use as they cipher key. For example, suppose the plaintext to be encrypted is:

ATTACKATDAWN

The sender chooses a keyword and repeats it until it matches the length of the plaintext. For example, the keyword “LEMON” would give the full encryption key:

LEMONLEMONLE

Use the plaintext letters as the row and the key letters as the column. Then replace each letter in the plaintext with the corresponding cell from the tabula recta. For the first letter in this example, the letter at row A column L is A. Next, the letter at row T column E is X. After that, the letter at row T column M is F. The complete ciphertext is then:

LXFOPVEFRNHR

Autokey

The autokey cipher is sometimes called the "Vigenère autokey" cipher to distinguish it from the "Vigenère" cipher. The autokey cipher is similar to the Vigenère cipher, but with a different method of constructing the key that makes the encryption method much stronger. Instead of repeating the key word over and over again, the key starts with the keyword followed by the plaintext message itself. So the key in the above example would be:

LEMONATTACKA

A polygraphic cipher is one that uses groups of letters instead of single letters as the basic units of encryption. For instance, AA could be replaced with QJ, AB with RU, etc. With single letters in a simple substitution cipher, there are only 26 possibilities for how each letter is encrypted, but with two-letter groups in a polygraphic cipher there are 676 possibilities. This makes such ciphers much more difficult to crack.

Polybius Square

Some common polygraphic ciphers make use of an arrangement of letters known as a Polybius square. The basic square lists the letters in order from left-to-right and top-to-bottom (I and J are treated as the same letter), like this:

Playfair is a Polybius square cipher that begins with a key word (with duplicate letters removed), then followed by all remaining letters in the alphabet in order. The letter J is not used, and the letter J in the plaintext is replaced with the letter I before encryption. For example, the Playfair square using the word "CIPHER" as the key would look like this:

See the Wikipedia page on Playfair for details of how to use the cipher.

A transposition cipher changes the position of letters in the plaintext to form the ciphertext. For instance, suppose the plaintext is:

FORTYNINETEEN

One way to encrypt it is to write the plaintext evenly divided across three lines, like so (padding it with random letters at the end to make the lines even):

FORTY
NINET
EENXX

Now read the letters down each column to create the ciphertext:

FNEOIERNNTEXYTX

Other patterns include spirals, alternating left-to-right and right-to-left rows, and more. Any pattern can work as long as the sender and receiver agree.

Book Cipher (Beale Cipher)

A book cipher uses some lengthy text as an encryption key. Common texts include dictionaries, religious books (such as the Bible), government documents (such as the Declaration of Independence), and more.

A book cipher encrypts each letter in the plaintext by referencing the same letter at some position in the key document. To encrypt a plaintext letter, replace it with a set of numbers that can be used to locate the letter in the document. A triplet of numbers could indicate the page number, line number, and word number in the line.

Cryptanalysis

The ultimate goal of cryptanalysis is to reveal the hidden message. This means determining both the cipher and the key.

Experiment

Don’t be afraid to try anything. Most of your experiments will not yield the plaintext message, so don’t be discouraged just because one attempt didn’t work.

Guess the Cipher and the Key

The cache itself may have hints as to what the cipher and key are. In fact, it may tell you explicitly one or both of those things. If not, it might strongly suggest through hints as to what they might be.

Look for Pigpen, Morse code, and semaphore symbols – they’re easy to identify and tell you right away what the cipher and key are.

Guess the Plaintext

If you haven't completed GC1YM11 (Strategy and Objectives), review the tips there: Begin with the end in mind. You ultimately want a set of coordinates, so look for ways in which that could be expressed. If you see two rows of letters, the first row might be the latitude and the second row the longitude. Try replacing the first or last letters of a row with “NORTH” or “WEST”. Look for “NORTH FORTY”, “NORTH FOUR ZERO, or “FORTY DEGREES” in the first row … if the message was encrypted using a simple substitution cipher, that may give you enough information to unlock much of the rest of the key.

Look for Fragments of Plaintext

It is rare for cryptanalysis methods to yield the entire key and plaintext message all at once. Cryptanalysis usually involves piecing together fragments of the plaintext message slowly. Look for words or pieces of words to appear as you experiment. And try to find words or phrases you expect to appear, based upon what you think the message contains.

Look for Words and Sentences

If the letters in the ciphertext appear look like badly spelled words, they probably are. A ciphertext with spaces, punctuation, and capitalization are all indicators that the message has been encrypted using a simple substitution cipher. Look for common words, such as single letter words like “A” and “I”, or three-letter words like “THE” and “AND”. Try replacing the letters in your ciphertext with those letters and see if any other words start to appear.

With that in mind, one way a puzzle creator might make a cipher for difficult to solve is to remove the spaces and punctuation and to capitalize all of the letters.

Perform a Frequency Analysis

Count the number of times each letter appears in the ciphertext. The most common letters in the English language are (in order of decreasing frequency):

E T A O I N S H R D L U

Replace the most common ciphertext letter with E, the next most common ciphertext letter with T, the next with A, and so forth. If that doesn’t work, try shuffling the T, A, O, I, and N letters around until the cleartext starts to make things that look like real words.

(Those letters should look familiar to you – they’re probably the ones you guess first when playing Hangman. And if they’re not, they should be.)

Look for Pairs of Letters

If the ciphertext consists of pairs of letters, that’s a strong sign it might be encrypted with Playfair. Also, if none of the pairs of letters contains a duplicate letter (such as “EE” or “QQ”), that’s another sign it might be using Playfair, since that cipher never generates a pair of duplicate letters. If the ciphertext is not grouped into pairs, try grouping it into pairs to see if there are any duplicate letter pairs or not.

Look for Groups of Numbers

If your ciphertext is made up of pairs or triplets of numbers, it is possibly a book cipher, especially if the ciphertext is related to a reference document in some way. Look for clues in the puzzle as to what the reference document might be.

Resources

Wikipedia's cryptography portal is an excellent starting point for looking up cipher information. Wikipedia also has an entire category devoted to classical ciphers.

There are a number of free online tools available to help you analyze and use all of these ciphers and more.

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