# Caesar Cipher

The Caesar cipher (also called a Caesar shift or rot-n cipher) is a encryption method that involves shifting the alphabet forward by a certain number of positions.

## Puzzle Application

The way that a Caesar cipher functions, is that a simple substitution cipher is applied, with the encrypted alphabet simply being the regular alphabet shifted forward a certain number of places, and wrapping around. For example, a shift of 13, or ROT13 (the most common type, and the only one where encoded and decoded letters pair up exactly), would look like this:

```ABCDEFGHIJKLMNOPQRSTUVWXYZ                                                             NOPQRSTUVWXYZABCDEFGHIJKLM
```

Using this, a message like "THE ANSWER TO THIS PUZZLE IS..." would be encoded like the following:

```THE ANSWER TO THIS PUZZLE IS...                                                                                   GUR NAFJRE GB GUVF CHMMYR VF...
```

Notice that in this cipher, A=N, but N=A. All other letters are similarly paired up with each other. With any shift other than 26 (which just wraps around to 0) and 13, this pairing does not happen, and the decryption becomes slightly more involved, although still not much more as long as the order of the plaintext alphabet and cipher alphabet are remembered.

In puzzle hunts, Caesar ciphers of variable shift value are often used to encode multiple streams of information within the same puzzle (and those values can then be used as a further aspect of the puzzle, possible as indexes). However, due to being relatively easy-to-decrypt, most puzzles will not involve a Caesar cipher and nothing else, and will often incorporate multiple encryption methods or addition solve path elements to create a more comprehensive (and difficult) puzzle.

Another method for making a Caesar cipher more difficult to solve is to apply different ciphers to each word in the sentence. This method forces the solver to realize that this cipher is in use, rather than trying to solve it as a cryptogram, as the sentence as a whole won't have consistent encryption for each letter. However, if the solver is not aware of caesar ciphers, it should be hinted at in another way, such as through flavortext.

Caesar ciphers are also often used outside of puzzles themselves, as the ROT13 cipher an easy way to encode hints for puzzles, and are actively used as the primary hint obfuscation method for geocaching. There have also been times where these two fields have overlapped, and Caesar ciphers have been used as hints in geocaching-themed puzzles.

## Background

The name "Caesar cipher" refers to the code's use by Julius and Augustus Caesar in their private correspondence, particularly with messages relating to military operations. The intention was reportedly that between a low literacy rate amongst their enemies and the lack of awareness of the code, it would be functionally unbreakable by those who may intercept them. This origin does make it one of the earliest uses of a basic substitution cipher (although predated by the Atbash cipher), and it has since been used by many other parties (with varying success).

Modern users include:

• Cereal box toy manufacturers ("Decoder Rings" tend to use caesar ciphers)
• Fugitives (Bernardo Provenzano was caught after his messages were decoded)
• Secret lovers (Classified ads were sometimes used to send secret messages)
• The Russian military (Used during WW1 as a replacement for more complicated codes)

However, due to the relative ease of decryption for the Caesar cipher (at least now that it's well-known), it has quickly fallen out of fashion amongst people actively wishing to conceal their messages. Today, there are multiple tools that can be used to easily run through every possible shift value at once, providing all results simultaneously so that the user can quickly determine both the message and shift amount.

## Strategy

### Identification

Identification of a Caesar-encrypted piece of text isn't too difficult, as long as a solver is well-aware of how cryptograms work. In fact, if they spot something that looks like it may be a cryptogram (spacing that seems normal for a regular sentence, presence of punctuation in normal spots, etc.), it may be wise to check if it's a piece of ciphertext. Besides, that, a puzzle's flavortext and title hinting towards a caesar cipher being used somewhere in a puzzle is somewhat common. Words to look out for include the names "Julius" and "Caesar" (along with other people named Julius X or X Caesar) as well as the words "Shift" and "Cycle" (along with any synonyms).

### Decryption

The easiest way to approach a caesar cipher, particularly if the shift length is unknown, is to run it through a rot-n solver. This has the added benefit of checking whether a piece of text has actually been caesar-shifted, or if it's a full substitution cryptogram (or something more difficult).

If this is done and none of the shifts result in a complete sentence, check if some of them result in individual words. It may be the case that each word was encrypted differently, requiring multiple single-word decryptions rather than one big one.

## Notable Examples

• The Comedy Planet, Featuring Sid! (MITMH 2009) (web) - Click to revealUses both a unique pair of clues for using Caesar cipher (Sid Caesar being a famous comedian, and the planet in question being "Cynarg", which is a Caesar shift of the word "Planet"), and it utilizes pronunciation of Caesar-shifted words, forcing solvers to try and determine how to spell nonsense words.
• Czar Cycle (MITMH 2013) (web) - Click to revealUses a Caesar cipher in non-Latin alphabets (specifically Greek and Cyrillic).