where x is the value of the original letter in the alphabet’s order, n is the value of the shift and 26 is the number of letters in the alphabet. In this formula n is positive during encryption and negative during decryption. A shift of 0 or 26 would obviously result in no encryption at all as the letters would simply shift to their original positions. Additionally, shifting twice equates to no extra security as shift values can be added together and be used as n.
Keyed Caesar Cipher: Due to its versatility, and ability to work with any alphabet, history has seen several variants of the Caesar cipher arise. In a slightly more secure variant, known as a keyed Caesar cipher, one writes a passphrase out, ignoring duplicate letters, then writes the remaining letters of the alphabet. Finally, all letters are shifted like in an ordinary Caesar cipher. This is more secure due to the more complex pattern of the letters.
A Keyed Caesar Cipher tool is available here.
Below is an example using the key BOXENTRIQ and a right shift of 3.
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Breaking the Cipher: The Caesar cipher, while reasonably effective in its Roman heyday, is now one of the easiest cryptographic codes to break. Breaking a Caesar cipher usually follows one of three scenarios depending on the amount of knowledge the attacking cryptanalyst has:
The cryptanalyst knows that the cipher is a Caesar cipher.
The cryptanalyst knows that the cipher is substitution cipher, but not a Caesar cipher.
The cryptanalyst is totally unaware of the kind of cipher they are faced with.
In all of the proposed scenarios, assuming that the cipher relies on a simple numerical shift, the code can very easily be cracked using a brute force attack (trying all possible shifts and determining which one works). In the case of a keyed Caesar cipher, matters become considerably more difficult, though still relatively easy to break. In this instance there are two primary methods of breaking the code:
One method is to create a list of words that could be the possible key phrase and attempt a brute force attack.
Another method is to employ frequency analysis. This is the standard method for breaking any substitution cipher. The most common letters in the English alphabet are E and T, so they are likely represented by the most common letters in the cipher.
History: The Caesar cipher is one of the oldest forms of cryptography in recorded history, with instances stretching back long before it was first named. Nonetheless, the cipher finds itself named for, and consistently associated with, the famous Roman Emperor and General Julius Caesar. It was Caesar who reportedly used the cipher with a shift of three to encode military messages to his commanders while on a campaign. After his assassination, Caesar's nephew Augustus carried on his uncle’s usage of the cipher to protect his correspondence but changed his messages encryption to a right shift of one. Despite the presence of more complex codes, the cipher was likely favored by the emperors due to its simplicity. The cipher was likely effective due to the illiteracy of many of those who would intercept them and a common misconception that the messages were written in a mysterious foreign language. Enforcing this assumption of security is the lack of evidence that any methods for solving substitution ciphers existed at the time, with the first instances of frequency analysis appearing in the middle east centuries after Caesar's death.
