This consists of counting how many times each letter appears. Natural English text has a very distinct distribution that can be used to help crack codes.
From the analysis the letter E is the most common, and appears almost 13% of the time, whereas Z appears far less than 1% of the time.
Application of the Caesar cipher does not change these letter frequencies, it simply shifts them along a bit (for a shift of 1, the most frequent ciphertext letter becomes F).
A code-breaker just has to find the shift that causes the ciphertext frequencies to match up closely with the natural English frequencies, then decrypt the text using that shift.
This method can be used to easily break Caesar ciphers by hand.
For a method that works well on computers, we need a way of figuring out which of the 25 possible decryptions looks the most like English text.
For that we need to use quadgram statistics as a fitness measure. This technique is used to characterize text by adding up the likelihoods of all length 4 blocks of ciphertext.
A high number means the text is very similar to English, a low number means it is not.
The key (or shift) that results in a decryption with the highest likelihood of being English text is most probably the correct key.
Of course, the more ciphertext you have, the more likely this is to be true (this is the case for all statistical measures, including the frequency approach above).
So, the method used is to take the ciphertext, try decrypting it with each key, then see which decryption looks the best.
This simplistic method of cryptanalysis only works on very simple ciphers such as the Caesar cipher and the rail fence cipher, even slightly more complex ciphers can have far too many keys to check all of them.
How to decipher Caesar without knowing the shift?
The easiest keyless/shiftless method consist in testing all shifts, if the alphabet has 26 letters, it takes only 25 tries.
History and usage
The Caesar cipher is one of the oldest forms of cryptography in recorded history. The cipher itself is named after the famous Roman Emperor and General Julius Caesar.
Julius used the cipher with a shift of 3 to encode military messages to his commanders.
After his death, 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.
Though there were more complex codes back then, the cipher was favored by the emperor due to its simplicity.
Also, 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.
The only thing that enforced this assumption of security was the lack of evidence that any methods for solving substitution ciphers existed at the time.
However, centuries after Caesar’s assassination, the first instances of frequency analysis appeared in the middle east.
Despite its growing ineffectiveness in the modern era, the cipher would nonetheless remain in use by various groups for less vital communications such as by Jewish groups to encrypt the name of god on the mezuzah.
And later by people wishing to exchange messages in plain view by posting encoded passages in newspapers.
The last major use of this cipher for warfare was by imperial Russian forces in the first world war due to the common solders struggling to understand more complex encryption methods.
A choice that was found to be a failure as contemporary German and Austrian code-breakers were easily able to decipher any messages sent in the code.
A more recent usage of this cipher was in 2006 when a Sicilian mob boss named Bernardo Provenzano was captured by the police due to his usage of an altered version of the Caesar cipher where letters were replaced by numbers after their shift.
Likewise, in 2011, a British counter-terrorist operation foiled a planned airplane bombing due to the usage of an easily breakable Caesar cipher by the perpetrators in their internet communications.
Despite its lack of security in helping to send confidential communications by itself the Caesar cipher still has several applications today in a variety of fields.
This is due to its versatility in acting as both a simple code for education and fun and as a building block for more complex encryptions.
For instance, the ROT13 system is a special case of the Caesar cipher that operates on a shift of 13.
ROT13 system started in the 1980s with the jokes newsgroup. Today is commonly used on online forums to hide, punchlines, plot points, and puzzle solutions in discussions to prevent spoilers.
In addition, Caesar ciphers are commonly used in children’s decoder rings to create simple codes and puzzles to teach cryptanalysis.
Particularly, decoder rings themselves are a modern version of the Caesar wheel, an early decryption tool used to break the Caesar cipher.
Though the Caesar cipher is easy to break for even the most beginner modern cryptanalyst, forms a key part of the much more difficult to break Vigenere cipher.
A cipher which took nearly 3 centuries to break and operates on interweaving different alphabets with several Caesar ciphers.