Decrypt Caesar Cipher Without Key
Janise Knollman
The Caesar cipher, also known as a shift cipher, Caesar's code, or Caesar shift is one of the oldest and most famous ciphers in history. While being deceptively simple, it has been used historically for important secrets and is still popular among puzzlers.
The easiest way to understand the Caesar cipher is to think of cycling the position of the letters. In a Caesar cipher with a shift of 3, A becomes D, B becomes E, C becomes F, etc. When reaching the end of the alphabet it cycles around, so X becomes A, Y becomes B and Z becomes C.
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.
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:
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:
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. Nevertheless, the last major use of the cipher for warfare was by imperial Russian forces in the first world war due to the common soldiers 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.
Regardless of being phased out well over a century ago, the Caesar cipher has not fallen out of use entirely. In 2006 a Sicilian mob boss named Bernardo Provenzano was captured by 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 airline 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:
Though advanced for its time the Caesar cipher is now one of the easiest codes to break. Using the information found here those stuck on any variant of the cipher will find that solving it is often as simple as a shift of one or two or three or perhaps 13! After all, no matter the complexity a true puzzler will find a solution.
See also: Code-Breaking overview Adfgvx cipher Adfgx cipher Affine cipher Atbash cipher Baconian cipher Beaufort cipher Bifid cipher Columnar transposition Cryptogram Double transposition Enigma machine Four-square cipher Gronsfeld cipher Keyed caesar cipher One-time pad Pigpen cipher Playfair cipher Rail fence cipher Rot13 Route transposition Trifid cipher Variant beaufort cipher Vigenere cipher
I have no clue how to do this can someone help me start and guide me write a program to decrypt the caesar encrypted text. I have a program which decrypts and prints all 26 shifts, but I want to print only one right one, thank you!
Frequency AnalysisIf your text is decently long, then it can be done by frequency analysis. Every english alphabet has certain frequency of occurring (for an example here). You know your most frequent letter in the encrypted text is likely to be E, T or A. You can substitute one letter at a time and see if 'words' skeleton starts to emerge. This will be a trial and error until the text is decrypted. Alternatively you can match the letters, in your encrypted text, with that of plain english letters depending upon the 'nearest' frequency of match.Your task will become harder if there are no spaces between the words (but I guess since you are dealing with simple Caesar cipher, that possibility is less).
If you do not have long enough text or a text whose pattern of letter usage is skewed w.r.t. 'usual' english, then I would have done as below. Please note this method will mean that you have access to English dictionary for a look up.
You can feed all the words (that are generated for each shift) into an english dictionary lookup. You can give +1 score if a match is found for every word or -1 otherwise. The correct shift will have maximum scores. This is more automated and comparatively less trial and error.
You could also get one step more sophisticated and do a frequency analysis: make histograms of ciphertext letters and compare those to the frequencies of English (e is the most common single letter; followed by t, a...just remember Etaoin Shrdlu and you'll be fine). Then you can do a $\chi$-squared test to compare your ciphertext frequencies to the expected ones from English.
As galvatron notes, there are only 26 possible keys, so you can just try them all even if you're working by hand. (If you have a computer, it's trivial.) Note that you don't need to decrypt the entire ciphertext with each key; just decrypting, say, the first word should be enough to rule out most if not all wrong keys.
In this particular case, since your ciphertext has word breaks, you can also exploit the fact that all normal English words contain at least one vowel (including Y). So, in particular, you can pretty safely assume that one of the two letters in the ciphertext word DI must decrypt to a vowel, which limits the possible keys to just 2 6 = 12 (including the null key that encrypts I to I, which we may immediately rule out). In fact, only one of these keys yields a common English word (IN), and indeed turns out to be the correct one.
(Of course, in general, you might be unlucky and the "word" you picked might turn out to be an acronym with no vowels at all. If so, you'll notice it when none of the decryptions makes any sense, at which point you can go back and try the other keys you previously ruled out as unlikely.)
As galvatron also notes, a good general method for breaking simple substitution ciphers is frequency analysis. Basically, you start by counting the number of times each letter occurs in the ciphertext, and then assume that the most common letters in the ciphertext most likely correspond to the most common letters in plain English text.
Unfortunately, for your specific example ciphertext, frequency analysis is somewhat less effective than usual. Not only is the ciphertext too short to yield reliable frequency data (most letters in it occur only once), but it looks as if the plaintext might even have been deliberately chosen to have an untypical frequency distribution. Of the three most common English letters, T is completely absent from the plaintext, and E and A both occurs only once, in the word EXAM, which also contains an X (one of the rarest letters in English, after Q and Z).
That said, if you persist with frequency analysis, trying to match the most common ciphertext letter (J, occurring 3 times) to the most common English letters (which can be easily memorized as the nonsense phrase ETAOIN SHRDLU), you're likely to hit the correct match (J = O) after only three false starts. If the author of the question had really wanted to mess up frequency analysis, they'd have replaced the word GOOD with MUCH. ;)
The Caesar cipher (or Caesar code) is a monoalphabetic substitution cipher, where each letter is replaced by another letter located a little further in the alphabet (therefore shifted but always the same for given cipher message).
Example: Crypt DCODEX with a shift of 3.
To encrypt D, take the alphabet and look 3 letters after: G. So D is encrypted with G.
To encrypt X, loop the alphabet: after X : Y, after Y : Z, after Z : A. So X is coded A.
DCODEX is coded GFRGHA
Example: Decrypt GFRGHA with a shift of 3.
To decrypt G, take the alphabet and look 3 letters before: D. So G is decrypted with D.
To decrypt X, loop the alphabet: before A: Z, before Z: Y, before Y: X. So A is decrypted X.
GFRGHA is decrypted DCODEX.
The code was named after Julius Caesar who was born in 100 BCE the first man which has testimonies (like Suetonius) proving that he used this type of substitution to protect his military communications.
We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the allotted time of 12 mins and will probably get a 0% score on the quiz.
When trying to break an unknown cipher, one first needs to figure out what kind of cipher one it is. Generally, a good starting point would be to start with the most common and well known classical ciphers, eliminate those that obviously don't fit, and try the remaining ones to see if any of them might work.
Compiling a letter (or symbol) frequency table of the ciphertext, and comparing it to the corresponding table of plain English text, can often yield information about the general type of cipher one is dealing with:
If the frequency distribution is closer to uniform than one would expect for natural language, you're probably looking at a polyalphabetic substitution cipher. With experience (and enough ciphertext), one may even be able to guess at the most likely cipher just based on the frequency distribution.
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