Monday, May 29th 2017    |   




Cipher  Simple Monoalphabetic Substitution


In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution. There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher.


Cryptanalysis  Frequency Analysis


Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies. Moreover, there is a characteristic distribution of letters that is roughly the same for almost all samples of that language. The phrase ETAOIN SHRDLU encodes the 12 most frequent letters in typical English language text.



The Basics: In a simple substitution cipher, each letter of the plaintext is replaced with another, and any particular letter in the plaintext will always be transformed into the same letter in the ciphertext. For instance, if all occurrences of the letter e turn into the letter X, a ciphertext message containing numerous instances of the letter X would suggest to a cryptanalyst that X=E.

More complex use of statistics can be conceived, such as considering counts of pairs of letters, or triplets (trigrams), and so on. This is done to provide more information to the cryptanalyst, for instance, Q and U nearly always occur together in that order in English, even though Q itself is rare.

Identify Common Pairs Of Letters: If the ciphertext appears to encode a message in English, but the plaintext does not reveal itself immediately, which is often the case, then focus on pairs of repeated letters. In English the most common repeated letters are ss, ee, tt, ff, ll, mm and oo. If the ciphertext contains any repeated characters, you can assume that they represent one of these.

Identify The Smallest Words First: If the ciphertext contains spaces between words, then try to identify words containing just one, two or three letters. The only one-letter words in English are a and I. The most common two-letter words are of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am. The most common three-letter words are the and and.

Tailor Made Frequency Tables: If possible, tailor the table of frequencies to the message you are trying to decipher. E.g., military messages tend to omit pronouns and articles, and the loss of words such as I, he, a and they will reduce the frequency of some of the commonest letters. If you know you are tackling a military message, you should use a frequency table generated from other military messages.

Play The Guessing Game: This can be one of the most useful skills for a cryptanalyst to employ - the ability to identify words, or even entire phrases, based on experience or sheer guesswork. Al-Khalil, an early Arabian cryptanalyst, demonstrated this talent when he cracked a Greek ciphertext. He guessed that the ciphertext began with the greeting 'In the name of God'. Having established that these letters corresponded to a specific section of ciphertext, he could use them as a crowbar to prise open the rest of the ciphertext. This is known as a crib.

Order Of Frequency Of Single Letters E T A O I N S H R D L U
Order Of Frequency Of Digraphs th er on an re he in ed nd ha at en es of or nt ea ti to it st io le is ou ar as de rt ve
Order Of Frequency Of Trigraphs the and tha ent ion tio for nde has nce edt tis oft sth men
Order Of Frequency Of Most Common Doubles ss ee tt ff ll mm oo
Order Of Frequency Of Initial Letters T O A W B C D S F M R H I Y E G L N P U J K
Order Of Frequency Of Final Letters E S T D N R Y F L O G H A K M P U W
One-Letter Words a, I.
Most Frequent Two-Letter Words of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am
Most Frequent Three-Letter Words the, and, for, are, but, not, you, all, any, can, had, her, was, one, our, out, day, get, has, him, his, how, man, new, now, old, see, two, way, who, boy, did, its, let, put, say, she, too, use
Most Frequent Four-Letter Words that, with, have, this, will, your, from, they, know, want, been, good, much, some, time


Messageabcdefghijklmnopqrstuvwxyz
Substitution
Final Letter                          


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Frequency %
* Message
Transformed
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