Concepts in Data Encryption:
A monoalphabetic substitution is one where a letter of plaintext always produces the same letter of ciphertext.
The operation is very similar to the Caesar Cipher and Atbash, with the exception that the cipher alphabet does not have the order which it had in the previous examples (which are just special cases of monoalphabetic substitutions). An example of a monoalphabetic substitution is shown below.
PLAINTEXT abcdefghijklmnopqrstuvwxyz CIPHERTEXT QRSKOWEIPLTUYACZMNVDHFGXJB
One may naïvely think that this cipher is secure, after all there are 26! different cipher alphabets ( 4 x 10 26) to choose from, however the letter frequencies and underlying patterns will be unchanged - and as such the cipher can be solved by pen and paper techniques. The best way to see how the cryptanalysis is performed is by doing some analysis.
These more advanced techniques include looking systematically at the position of letters in words in order to identify vowels, pattern words, and looking at the letter frequencies, though common pairings (TH, HE etc.) may come up.
The most common English letters are ETAIN. it is highly likely that in any ciphertext, we have some matches - though it is not a certainty. It should also be noted that the most commontrigraph (three letter sequence) in English is 'THE'.
When cryptanalysing more complex ciphers such as Vigenère one of the first steps could be to try and reduce the cipher into a series of monoalphabetic ciphers.
Of course the analysis may be done automatically by a computer program which observes letter positions and frequencies etc.
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