If you have got a message encrypted using the substitution cipher that you want to crack, you can use frequency analysis. In other words, if the sender has tried to disguise a letter by replacing with a different letter, you can still recognise the original letter because the frequency characteristics of the original letter will be passed on to the new letters.
To apply frequency analysis, you will need to know the frequency of every letter in the English alphabet, or the frequency of letters in whichever language the sender is using.
Below is a list of average frequencies for letters in the English language. So, for example, the letter E accounts for 12.7% of all letters in English, whereas Z accounts for 0.1 %. All the frequencies are tabulated and plotted below.
Please note, these frequencies are averages, and E will not always constitute 12.7 % of all the letters in a text, and may not even be the most common letter. The longer the message, the more likely it is that will obey the average distribution shown above. However, there are exceptions to this rule. In 1969, the French author Georges Perec managed to write a 200-page book called 'La Disparition' without using any words containing the letter E. Amazingly, the book was later translated into English by Gilbert Adair, again avoiding the use of the letter E.Count Letter Frequencies