Question

The Public Domain Graph - English Letter Frequency shows the frequencies of letters in the English language.

Explain how you would use this information to break a monolithic cipher like Caesar Cipher.

Answer 1

If we know the frequencies of letters in english language then we can easily break a monolithic cipher
i.e Caesar cipher because frequency tells us about how often a letter is occuring in english language
and in monolithic cipher each letter is shifted by certain unit decided by the key for eg if key is 2
then A becomes C, B becomes D and so on. If we know the frequency of each letter in the original english
language from the graph then we can plot the public domain graph for encrypted text also and find out
frequency of each letter in it. Suppose A has highest frequency in the original language and E has the
highest frequency in the encrypted language then it clearly indicates that A is encrypted to E that means
a shift of 4 and hence key is 4 which clearly breaks down our cipher text.