Playfair cipher is relatively easy to figure out because it still exposes much of the structure of the plaintext language by revealing the frequency of letters.

Explanation:

In a typical message or plaintext, we use some alphabets more frequently than others.for example, we use the alphabet ' e ' more frequently than ' z '

in fig 3.6 we can see the distribution of letters in a typical plain text. (in plaintext plot)

Let's see how the frequency distribution plot developed in fig 3.6 in the first place.

to find the frequency value of the letter, we take the number of occurrences of each letter in the text and divide it with the occurrences of the most frequently used letter.

such that value of letter ' e ' calculated as 12.702/12.072 = 1; (Acc to values of fig 3.5)
 value of letter ' t ' calculated as 9.056/12.072 = 0.72;

Generally, if the relative frequency of a letter's plot is straight, flat, and horizontal then it is said to be secure and cryptanalysis of ciphertext is nearly impossible.

But in fig 3.6 You can see the plot of the Playfair cipher plot nowhere near flat. and share much of the same relative frequency structure as plaintext. leaving enough data for attackers to work with. Generally, a few hundred letters of ciphertext are said to be sufficient.

This is the main drawback of Playfair encryption.