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The idea behind the use of frequency analysis in cryptanalysis is that all human languages have underlying statistical patterns and redundancies that can be exploited to help break a variety of ciphers. For the English language, it is well documented that the distribution of the most frequent characters is remarkably similar throughout texts of diverse style and length, as indicated in Table 2.1.

Table 2.1 Approximate frequencies of letters in the English language.

Letter	Frequency (%)	Letter	Frequency (%)
a	7.44	b	1.46
c	2.52	d	3.53
e	12.22	f	2.68
g	1.84	h	5.97
i	6.82	j	0.20
k	0.65	l	4.28
m	2.71	n	6.32
o	8.25	p	1.97
q	0.12	r	6.21
s	6.99	t	9.85
u	3.67	v	0.12
w	2.09	x	0.18
y	1.87	z	0.03

Source: These percentages are based on the book “The Tragical History of Doctor Faustus”, by Christopher Marlowe, which is a book of approximately 100 000 characters that was chosen randomly from the Project Gutenberg.

Frequency analysis can be used for cryptanalysis. However, one needs a lot of craft in its use, along with any information that can be gathered about the contents of the message and the sender.