Cryptography Reference
In-Depth Information
Table 1.1
Relative letter frequencies of the English language
Letter Frequency Letter Frequency
A
0.0817
0.0675
N
0.0150
0.0751
B
O
0.0278
0.0193
C
P
0.0425
0.0010
D
Q
0.1270
0.0599
E
R
0.0223
0.0633
F
S
0.0202
0.0906
G
T
0.0609
0.0276
H
U
0.0697
0.0098
I
V
0.0015
0.0236
J
W
0.0077
0.0015
K
X
0.0403
0.0197
L
Y
0.0241
0.0007
M
Z
Lesson learned
Good ciphers should hide the statistical properties of the encrypted
plaintext. The ciphertext symbols should appear to be random. Also, a large key
space alone is not sufficient for a strong encryption function.
1.3 Cryptanalysis
This section deals with recommended key lengths of symmetric ciphers and differ-
ent ways of attacking crypto algorithms. It is stressed that a cipher should be secure
even if the attacker knows the details of the algorithm.
1.3.1 General Thoughts on Breaking Cryptosystems
If we ask someone with some technical background what breaking ciphers is about,
he/she will most likely say that code breaking has to do with heavy mathematics,
smart people and large computers. We have images in mind of the British code
breakers during World War II, attacking the German Enigma cipher with extremely
smart mathematicians (the famous computer scientist Alan Turing headed the ef-
forts) and room-sized electro-mechanical computers. However, in practice there are
also other methods of code breaking. Let's look at different ways of breaking cryp-
tosystems
in the real world
(Fig. 1.6).
