Implementing-cryptography-using-python

Chapter 2 ■ Cryptographic Protocols and Perfect Secrecy 51
a perfectly secure Shannon cipher. That is, the perfectly secure Shannon cipher
ε has produced a ciphertext that has equal probability of being any message, i.e.,
the ciphertext c gives no information about the plaintext m.
We will explore perfect security more as we explore the one-time pad in the
next section.
One-Time Pad
Perfect secrecy is the concept that given a ciphertext (an encrypted message) from
a cipher or perfectly secure encryption system, nothing will be revealed about
the plaintext (unencrypted message) by the ciphertext.
A perfectly secret cipher has a couple of other equivalent properties:
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There is a key that encrypts every possible plaintext to every possible
ciphertext (perfect key ambiguity). However, this is true only if the keys
used are the same size as the messages.
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Even if given a choice of two plaintexts, one the real one, for a ciphertext,
you cannot distinguish which plaintext is the real one (perfect message
indistinguishability).
What perfect secrecy means in practice is that no amount of computation
applied to the ciphertext will give you any advantage in knowing anything
about the plaintext or key. This is obviously a desirable property of a cipher,
and perfectly secret ciphers do exist. For example, a one-time pad (OTP) is a
perfectly secret cipher.
When you examine the OTP, you may decide that it is the basis of modern
cryptography or you may determine it does not accomplish what we need.
Still, it is important that you understand the scheme and its subtleties. In this
section, you will gain a formal description of the algorithm and will explore
the OTP from Python; the goal is to show that it can be broken when used in
a repetitive scenario.
XOR, AND, and OR
The bitwise operators work on bits and perform bit-by-bit operations. Assume
a = 60 and b = 13. The binary format of a and b will be as follows:
a = 0011 1100
b = 0000 1101
Table 2.2 explains each bitwise operator and gives an example.