Implementing-cryptography-using-python

Chapter 5 ■ Stream Ciphers and Block Ciphers 141
contain an even number of hexadecimal digits. Since the output of the hexlify
function returns twice the length, unhexlify returns half the length. Here’s
an example of using unhexlify to convert back to binary from hexadecimal:
>>> unpasshex = binascii.unhexlify(passhex)
>>> unpasshex
b'apples are red'
Furthermore, Python allows you to use an x escape character to convert from
hexadecimal back to plaintext, as shown here:
>>> hx = '\x61'
>>> hx
'a'
Use Stream Ciphers
Let’s now start exploring private-key cryptography by looking at what stream
ciphers are and then examine them as a combination of the one-time pad (OTP)
and cryptographically secure pseudorandom number generators (CSPRNGs).
The concepts of the OTP that make it perfectly secure are what you capitalize
on to make a more robust cipher. As you may recall from Chapter 2, the onetime
pad takes a message of n bits and a uniformly random secret key of the
same length and generates a ciphertext by bitwise XOR. The encryption and
decryption methods are identical operations.
When you examine the elegance of the OTP, you will undoubtedly see its
weaknesses as well:
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You must arrange a key exchange between the sender and receiver of the
message; this is true in all symmetric schemes.
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You can use the key only once.
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The key length is equal to the message length; if you have a way to send
the key securely, then perhaps you could have just sent the message.
You could resolve one of the most significant OTP problems if both parties
could generate keys on the fly with the other party. Ideally, both parties would
use an unpredictable pseudorandom number generator that would start from
the same seed and generate a uniformly random stream of noise that acts like
the one-time pad key. As you witnessed in the previous chapter, PRNGs can
be predictable and therefore can be cracked. Thus, we need to find improved
cryptographically secure pseudorandom number generators.
The goal of this section is to guide you through the creation of your first
“do it yourself” stream cipher. We will utilize an insecure stream cipher as an