Implementing-cryptography-using-python

142 Chapter 5 ■ Stream Ciphers and Block Ciphers
experiment to gain an understanding of the underlying mechanics. Once you
complete it, you should feel comfortable with the use of XOR, seeds, and the
general structure of a stream cipher. This section builds the base in which we
add improvements with true security in mind.
In building your first stream cipher, you take the idea of the one-time pad
mixed with the convenience of PRGNs to create an encryption scheme. Since
we haven’t explored any nonbreakable PRNGs yet, we will work with an insecure
one, but the mechanics are identical when you have a more secure source
of pseudorandom bits. It is important to know that since you want to use the
stream cipher to work on real-world data, it is going to have to encrypt ASCII
characters, which means you need to think about the use of random bytes more
than the use of random bits. It is not much of a change, but it is worth mentioning.
To start, we select the C rand() function as our PRNG since we have code for
it from Chapter 4. It generates 31 bits at a time (the numbers are mod 2 31 ), and
you want 8 bits at a time. For the sake of this example, use the lowest 24 bits of
each number that comes out and generate three bytes of randomness for our
stream. Using these guidelines, this should allow you to generate a convention
that matches the partner with which you are communicating. A rand()-generated
number might look like temp = 2158094741372. If you want three bytes of randomness
from that, you could follow these steps:
>>> temp = 2158094741372
>>> temp % 2**8
124
>>> (temp >> 8) % 2**8
111
>>> (temp >> 16) % 2**8
120
The code should produce three numbers uniformly chosen between 0 and
255. Now let’s write our own simple PRNG function. It should produce four
output numbers, [1471611625, 1204518815, 463882823, 963005816]:
def crand(seed):
r=[]
r.append(seed)
for i in range(30):
r.append((16807*r[-1]) % 2147483647)
if r[-1] < 0:
r[-1] += 2147483647
for i in range(31, 34):
r.append(r[len(r)-31])
for i in range(34, 344):
r.append((r[len(r)-31] + r[len(r)-3]) % 2**32)
while True:
next = r[len(r)-31]+r[len(r)-3] % 2**32